Click on the "[show]" link in the box below.
Van Leeuwen distance
van Leeuwen 2007 distance of 152 ± 20 pc. [1]
Luminosity Calculations
Luminosity Calculations [2] EndLuminosityCalcs
The following calculations determine Betelgeuse's average radius in terms of solar units based on the following assumptions for angular diameter:
1) Weiner 2000 = 55.20 ± 0.50 mas (limb darkened)
2) Perrin 2004 = 43.33 ± 0.04 mas
The calculations begin with the formula for a star's angular diameter, as follows:
where
represents Betelgeuse's angular diameter in
arcseconds,
the Distance from Earth in
parsecs
Betelgeuse's diameter in
AU, and
Betelgeuse's Radius in AU. Therefore:
To convert the above into Solar units, the math is straightforward. Since 1 AU = 149,597,871 km and the mean diameter of the Sun = 1,392,000 km (hence a mean radius of 696,000 km), the calculation is as follows:
Luminosity Note
Nevertheless, since Betelgeuse is a pulsating variable star, there are times where the star's luminosity could theoretically exceed 200,000 L☉ [note 1]
Therefore:
Note: These calculations do not take into consideration any diminution caused by extinction, which in the case of Betelgeuse has been estimated at around 3.1%
The current debate revolves around which wavelength—the visible, near-infrared ( NIR) or mid-infrared ( MIR)—produces the most accurate angular measurement. [note 2] In 1996, Manfred Bester,working with the ISI in the mid-infrared, led a team at the Space Sciences Laboratory (SSL) at U.C. Berkeley to produce a solution showing Betelgeuse with a uniform disk of 56.6 ± 1.0 mas. In 2000, the SSL team produced another measure of 54.7 ± 0.3 mas, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared. [3] Also included was a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas. The Bester estimate equates to a radius of roughly 5.6 AU or 1,200 R☉, assuming the 2008 Harper distance of 197.0 ± 45 pc, [4] a figure roughly the size of the Jovian orbit of 5.5AU, published in 2009 in Astronomy Magazine and a year later in NASA's Astronomy Picture of the Day. [5]
Across the Atlantic, another team of astronomers working in the near-infrared and led by Guy Perrin of the Observatoire de Paris produced a 2004 document arguing that the more accurate photospheric measurement was 43.33 ± 0.04 mas. [6] The study also put forth an explanation as to why varying wavelengths from the visible to mid-infrared produce different diameters. The star is seen through a thick, warm extended atmosphere. At short wavelengths (the visible spectrum) the atmosphere scatters light thus slightly increasing the star's diameter. At near-infrared wavelengths ( K and L bands), the scattering is negligible, so the classical photosphere can be directly seen; in the mid-infrared the scattering increases once more causing the thermal emission of the warm atmosphere to increase the apparent diameter. [6] Comparing studies, however, is problematic since each wavelength produces a different view of the star, requiring different interpretations. [7]
Studies done in 2009 with the IOTA and VLTI brought strong support to Perrin's analysis yielding diameters ranging from 42.57 to 44.28 mas with comparatively insignificant margins of error. [9] [10] In 2011, Keiichi Ohnaka from the Max Planck Institute for Radio Astronomy produced a third estimate in the near-infrared corroborating Perrin's numbers, this time showing a limb-darkened disk diameter of 42.49 ± 0.06 mas. [11] Consequently, if we combine the smaller Hipparcos distance from van Leeuwen of 152 ± 20 pc with Perrin's angular measurement of 43.33 mas, a near-infrared photospheric estimate would equate to 3.4AU or roughly 730 R☉. [12]
Central to this discussion is another paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate. [13] [14] Unlike most papers heretofore published, this study encompassed a 15-year period at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in Betelgeuse's apparent size equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9 AU in 15 years. What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star's photosphere as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggested that if a cycle does exist, it is probably a few decades long. [13] Other possible explanations are photospheric protrusions due to convection or a star that is not spherical but rather asymmetric causing the appearance of expansion and contraction as the star rotates on its axis. [15]
In conclusion, the current debate between measurements in the mid-infrared, which suggest a possible expansion and contraction of the star, versus the relatively constant photosphere observed in the near-infrared is yet to be resolved. In a recent paper published in 2012, however, the Berkeley team reported that their measurements were "dominated by the behaviour of cool, optically thick material above the stellar photosphere", indicating that the apparent expansion and contraction may be due to activity in the star's outer shells and not the photosphere itself. This conclusion, if further corroborated, would suggest an average angular diameter for Betelgeuse closer to Perrin's estimate at 43.33 arcseconds, hence a stellar radius of about 3.4 AU (730 R☉), if we assume the shorter Hipparcos distance of 498±73ly in lieu of Harper's estimate at 643±146ly. [note 3] [16] The upcoming Gaia Mission will likely clarify many of the assumptions presently used in calculating the size of Betelgeuse's stellar disk, yielding a much wider consensus among astronomers.
Once considered as having the largest angular diameter of any star in the sky after the Sun, Betelgeuse lost that distinction in 1997 when a group of astronomers measured R Doradus with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse. [17]
In 1985, Margarita Karovska, in conjunction with other astrophysicists at the CfA, announced the discovery of two close companions orbiting Betelgeuse. Analysis of polarization data from 1968 through 1983 indicated a close companion with a periodic orbit of about 2.1 years. Using speckle interferometry, the team concluded that the closer of the two companions was located at 0.06 ± 0.01 " (~9 AU) from the main star with a position angle (PA) of 273 degrees, an orbit that would potentially place it within the star's chromosphere. The more distant companion was estimated at 0.51 ± 0.01 " (~77 AU) with a PA of 278 degrees. [18] [19]
In the years that followed no confirmation of Karovska's discovery was ever published. Rather in 1992, a team of collaborators from the Cavendish Astrophysics Group published a paper noting that the brightness features on the surface of Betelgeuse appear to be "too bright to be associated with a passage of the suggested companions in front of the red giant." They also noticed that these features were fainter at 710 nanometers compared to 700 by a factor of 1.8, indicating that such features would have to reside within the molecular atmosphere of the star. [20]
That same year, Karovska published a new paper reconfirming her team's exegesis, but also noting that there was a meaningful correlation between the calculated position angles of the orbiting companion and the reported asymmetries, suggesting a possible connection between the two. [21] Since then, researchers have turned their attention to analyzing the intricate dynamics of the star's extended atmosphere and little else has been published on the possibility of orbiting companions, although as Xavier Haubois and his team reiterate in 2009, the possibility of a close companion contributing to the overall flux has never been fully ruled out. [10] Dommanget's double star catalog (CCDM) lists at least four adjacent stars, all within three arcminutes of this stellar giant, yet aside from apparent magnitudes and position angles, little else is known. [22] As the decade unfolds and new technologies are brought to unraveling the star's enigmatic past, conclusive evidence will likely emerge of any potential star system. Given the planned capabilities of the upcoming Gaia mission, a confirmation could occur any time after the mission's scheduled launch in August 2013. [23]
Because the original discovery of a companion star in 1985 by Karovska was never confirmed, there is no direct method of measuring Betelgeuse's mass. [24] A mass estimate is only possible using theoretical modeling, a situation which has produced mass estimates ranging from 5—30 M☉ in the last decade. [25] [26] The current estimate that is widely accepted is 17.5±2.5M⊙, based on a photospheric measurement of 5.6AU or 1,200R⊙. However, a novel method of determining the supergiant's mass was proposed in 2011 by Hilding Neilson et al. arguing for a stellar mass of 11.6M⊙, based on observations of the star’s intensity profile from narrow H-band interferometry and using a photospheric measurement of 4.3AU or 995R⊙. How the debate is resolved is still open to question—at least until a companion star is identified allowing for a direct calculation of the star's mass.
These uncertainties also mean that no consensus has yet emerged regarding the star's mass. Estimates range from 5 to 30 solar masses (M☉) with most investigators showing a preference for a relatively large mass ranging from 10 to 20M☉. One model reports a mass at the lower end of the scale at 14M☉, although a mass ranging from 18 to 20 is more commonplace. [25] [27]
Across the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement. [6] "A consistent scenario to explain the observations of this star from the visible to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slightly increasing its diameter. The scatter becomes negligible above 1.3 μm. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." The argument has yet to receive widespread support among astronomers. [7]
Across the Atlantic, another team of astronomers led by Guy Perrin of the
Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement.
[6] The study also puts forth an explanation as to why varying wavelengths from the visible to mid-infrared produce different diameters. The star is seen through a thick, warm extended atmosphere. At short wavelengths (the visible spectrum) the atmosphere scatters light thus slightly increasing its diameter. At infrared wavelengths (
K and L bands), the scattering is negligible, so the classical photosphere can be directly seen; in the mid-infrared the scattering increases once more causing the thermal emission of the warm atmosphere to increase the apparent diameter.
[6] Comparing studies at different wavelengths, however, is problematic, since each wavelength produces a different view of the star, requiring different interpretations.
[7]
I reworked paragraph pursuant to observations
To compute these density ratios, the first step is to calculate the mass and volume of each component from which density calculations can be made. Given the density of the Sun, Betelgeuse, and the Earth's atmosphere, dividing gives the appropriate ratios.
Calculations for the Sun:
Calculations for Betelgeuse:
Calculations for the Earth's atmosphere:
Average Density Ratios:
Conclusion:
The following calculations determine Betelgeuse's average radius in terms of solar units based on the following assumptions for angular diameter:
1) Weiner 2000 = 55.20 ± 0.50 mas (limb darkened)
2) Perrin 2004 = 43.33 ± 0.04 mas
The calculations begin with the formula for a star's angular diameter, as follows:
where
represents Betelgeuse's angular diameter in
arcseconds,
the Distance from Earth in
parsecs
Betelgeuse's diameter in
AU, and
Betelgeuse's Radius in AU. Therefore:
To convert the above into Solar units, the math is straightforward. Since 1 AU = 149,597,871 km and the mean diameter of the Sun = 1,392,000 km (hence a mean radius of 696,000 km), the calculation is as follows:
Luminosity is generally understood as a measurement of brightness. Each discipline, however, defines the term differently, depending on what is being measured.
In astronomy, luminosity measures the total amount of energy emitted by a star or other astronomical object in joules per second, which are watts. Watts are the unit of power, and so just as a light bulb is measured in watts, so too is the Sun, the latter having a total power output of 3.846×1026 W. It is this number which constitutes the basic metric used in astronomy and is known as 1 solar luminosity, the symbol for which is . Radiant power, however, is not the only way to conceptualize brightness, so other metrics are also used. The most common is apparent magnitude, which is the perceived brightness of an object from an observer on Earth at visible wavelengths. Other metrics are absolute magnitude, which is an object's intrinsic brightness at visible wavelengths, irrespective of distance, while bolometric magnitude is the total power output across all wavelengths.
The field of optical photometry uses a different set of distinctions, the main ones being luminance and illuminance. Astronomical photometry, by contrast, is concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation. In the field of computer graphics the concept of luminosity is different altogether and is typically used as a synonym for the concept of lightness, otherwise known as the value or tone component of a color.
In astronomy, luminosity is the amount of electromagnetic energy a body radiates per unit of time. It is measured in two forms: apparent (visible light only) and bolometric (total radiant energy). A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term "luminosity" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities. A star also radiates neutrinos, which carry off some energy, about 2% in case of our sun, producing a stellar wind and contributing to the star's total luminosity.[ citation needed]
A star's luminosity is determined primarily by two stellar characteristics: size and temperature. The former is typically represented in terms of solar radii, , while the latter is represented in degrees Kelvin. To determine a star's radius, however, two other metrics are needed: the star's angular diameter and its distance from Earth, usually calculated using parallax measurements. A third metric is the degree of interstellar extinction that is present, a condition that usually arises because of gas and dust present in the interstellar medium (ISM), the Earth's atmosphere, and circumstellar matter. Consequently, one of astronomy's central challenges in determining a star's luminosity is to derive accurate measurements for each of these components, without which an accurate luminosity figure remains elusive.
In the current system of stellar classification, stars are grouped according to temperature, with the very young and energetic Class O stars boasting temperatures in excess of 30,000 K while the older Class M stars exhibit temperatures less than 3,700K—a vast difference that has a huge impact on the star's luminosity. As a result, the most luminous stars in the galaxy are usually the youngest; as stars evolve, their stellar wind causes them to lose mass, a phenomenon which varies greatly with each class of stars but which nevertheless causes a diminution in the star's luminosity over its lifetime. In the Hertzsprung-Russel diagram, the vast majority of stars are found along the main sequence with blue Class 0 stars found at the top left of the chart, given their high luminosity, while red Class M stars fall to the right. The reason certain stars like Deneb and Betelgeuse are found off the main sequence is because of their extremely high luminosity resulting from their enormous size. A star like Deneb, for instance, has a radius that is 203, yielding a mass of 19 and luminosity of 196,000, which means that this blue-white supergiant radiates one hundred and ninety-six thousand times as much energy as the Sun. [30] By contrast, the much cooler Betelgeuse has a luminosity of approximately 140,000, a figure which is only possible because it is considerably larger than Deneb; with a radius of 995, Betelgeuse is about 15 times its size. The most brilliant star ever discovered is a Wolf-Rayet star known as R136a1, spotted in the Large Magellanic Cloud in 2010. At 265 it is one of the most massive and most luminous stars yet identified, boasting a total bolometric luminosity 8,700,000 times that of the Sun.
To conceptualize the range of magnitudes in our own galaxy, the
Milky Way, the smallest star to be identified has about 8% of the Sun’s mass and glows feebly at absolute magnitude +19. Compared to the Sun, which has an absolute of +4.8, this faint star is 14 magnitudes or 400,000 times dimmer than our Sun. Our galaxy's most massive stars begin their lives with masses of roughly 100 times solar, radiating at upwards of absolute magnitude –8, over 160,000 times the solar luminosity. The total range of stellar luminosities, then, occupies a range of 27 magnitudes, or a factor of 60 billion!
The most common distinctions encountered are apparent magnitude, which is the measurement of a celestial body's visible light as seen by an observer from Earth. By contrast, absolute Magnitude is a visible light measurement irrespective of distance and so measures a luminous body's intrinsic brightness. Finally, bolometric magnitude measures light at all wavelengths in the electromagnetic spectrum. Every star will thus have three different measurements. Betelgeuse has an median apparent magnitude of 0.42, but given a distance of 197 parsecs has an absolute magnitude of -6.02. Being a cool, red star, its bolometric luminosity is -7.99, which means the visible light that reaches our eye is only 6% of the star's total radiant energy. If the human eye could see light at all wavelengths, Betelgeuse would show up as the brightest star in the sky.
The word "luminosity" may also refer to spectral luminosity, measured either in W/Hz or W/nm.
In the late phase of stellar evolution, massive stars like Betelgeuse exhibit high rates of mass loss, possibly as much as 1M☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux. [31] All stars exhibit mass loss. Rates vary from about 10−14 to 10−4 yr −1 depending on spectral type, luminosity class, rotation rate, companion proximity, and evolutionary stage. [32] Exactly how this mass loss occurs, however, has been a mystery confronting astronomers for decades. When Schwarzschild first proposed his theory of monster convection cells, he argued it was the likely cause in red supergiants. Prior attempts to explain mass loss in terms of solar wind theory had proven unsuccessful as they led to a contradiction with observations involving circumstellar shells. [33] Other theories that have been advanced include magnetic activity, global pulsations and shock structures as well as stellar rotation. [34]
As a result of work done by Pierre Kervella and his team at the Paris observatory in 2009, astronomers may be close to solving this mystery. Kervella noticed a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions. [35] [31] The plume's presence, in fact, implies that the spherical symmetry of its photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation. [35] Probing deeper still with ESO's AMBER, Keiichi Ohnaka observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella. [31] [36]
In addition to the photosphere, six other components of Betelgeuse's atmosphere have now been identified. Extending outward, one encounters a molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide (CO). There is also evidence of coronal plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence. [25] Some of these elements are known to be asymmetric while others overlap. [10]
At about .45 stellar radii (~2–3 AU) above the photosphere, the closest membrane appears to be the molecular layer known as the MOLsphere. Studies show it to be composed of water-vapor and carbon monoxide with an effective temperature of about 1500 ± 500K. [10] [37] Water-vapor had been originally detected in the supergiant's spectrum back in the 1960s with the two Stratoscope projects but had been ignored for decades. Recent studies suggest that the MOLsphere may also contain SiO and Al2O3—molecules which could explain the formation of dust particles.
Between two and seven stellar radii (~10–40 AU), astronomers have identified another region known as an asymmetric gaseous envelope composed of elemental abundances C, N and O. [10] [38] The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense atmosphere with a "remarkably complex structure". [39] Observations show the atmosphere to be boiling with a temperature of 3,450 ± 850K—similar to the temperature recorded on the star's surface but much lower than surrounding gas in the same region. [39] [40] The VLA images also showed this lower-temperature gas progressively decreasing in temperature as it extends outward—the existence of which, although unexpected, turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere." [39] This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing C N and extending at least six photospheric radii in the southwest direction of the star, is believed to exist. [10]
The chromosphere, as mentioned earlier, was resolved in 1996 at about 2.2 times the optical disk (~10 AU) at ultraviolet wavelengths and is reported to have a temperature no higher than 5,500K. [41] [10] The image was taken with the Faint Object Camera on board the Hubble Space Telescope and also revealed a bright area in the southwest quadrant of the disk. However, in 2004 observations with the STIS, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of 197 pc, the size of the chromosphere could be 200 AU [42] The CfA team led by Alex Lobel concluded that the spatially resolved STIS spectra directly demonstrate the co-existence of warm chromospheric plasma with cool gas in Betelgeuse's circumstellar dust envelope. [42]
The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton and colleagues, who noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, they concluded that the red supergiant emits most of its excess beyond 12 stellar radii or roughly the distance of the Kuiper belt at 50 to 60 AU, depending on the assumed stellar radius. [10] [45] Since then, there have been numerous studies done of this dust envelope at varying wavelengths yielding decidedly different results. More recent studies have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, or 100 to 200 AU. [46] [47] What these studies point out is that the dust environment surrounding Betelgeuse is anything but static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. A few years later, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in just one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots. [46] The 1984 report of a giant asymmetric dust shell located 1 pc (206,265 AU) from the star has not been corroborated in recent studies, although another report published the same year said that three dust shells were found extending four light years from one side of the decaying star, suggesting that, like a snake, Betelgeuse sheds its outer layers as it journeys across the heavens. [48] [49]
Although the exact size of the two outer CO shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0 arcseconds with the other expanding as far as 7.0 arcseconds. [50] Using the Jovian orbit of 5.5 AU as the "average" radius for this gargantuan star, the inner shell would extend roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer one as far as 250 stellar radii (~1400 AU). With the heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of the Solar System.
Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~6.3 AU per year) creating a bow shock. [51] The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first imaged. The cometary structure is estimated to be at least 1 parsec, assuming a current distance of 643 light years. [52]
Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young—less than 30,000 years old—suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. [53] In their 2012 paper, Mohammed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks." [54] [55] Moreover, if future research bears out this hypothesis, Betelgeuse may well prove to have traveled close to 200,000 AU as a red supergiant scattering as much as 3 along its trajectory.
Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as ὑπόκιρρος. It was described by a translator of Ulugh Beg's Zij-i Sultani as rubedinem "ruddiness". [56]
The variation in Betelgeuse's brightness was first described in 1836 by Sir John Herschel, when he published his observations in Outlines of Astronomy; he noted an increase in activity from 1836–1840, followed by a subsequent reduction. In 1849, he noted a shorter cycle of variability which peaked in 1852. Later observers recorded unusually high maxima with an interval of several years, but only small variations from 1957 to 1967. The records of the American Association of Variable Star Observers (AAVSO) show a maximum apparent magnitude (brightness) of 0.2 in the years 1933 and 1942, with a minimum fainter than magnitude 1.2 in both 1927 and 1941. [28] [29] This variability in brightness may explain why Johann Bayer designated the star alpha as it may have rivalled the usually brighter Rigel (beta). [57]
In 1919, Albert Michelson and Francis Pease mounted a 6-metre (20 ft) interferometer on the front of the 2.5 metre (100 inch) telescope at Mount Wilson Observatory. Helped by John Anderson, in December 1920 the trio measured the angular diameter of α Orionis as 0.047 "—a figure which resulted in a diameter of 3.84 × 108 km (240 million miles or 2.58 AU) based on the then-current parallax value of 0.018 ". [58] However, there was known uncertainty owing to limb darkening and measurement errors—a central theme which would be the focus of scientific inquiry for a whole century. Beginning with this first measurement at visible wavelengths, researchers have conducted multiple experiments ranging from the ultraviolet to the far infrared, with each producing different results fueling much debate.
The 1950s and '60s saw several important developments, the two Stratoscope projects and the 1958 publication of Structure and Evolution of the Stars, both the work of Martin Schwarzschild. [59] The book taught a generation of astrophysicists how to use emerging computer technology to create stellar models while the Stratoscope projects, by taking instrumented balloons above the Earth's turbulance, produced some of the finest images of solar granules and sunspots ever seen, thus confirming the existence of convection in the solar atmosphere. Both developments would prove to have a significant impact on our understanding of the structure of red supergiants like Betelgeuse.
In the late 1980s and early 1990s, Betelgeuse became a regular target for aperture masking interferometry visible-light and infrared imaging, which revealed a number of bright spots on the star's surface, thought to result from convection. [61] [62] These were the first optical and infrared images of the disk of a star other than our Sun, and generally showed one or more bright patches—indicating the location of hotspots in the star's photosphere. [63]
In 1995, the Hubble Space Telescope's Faint Object Camera captured an ultraviolet image of comparable resolution—this was the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star. The image was taken at ultraviolet wavelengths since ground-based instruments cannot produce images in the ultraviolet with the same precision as Hubble. Like earlier images, this ultraviolet image also contained a bright patch, indicating a hotter region of about 2,000 Kelvin, in this case on the southwestern portion of the star's surface. [64] Subsequent utraviolet spectra taken with the GHRS suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a position angle from celestial North of about 55°. [65]
Recent ground-based measurements of the disk of Betelgeuse in the mid- infrared, at 11.15 μm, gave an angular diameter of 54.7 ± 0.3 milliarcseconds (mas) in November 1999, slightly smaller than the typical visible-light angular diameter. This measurement assumes a uniform disk model and ignores any possible contribution from hotspots (which are less noticeable in the mid-infrared.) Allowing for the limb darkening effect, whereby the intensity of a star's image diminishes near the edge, gives an increase of approximately 1%, to 55.2 ± 0.5 milliarcseconds (mas). It is difficult to define the precise diameter of Betelgeuse as the photosphere has no "edge"—instead the gas making up the photosphere gets gradually thinner with distance from the star. [3]
On June 9, 2009, Nobel Laureate Charles Townes announced that the star has shrunk 15% since 1993 with an increasing rate. He presented evidence that UC Berkeley's Infrared Spatial Interferometer (ISI) atop Mt. Wilson Observatory had observed 15 consecutive years of stellar contraction. The average speed at which the radius of the star has been shrinking during this period is around 1,000 km/hr. [note 4] Despite Betelgeuse's diminished size, Townes and his colleague, Edward Wishnow, pointed out that the star's visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers (AAVSO), had shown no significant dimming over that same time frame. According to the university, Betelgeuse's diameter is about 5.5 AU, and the star's radius has shrunk by a distance equal to half an astronomical unit, or about the orbit of Venus. [7] [66] Townes concluded it was unclear whether the shrinkage is part of a periodic process or not; if it is, then the cycle is likely to be decades long. [67]
In July 2009, images released by European Southern Observatory, taken by the ground based Very Large Telescope, gave a more detailed view of the surface of the star. [31] In the picture a plume of gas six times its photospheric diameter is seen extending from the star. [35] This is comparable to the distance between the Sun and Neptune.
A series of spectropolarimetric observations, obtained in 2010 with the Bernard Lyot Telescope at Pic du Midi Observatory, revealed the presence of a weak magnetic field at the surface of Betelgeuse, suggesting that the giant convective motions of supergiant stars are able to trigger the onset of a small-scale dynamo. [68]
(already exported)
(already exported)
The uncertainty of Betelgeuse's distance has puzzled astronomers for centuries, and has made reliable estimates difficult for many other stellar parameters such as luminosity. When combined with the angular diameter of the star, one can estimate the radius and its effective temperature; luminosity combined with an understanding of isotopic abundances provides an estimate of the stellar age and mass. [69] In 1920, when the first interferometric studies were performed, the assumed parallax was 0.180 arcseconds. That equates to a distance of 56 pc or roughly 180 ly. Since then, there has been an ongoing inquiry as to the actual distance of this mysterious star with estimated distances as high as 900 light years being proposed. [69]
Before the publication of the Hipparcos Catalogue (1997), there were two respected publications with up-to-date parallax data on Betelgeuse. The first was the Yale University Observatory (1991) with a published parallax of π = 9.8 ± 4.7 milliarcseconds, yielding a distance of roughly 102 pc or 330 ly. [70] The second was the Hipparcos Input Catalogue (1993) with a trigonometric parallax of π = 5 ± 4 mas, a distance of 200 pc or 650 ly—almost twice the Yale estimate. [71] With such uncertainty, researchers were adopting a wide range of distance estimates, a phenomenon which fueled much debate—not only in terms of the star's distance, but also in terms of its many other implications. [69]
The long-awaited results from the Hipparcos mission were finally released in 1997. Instead of resolving the issue, a new parallax figure was published of π = 7.63 ± 1.64 mas, which equated to a distance of 131 pc or roughly 430 ly. [72] Because stars like Betelgeuse vary in brightness, they raise specific problems in quantifying their distance. [73] As a result, the large cosmic error in the Hipparcos solution may be of stellar origin, perhaps related to movements of the photocenter, of order 3.4 mas, in the Hipparcos photometric Hp band. [69] [74]
Where a breakthrough in this debate appears to have come is with the advances in radio astronomy. New high spatial resolution, multiwavelength, NRAO Very Large Array (VLA) radio positions of Betelgeuse have produced a more precise estimate, which combined with the recent Hipparcos data furnished a new astrometric solution: π = 5.07 ± 1.10 mas, a tighter error factor yielding a distance of 197 +/- 45 pc or 643 +/- 146 ly. [69]
In 2012, the
European Space Agency's (ESA) upcoming
Gaia mission will explore the detailed physical properties of each star observed, revealing luminosity, temperature, gravity and composition. Gaia will achieve this by repeatedly measuring the positions of all objects down to magnitude 20, and those brighter than magnitude 15, to an accuracy of 24
microarcseconds—akin to measuring the diameter of a human hair from 1000 km away. On-board detection equipment will ensure that variable stars like Betelgeuse will all be detected and catalogued to this faint limit, thus addressing most of the limitations of the earlier Hipparcos mission. The nearest stars, in fact, will have their distances measured to the extraordinary accuracy of 0.001%. Even stars near the Galactic centre, some 30 000 light-years away, will have their distances measured to within an accuracy of 20%.
[75]
Distance is not the only mystery that has bewitched astronomers. Known as a semiregular pulsating variable, researchers have offered different hypotheses to explain α Ori's volatile choreography. Our current understanding of stellar structure suggests that the outer layers of this supergiant gradually expand and then contract causing the surface area ( chromosphere) to alternately increase and decrease, the temperature to rise and fall — thus eliciting the measured cadence in the star's brightness between its dimmest magnitude of 1.2, seen as early as 1927, and its brightest of 0.2, seen in 1933 and 1942. A red supergiant like Betelgeuse will pulsate like this because its stellar atmosphere is inherently unstable. As the star contracts, it absorbs more and more of the energy that passes through it, causing the atmosphere to heat up and expand. Conversely, as the star expands, its atmosphere becomes less dense allowing the energy to escape and the atmosphere to cool, thus initiating a new contraction phase. [28] What's been particularly challenging in this regard has been apprehending the irregularity of the star's pulsations and modeling its periodicity, as it appears there are several cycles interlaced.
*Paragraph Purpose: Explain periodicity Next Step: Need to pull together all the different periodicity estimates from different articles.
Let us first recall that Alpha Orionis varies in both visual brightness and radial velocity on time scales of a few weeks ormonths. As shown in classical papers, the short term fluctuations modulate a regular, cyclic variation with a period of nearly six years. Stebbins (1931) monitored the brightness of Betelgeuse photoelectrically more or less continuously, weather and seasons permitting, from 1917 to 1931. He concluded that "there is no law or order in the rapid changes of Betelgeuse" and used a drastic smoothing process to derive a mean cure that was best fitted by a period of 5.40 days, although the period of 5.781 days, which had been obtained by Spencer Jones (1928) from radial-velocity data, represented the brightness data nearly as well. The observations from Stebbins and Sanford indicate a possible sub-period of about 100 days in the brightness variations. [76] It is noteworthy that on at least 3 occasions during the past 60 years, unusually large and relatively rapid decreases in the radial velocity of Alpha Ori were followed by suddendecreases of about half a magnitude in visual brightness. Such events seem to have a tendency to occur just after the time of the minimum in the radial velocity connected with the star's pulsations. As the pulsating star reaches maximum velocity of expansion and begins to decelerate there may be instabilities set up in the atmosphere which trigger mass ejection adn the formation of dust grains. The correlation between the observed angular diamter of Betelgeuse and the opacity of the TiO (Balega et al. 1982) indicates that the scale height of the region in which the Fraunhofer spectrum is formed is on the order of one stellar radius. The extrem widths of the line in the Alpha Ori and other supergians suggest that the levitation of the atmosphere may be caused by nonthermal gas pressure, but no matter what holds up the atmosphere, its coupling to the pulsating star cannot be very tight and there will be a tendency for the atmosphere to continue accelerating and to over shoot the minimum, as may have been observed. A survey of variations in the radial-velocity and visual brightness of the star Betelgeuse (alpha-Orionis) over the last six decades is presented. On the basis of a comparison of the results of several observations, it is suggested that major disturbances in Betelgeuse's atmosphere are likely to occur in the year or two following the minimum in the six-year velocity curve. A coordinated observing program is proposed to take place during and after the next minimum, which is predicted to take place in early 1984. The desirable observations include multicolor photometry (particularly in the infrared), polarization measurements, and spectroscopy.
As discussed in classical papers by Stebbins and Sanford in the 1930s, there are short term variations of roughly 150 to 300 days that modulate a regular cyclic variation with a period of roughly 5.7 years. In 2000, astronomers announced the detection of statistically significant cycles in "longwave" (1900-3200 Å) ultraviolet light radiated by the star at 0.531 years (just over six months) and 3.46 years Rinehart et al., 2000). [77]
In fact, the supergiant consistently displays irregular photometric, polarimetric and spectroscopic variations, which points to the occurrence of complex activity on the star's surface and in its extended atmosphere. [61] In marked contrast to most giant stars that are typically long-period variables with reasonably regular periods, red giants and supergiants like Betelgeuse and Antares are generally semi-regular or irregular with pulsating characteristics. In a landmark paper published in 1975, Martin Schwarzschild attributed these brightess fluctuations to the changing granulation pattern formed by a few giant convection cells covering the surface of these stars. [33] [78] For the Sun, these convection cells, otherwise known as solar granules, represent the foremost mode of heat transfer — hence those convective elements which dominate the brightness variations in the solar photosphere. [33] The typical diameter for a solar granule is about 2,000 km (yielding a surface area roughly the size of India), with an average depth of 700 km. With a surface of roughly 6 trillion square kilometers, there are about 2 million of such granules lying on the Sun's photosphere. Beneath these granules, it is conjectured that there are 5-10 thousand supergranules, the average diameter of which is 30,000 km with a depth of about 10,000 km. [79] By contrast, stars like Betelgeuse, Schwardschild argues, may have only a dozen monster granules with diameters of 180,000,000 km or more dominating the surface of the star with depths of about 60,000,000 kilometers, which, because of the very low temperatures and extremely low density found in red giant envelopes, result in convective inefficiency. Consequently, if only one-third of these convective cells are visible to us at any one time, the time variations in their observable light may well be reflected in the brightness variations of the integrated light of the star. [33]
Schwarzschild's hypothesis of gigantic convection cells dominating the surface of red giants and supergiants seems to have stuck with the astronomical community. In 1995, the Hubble Space Telescope captured the first direct picture of the supergiant's surface. The image revealed an extended chromosphere of roughly twice the star's angular diameter with a mysterious hot spot located in the southwest quadrant of the disk that dominated the total ultraviolet flux. The hot spot appeared to be hotter than the surrounding chromosphere by at least 2,000K. "Such a major single feature is distinctly different from scattered smaller regions of activity typically found on the Sun", authors Andrea Dupree and Ronald Gilliland reported, "although the strong ultraviolet flux enhancement is characteristic of stellar magnetic activity. This inhomogeneity may be caused by a large scale convection cell or result from global pulsations and shock structures that heat the chromosphere." [80] [28] Two years later in 1997, astronomers observed intricate assymetries in the brightness distribution of the star, revealing at least three bright spots on the stellar disk. [81] And then in the year 2000, another team of astronomers led by Alex Lobel of the Harvard–Smithsonian Center for Astrophysics (CfA) noted that Betelgeuse exhibits raging storms of hot and cold gas in its turbulent atmosphere, a potential cause being asymmetric pulsations in the star's chromosphere. The team surmised that huge areas of the star's photosphere will vigorously bulge out in different directions at times, ejecting long plumes of warm gas into the cold dust envelope. Another explanation that was also given was the occurrence of shockwaves caused by warm gas traversing cooler regions of the star. [82] [77] The team investigated the atmosphere of Betelgeuse, over a period of five years between 1998 and 2003 with the STIS instrument aboard Hubble. They found that the bubbling action of the chromosphere tosses gas out one side of the star, while it falls inward at the other side, similar to the slow-motion churning of a lava lamp.
*Paragraph Purpose: Latest findings that explain variability Next Step: Read Dupree and other recent articles; integrate
A Harvard-Smithsonian Center for Astrophysics Press Release announcing the first direct image of a star (Betelgeuse), suggests that this event holds new implications for stellar research. Researchers said that this new picture of Betelgeuse suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. "What we see on Betelgeuse is completely different from what occurs on the surface of the Sun," said Dupree, a senior scientist at the CfA. "Instead of lots of little sunspots, we find an enormous bright area more than 2,000 K degrees hotter than the surrounding surface of the star." Until follow-up observations of Betelgeuse are completed, however, astronomers won't know the mystery spot's true nature--or how it formed. [28]
Dupree explains that the spot might change position over time. Dupree and colleagues using the International Ultraviolet Explorer found a 420-day period, during which the star oscillates, or "rings" like a bell. The oscillations, thought to be caused by turbulence below the surface of the star, might cause changes in the bright spot. Future oscillations might spur other bright spots on different regions of the star's surface, causing it to wink on and off like blinking lights on a Christmas tree, said Dupree. Alternatively, the spot might move systematically across the star's surface, which would imply that the star has magnetic fields strong enough to hold the bright spot's hot gas in position. Either scenario would lead astronomers to re-think completely current ideas of how stars evolve. "We hope this work will also pave the way for a generation of space interferometers," said Dupree. Such instruments would greatly improve the resolution of structures on stellar surfaces. [28]
The major question this hot spot raised was whether it was linked to oscillations previously detected in the giant star, or whether it somehow moves systematically across the star's surface under the grip of powerful magnetic fields. [28]
As a variable star, Betelgeuse has the sub-classification "SRC": supergiants with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days. If we assume a distance of 197 pc (± 640 ly), the peak absolute magnitude of Betelgeuse would be -6.27, while its minimum would be -5.27, and its mean -6.05.
A third challenge that has confronted astronomers has been measuring this behemoth's angular diameter. As noted earlier, Betelgeuse became the first star to have its diameter measured on December 13, 1920 by means of a 20-foot beam interferometer. [58] Though interferometry was still very much in its infancy, the experiment proved to be a success and the angular diameter was found to average .047 arcseconds. What was noteworthy were the astronomers' insights on limb darkening. In addition to a measurement error of 10%, Michelson and Pease estimated the actual size of the star to be about 17% larger because of the diminishing intensity of light around the edges—hence an angular diameter as large as .055 ". [58] [13] Since that time, there have been many other studies conducted with angles ranging anywhere from .042 to .069 arcseconds. [3] [83] [84] If we simply take that data and combine it with historical distance estimates of 180 to 815 ly, the projected diameter of the stellar disk could be anywhere from 2.4 to 17.8 AU, hence radii of 1.2 to 8.9 AU respectively. [note 2] That's a wide margin—hence one of the reasons Betelgeuse has been such a mystery. Using the Solar System as a yardstick, the orbit of Mars is approximately 1.5 AU, Ceres in the asteroid belt 2.7 AU, Jupiter 5.5 AU—hence a photosphere which, depending on Betelgeuse's actual distance from Earth, could well extend beyond the orbit of Jupiter but not quite as far as Saturn at 9.5 AU.
The precise diameter has been hard to define for several reasons:
To overcome these challenges, researchers have employed a number of solutions. Astronomical interferometry was first imagined by Hippolyte Fizeau in 1868. [86] He proposed the observation of stars through two apertures to obtain interferences that would provide information on the spatial intensity distribution of the source. Since then, the science of interferometry has evolved considerably where multiple-aperture interferometers are now used consisting of a large number of images superimposed on each other. These "speckled" images are then synthesized using Fourier analysis—a method which has been used for a wide array of astronomical objects including the study of binary stars, quasars, asteroids and galactic nuclei. [87] Space observatories like Hipparcos, Hubble and Spitzer have produced significant breakthroughs and recently another instrument, the Astronomical Multi-BEam Recombiner (AMBER), is yielding new insights. As part of the VLTI, AMBER is capable of combining the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond spatial resolution. Also by combining three baselines instead of two, which customary with conventional interferometry, AMBER enables astronomers to compute the closure phase—an important element in astronomical imaging. [85] [9]
The current debate centers around which wavelength—the visible, near-infrared ( NIR) or mid-infrared ( MIR)—produces the most accurate angular measurement. [note 2] The solution that has been most widely adopted, it appears, is the one performed with the ISI in the mid-infrared by astronomers from the Space Sciences Laboratory at U.C. Berkeley. In the epoch year 2000, the group, under the leadership of John Weiner, published a paper showing Betelgeuse as having a uniform disk of 54.7 ± 0.3 mas. [3] The paper also included a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas—a figure which equates to a radius of roughly 5.5 AU (1,180 times solar), assuming a distance of 197.0 ± 45 pc. [note 3] Nevertheless, given the angular error factor of ± 0.5 mas combined with a parallax error of ± 45 pc found in Harper's numbers, the photosphere's radius could actually be as small as 4.2 AU or as large as 6.9 AU.
Crossing the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43:33 ± 0:04 mas was a more accurate photospheric measurement. "A consistent scenario to explain the observations of this star from the visible to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slighty increasing its diameter. The scatter becomes negligible above 1.3 μm. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." It's a compelling argument but one which has yet to receive widespread support among astronomers. [7]
More recent studies done in the near-infrared with the IOTA and VLTI have brought strong support to Perrin's analysis yielding diameters that range from 42.57 to 44.28 mas with impressively tight error factors of no more than 0.04 mas. [9] [10] Of pivotal importance in this discussion, however, is a second paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate. [13] [14] Unlike most papers heretofore published, this seminal paper represented a systematic study of the star over a 15-year horizon at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in angular separation equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9 AU. [note 4] What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggests that if a cycle does exist, it is likely a few decades long. [13] Consequently, until a full cycle of data has been gathered, we will not know whether the 1993 figure of 56.0 mas represents the maximum extension of the star or its mean, or whether the 2008 figure of 47.0 in fact represents a minimum. It will likely take another 15 years or longer (2025 C.E.) before we know with any certainty, meaning that the Jovian orbit of 5.5 AU will likely serve as the star's "average" diameter for some time. [5]
Once considered as having the largest angular diameter of any star in the sky after the Sun, in 1997 Betelgeuse lost that distinction when a group of astronomers measured R Doradus with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse. [17] _____________END OF SECTION______________________________________________________________
Because of the size and proximity of this star it has the third largest angular diameter as viewed from Earth, being smaller than only the Sun and R Doradus. The angular size of Betelgeuse was one of the first to be measured with an astronomical interferometer and the apparent diameter was found to be variable. Between 1993 and 2009, the star's diameter has contracted by over 15 percent. [88] The precise diameter is hard to define since optical emissions decrease very gradually with radius from the center of Betelgeuse and the color of these emissions also vary with radius.
If we assume a compromise distance of 570 light years, the star's diameter would be about 950 to 1,000 times that of the Sun. If the Sun were the size of a beach ball, then Betelgeuse would be as large as a professional sports stadium. Although only 20 times more massive than the Sun, [27] this star could be hundreds of millions times greater in volume.
Betelgeuse is not only one of the most eminent stars in the heavens, but also one of the most luminous. In the SIMBAD astronomical database the star's spectral class is listed as M2Iab. The "ab" suffix is derived from the Yerkes spectral classification system signifying that Betelgeuse is an intermediate luminous supergiant, less luminous than other supergiants like Deneb. However given some of the recent findings, it may be that this classification is outdated, as Betelgeuse, it appears, is actually much brighter than Deneb and other stars in its class.
If we assume an average radius of 5.5 AU and a distance of 197 pc, Betelgeuse has a luminosity in excess of 180,000 Suns at maximum. When the star contracts as it has since 1993, its luminosity diminishes to about 130,000 Suns. Either way, that amount of electromagnetic energy certainly dwarfs Deneb's output of about 50,000 Suns. [note 1] However, with most of the star's radiant energy occurring in the infrared and huge amounts of it being absorbed by circumstellar matter in the star's outer shell, we simply don't experience the star's total luminosity.
Betelgeuse is a cool star, typical of red supergiants, with a surface temperature of about 3,500 degrees Kelvin. It's also a slow rotator, with the most recent velocity recorded at 5 km/s. At 5.5 AU, it takes the star roughly 32.3 years to turn on its axis — extremely slow when you compare it to a fast rotator like Pleione in the Pleiades star cluster which turns on its axis once every 11.8 hours.
Source Data
The kinematics of Betelgeuse are intriguing yet not easily explained. The age of a Type M supergiant with an initial mass of 20 times solar is roughly 10 million years. [89] [69] Given its current space motion, a projection back in time would take Betelgeuse around 290 parsecs farther from the galactic plane where there is no star formation region — an implausible scenario. Although the space motion for the 25 Ori group has yet to be measured, α Ori’s projected path does not appear to intersect with it either. Also, formation close to the far younger Orion Nebula Cluster (ONC, also known as Ori OB1d) is doubtful. VLBA astrometry yields a distance to the ONC between 389 and 414 parsecs. Consequently, it is likely that Betelgeuse has not always had its current motion through space and has changed course at one time or another. [69]
The most likely star-formation scenario for Betelgeuse is that it's a runaway star from the Orion OB1 Association. Originally a member of a high-mass multiple system within Ori OB1a, which includes the late type O and B stars in Orion's belt — Alnitak, Alnilam and Mintaka — Betelgeuse was probably formed about 10-12 million years ago from the molecular clouds observed in Orion but has evolved rapidly due to its unusually high mass. [69]
Like many of the stars of Orion where massive young stars with over 10 times the Sun's mass can be found in abundance, Betelgeuse will use its fuel quickly and not live very long. On the Hertzsprung-Russell diagram, Betelgeuse has moved off the main sequence and has swelled and cooled to become the red supergiant that it known as. Although this titanic star may have only been in existence for ten million years, unlike its OB cousins born about the same time, Betelgeuse has probably exhausted the hydrogen in its core causing it to contract under the force of gravity into a hotter and denser state. As a result, it has begun to fuse helium into carbon and oxygen producing enough radiation to unfurl its outer envelopes of hydrogen and helium. Its extreme luminosity is being generated by a mass so large that the star will eventually fuse higher elements through neon, magnesium, sodium, and silicon all the way to iron, at which point it will likely collapse and explode as a supernova [27] [77]
As an early M-type supergiant, Betelgeuse is one of the largest and most luminous stars of its class. A radius of 5.5 AU is roughly 1,180 times the radius of the Sun — a sphere so huge that it could contain over 2 quadrillion Earths (2.15 × 1015) or more than 1.6 billion (1.65 × 109) Suns. That's the equivalent of Betelgeuse being a giant football coliseum like Wembley Stadium in London with the Earth a tiny pearl, 1 millimeter in diameter, orbiting a Sun the size of a mango. [note 5] Of particular interest in this respect is the impact of a 15% reduction in the star's radius as reported. That equates to a shortening of the star's radius from about 5.5 AU to 4.6 AU, and a diminution in the star's photospheric volume of approximately 41% or 680 million Suns. [note 6]
Not only is the photosphere enormous, but the star is surrounded by a circumstellar gaseous envelope that extends well over a trillion kilometers from the star. As a result, it takes over two months for light to escape its own shell. In the outer reaches of the photosphere, the density is extremely low. In volume, Betelgeuse exceeds the Sun by a factor of about 1.6 billion Suns. Yet the actual mass of the star is no more than 18-19 solar masses, since the star is estimated to have lost 1-2 solar masses since its birth. [27] Consequently, the average density of this stellar mystery is less than one-sextillionth (1.116 × 10−23) the density of our Sun. If we compare such star matter to the density of ordinary air at sea level, the ratio is roughly 1.286 × 10−5 — a density so tenuous, one would have to get above the noctilucent clouds in the Earth's mesosphere to experience it. [note 7] Such star matter is so ethereal that Betelgeuse has often been called a "red-hot vacuum". [29] [28]
In the late phase of stellar evolution, massive stars like Betelgeuse exhibit high rates of mass loss, possibly as much as 1M☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux. [31] All stars exhibit mass loss. Rates vary from about 10−14 to 10−4 yr −1 depending on spectral type, luminosity class, rotation rate, companion proximity, and evolutionary stage. [32] Exactly how this mass loss occurs, however, has been a mystery confronting astronomers for decades. When Schwarzschild first proposed his theory of monster convection cells, he argued it was the likely cause in red supergiants. Prior attempts to explain mass loss in terms of solar wind theory had proven unsuccessful as they led to a contradiction with observations involving circumstellar shells. [33] Other theories that have been advanced include magnetic activity, global pulsations and shock structures as well as stellar rotation. [34]
As a result of work done by Pierre Kervella and his team at the Paris observatory in 2009, astronomers may be close to solving this mystery. Kervella noticed a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions. [35] [31] The plume's presence, in fact, implies that the spherical symmetry of its photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation. [35] Probing deeper still with ESO's AMBER, Keiichi Ohnaka observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella. [31] [36]
Evidence of circumstellar shells surrounding M supergiants was first proposed by Walter Adams and Elizabeth MacCormack in 1935 when they observed anomalies in the spectral signature of such stars and concluded that the likely cause was an expanding gaseous envelope. [90] [91] In 1955, Armin Deutsch noticed in the Rasalgethi system that spectroscopic peculiarities were mysteriously occurring in the G star companion, α2 Her, from which he concluded that the whole system had to be enveloped by a circumstellar shell composed of matter being ejected by the main star, M supergiant α1 Her, and extending to at least 170 stellar radii. [90] [92] In the mid 1970s, Andrew Bernat undertook a detailed analysis of four circumstellar shells, Betelgeuse, Antares, Rasalgethi and Mu Cephei, concluding that red stars dominate mass return to the Galaxy. [90]
In addition to the photosphere, six other components of Betelgeuse's atmosphere have now been identified. Extending outward, one encounters a molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide (CO). There is also evidence of coronal plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence. [25] Some of these elements are known to be asymmetric while others overlap. [10]
At about .45 stellar radii (~2–3 AU) above the photosphere, the closest membrane appears to be the molecular layer known as the MOLsphere. Studies show it to be composed of water-vapor and carbon monoxide with an effective temperature of about 1500 ± 500K. [10] [37] Water-vapor had been originally detected in the supergiant's spectrum back in the 1960s with the two Stratoscope projects but had been ignored for decades. Recent studies suggest that the MOLsphere may also contain SiO and Al2O3—molecules which could explain the formation of dust particles.
Between two and seven stellar radii (~10–40 AU), astronomers have identified another region known as an asymmetric gaseous envelope composed of elemental abundances C, N and O. [10] [38] The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense atmosphere with a "remarkably complex structure". [39] Observations show the atmosphere to be boiling with a temperature of 3,450 ± 850K—similar to the temperature recorded on the star's surface but much lower than surrounding gas in the same region. [39] [40] The VLA images also showed this lower-temperature gas progressively decreasing in temperature as it extends outward—the existence of which, although unexpected, turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere." [39] This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing C N and extending at least six photospheric radii in the southwest direction of the star, is believed to exist. [10]
The chromosphere, as mentioned earlier, was resolved in 1996 at about 2.2 times the optical disk (~10 AU) at ultraviolet wavelengths and is reported to have a temperature no higher than 5,500K. [41] [10] The image was taken with the Faint Object Camera on board the Hubble Space Telescope and also revealed a bright area in the southwest quadrant of the disk. However, in 2004 observations with the STIS, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of 197 pc, the size of the chromosphere could be 200 AU [42] The CfA team led by Alex Lobel concluded that the spatially resolved STIS spectra directly demonstrate the co-existence of warm chromospheric plasma with cool gas in Betelgeuse's circumstellar dust envelope. [42]
The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton and colleagues, who noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, they concluded that the red supergiant emits most of its excess beyond 12 stellar radii or roughly the distance of the Kuiper belt at 50 to 60 AU, depending on the assumed stellar radius. [10] [45] Since then, there have been numerous studies done of this dust envelope at varying wavelengths yielding decidedly different results. More recent studies have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, or 100 to 200 AU. [46] [47] What these studies point out is that the dust environment surrounding Betelgeuse is anything but static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. A few years later, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in just one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots. [46] The 1984 report of a giant asymmetric dust shell located 1 pc (206,265 AU) from the star has not been corroborated in recent studies, although another report published the same year said that three dust shells were found extending four light years from one side of the decaying star, suggesting that, like a snake, Betelgeuse sheds its outer layers as it journeys across the heavens. [48] [49]
Although the exact size of the two outer CO shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0 arcseconds with the other expanding as far as 7.0 arcseconds. [50] Using the Jovian orbit of 5.5 AU as the "average" radius for this gargantuan star, the inner shell would extend roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer one as far as 250 stellar radii (~1400 AU). With the heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of the Solar System.
Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~6.3 AU per year) creating a bow shock. [51] The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first imaged. The cometary structure is estimated to be at least 1 parsec, assuming a current distance of 643 light years. [52]
Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young—less than 30,000 years old—suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. [53] In their 2012 paper, Mohammed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks." [54] [55] Moreover, if future research bears out this hypothesis, Betelgeuse may well prove to have traveled close to 200,000 AU as a red supergiant scattering as much as 3 along its trajectory.
______________________________ END OF SUB-SECTION______________________________________
1.7 The spots observed on the surface of Betelgeuse
Thanks to the pupil masking technique, Wilson et al. (1992,1997) and Buscher et al. (1990) have detected the presence of spots on the surface of Betelgeuse that represent up to 15-20% of the observed flux in the visible. The most likely origin of these asymmetries is a signature of the convection phenomenon. However, the hypotheses of inhomogeneities in the molecular layers or the presence of a transiting companion are not ruled out (Buscher et al. 1990). These observed spots are few, 2 or 3, and their characteristrics change with time (Wilson et al. 1992). Confirming previous results, Young et al. (2000) observed spots in the visible but show a fully centro-symmetric picture of Betelgeuse in the near infrared (at 1.29 m). Lately Tatebe et al. (2007) also report the observation of an asymmetry at 11.15 m located on the southern edge of the disk of Betelgeuse. In order to deepen our knowledge on RSGs, it is critical to observe spots at high angular resolution. Many unknowns remain such as their size, location, chemical composition, dynamical properties, lifetime and origin.
Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~1 billion km per year) creating a bow shock. The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first successfully imaged. The cometary structure is estimated to be 3 light-years wide.
Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young, less than 30,000 years old, suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. In their 2012 paper, Mohammed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, recent evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks."
Need to handle all this speculation that Betelgeuse is going to blow up soon. Sky & Telescope says "anytime" in the next million years. In stellar time, that's soon.
The future fate of Betelgeuse depends on its mass; as it probably contains more than 15 solar masses, it will continue to burn and fuse elements until its core is iron, at which point Betelgeuse will explode as a type II supernova. During this event the core will collapse, leaving behind a neutron star remnant some 20 km in diameter. [57]
Betelgeuse is already old for its size class and will explode relatively soon compared to its age of several million years. [93] At the current distance of Betelgeuse from the Earth, such a supernova explosion would be the brightest recorded; outshining the Moon in the night sky and becoming easily visible in broad daylight. [93] Professor J. Craig Wheeler of The University of Texas at Austin predicts the supernova will emit 1053 ergs of neutrinos, which will pass through the star's hydrogen envelope in around an hour, then reach the solar system several centuries later. Since its rotational axis is not pointed toward the Earth, Betelgeuse's supernova is unlikely to send a gamma ray burst in the direction of Earth large enough to damage Earth's ecosystems. [94] The flash of ultraviolet radiation from the explosion will be weaker than the ultraviolet output of the Sun.
The supernova would brighten to an apparent magnitude of –12 over a two-week period, then remain at that intensity for two or three months before rapidly dimming. The year following the explosion, radioactive decay of cobalt to iron will dominate emission from the supernova remnant, and the resulting gamma rays will be blocked by the expanding envelope of hydrogen. If the neutron star remnant became a pulsar, then it might produce gamma rays for thousands of years. [95]
In 1985, Margarita Karovska, in conjunction with other astrophysicists at the Harvard–Smithsonian Center for Astrophysics, announced the discovery of two close companions orbiting Betelgeuse. Analysis of polarization data from 1968 through 1983 indicated that Alpha Ori had a close companion with a periodic orbit of about 2.1 years. The team realized that the observed polarization could be caused by a systemic asymmetry created by the close companion orbiting Alpha Ori inside its extended dust envelope. Using speckle interferometry, the team concluded that the closer of the two companions was located at 0.06 ± 0.01 arcseconds from the main star with a position angle of 273 degrees. The more distant companion was estimated at 0.51 ± 0.01 " with a PA of 278 degrees. The magnitude differences with respect to the primary, measured at 656.3 (Hα) and 656.8 nm (red continuum), were 3.4 and 3.0 for the close component and 4.6 and 4.3 for the distant component. [18] [19]
In the years that followed, different teams of astronomers began to monitor the data in the hope of obtaining additional confirmation. In 1987, Andrea Dupree made the following observation: "Periastron of the recently discovered close optical companion to Alpha Ori is predicted to be 1986.7; detection of atmospheric disturbances similar to those found subsequent to the last periastron (~ 1984.6) would give strong support to the presence of a companion." [96] However it appears that such detection never materialized. Rather, in 1990, David F. Buscher, John E. Baldwin and a team of collaborators from the Cavendish Astrophysics Group made a number of high-resolution images of the supergiant at wavelengths of 633, 700, and 710 nm using the nonredundant masking method. At all these wavelengths, they remarked, there was unambiguous evidence for an asymmetric feature on the surface of the star, which contributed 10-15 percent of the star's total observed flux. Their conclusion was that such a phenomenon could be caused by a close companion passing in front of the stellar disk, differential photospheric brightening due to the effects of stellar rotation or the more likely scenario of "large-scale convection in the stellar atmosphere" as suggested by Schwarzschild. [61]
Two years later in 1992, the Cavendish colleagues published another paper, this time under the helm of Richard. W. Wilson, noting that the brightness features on the surface of Betelgeuse appear to be "too bright to be associated with a passage of the suggested companions in front of the red giant." They also noticed that these features were fainter at the 710 than at 700 nm, by a factor of 1.8, indicating that they would have to reside within the molecular atmosphere of the star. [20]
That same year, Karovska published a new paper reconfirming her and her colleagues' interpretation of the data, but also noting that "the correlation between the calculated position angles of the companion and the measured position angles of the asymmetries suggests that there is a possible connection between the asymmetries and the companion. The asymmetry in the images of α Ori could be caused by the unresolved companion, by tidal distortion of the supergiant's atmosphere, or possibly by an unresolved bright spot on the stellar surface facing the companion. To determine the nature of the companion (which presently remains a puzzle), it is crucial to obtain further speckle observations using large aperture telescopes, coordinated with other ground-based observations and the observations from space." [21]
Since then, researchers turned their attention to analyzing the intricate dynamics of the star's extended atmosphere and little else has been published on the possibility of orbiting companions. As the decade unfolds and new technologies are brought to unraveling the star's enigmatic past, we will likely see conclusive evidence, one way or another, of any potential star system. Given the planned capabilities of the upcoming Gaia mission, a confirmation could occur any time after the mission's scheduled launch in December 2012.
Article | Year1 | Telescope | # | Spectrum | λ ( μm) | ∅ ( mas)2 | Radii3 @ 197±45 pc |
Notes |
---|---|---|---|---|---|---|---|---|
Michelson | 1920 | Mt-Wilson | 1 | Visible | 0.575 | 47.0 ± 4.7 | 3.2 - 6.3 AU | Limb darkened +17% = 55.0 |
Bonneau | 1972 | Palomar | 8 | Visible | 0.422-0.719 | 52.0 - 69.0 | 3.6 - 9.2 AU | Strong correlation of ∅ with λ |
Balega | 1978 | ESO | 3 | Visible | 0.405-0.715 | 45.0 - 67.0 | 3.1 - 8.6 AU | No correlation of ∅ with λ |
1979 | SAO | 4 | Visible | 0.575-0.773 | 50.0 - 62.0 | 3.5 - 8.0 AU | ||
Buscher | 1989 | WHT | 4 | Visible | 0.633-0.710 | 54.0 - 61.0 | 4.0 - 7.9 AU | Discovered asymmetries/hotspots |
Wilson | 1991 | WHT | 4 | Visible | 0.546-0.710 | 49.0 - 57.0 | 3.5 - 7.1 AU | Confirmation of hotspots |
Tuthill | 1993 | WHT | 8 | Visible | 0.633-0.710 | 43.5 - 54.2 | 3.2 - 7.0 AU | Study of hotspots on 3 stars |
1992 | WHT | 1 | NIR | 0.902 | 42.6 ± 0:03 | 3.0 - 5.6 AU | ||
Weiner | 1999 | ISI | 2 | MIR ( N Band) | 11.150 | 54.7 ± 0.3 | 4.1 - 6.7 AU | Limb darkened = 55.2 ± 0.5 |
Perrin | 1997 | IOTA | 7 | NIR ( K Band) | 2.200 | 43:33 ± 0:04 | 3.3 - 5.2 AU | K&L Band,11.5μm data contrast |
Haubois | 2005 | IOTA | 6 | NIR ( H Band) | 1.650 | 44.28 ± 0.15‡ | 3.4 - 5.4 AU | Rosseland diameter 45.03 ± 0.12 |
Hernandez | 2006 | VLTI | 2 | NIR ( K Band) | 2.099-2.198 | 42:57 ± 0:02 | 3.2 - 5.2 AU | High precision AMBER results. |
Ohnaka | 2008 | VLTI | 3 | NIR ( K Band) | 2.280-2.310 | 43.19 ± 0.03 | 3.3 - 5.2 AU | Limb darkened 43.56 ± 0.06 |
Townes | 1993 | ISI | 17 | MIR ( N Band) | 11.150 | 56.00 ± 1.00 | 4.2 - 6.8 AU | Systematic study involving 17 measurements at the same wavelength from 1993-2009 |
2008 | ISI | MIR ( N Band) | 11.150 | 47.00 ± 2.00 | 3.6 - 5.7 AU | |||
2009 | ISI | MIR ( N Band) | 11.150 | 48.00 ± 1.00 | 3.6 - 5.8 AU | |||
Harper | 2004 | VLA | Also noteworthy, Harper et al in the conclusion of their paper make the following remark: "In a sense, the derived distance of 200 pc is a balance between the 131 pc (425 ly) Hipparcos distance and the radio which tends towards 250 pc (815 ly)"—hence establishing ± 815 ly as the outside distance for the star. |
1The final year of observations, unless otherwise noted. 2Uniform disk measurement, unless otherwise noted. 3Radii calculations use the same methodology as outlined in Note #2 below ‡Limb darkened measurement.
{{
cite journal}}
: Unknown parameter |duplicate-journal=
ignored (
help)CS1 maint: date and year (
link)
{{
cite journal}}
: Explicit use of et al. in: |author=
(
help)CS1 maint: multiple names: authors list (
link) Cite error: The named reference "WEINER" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
The measurements cannot be compared anyway, because the star's size depends on the wavelength of light used to measure it, Townes said. This is because the tenuous gas in the outer regions of the star emits light as well as absorbs it, which makes it difficult to determine the edge of the star.
{{
cite web}}
: Unknown parameter |month=
ignored (
help) Cite error: The named reference "UC BERKELEY" was defined multiple times with different content (see the
help page).
{{
cite web}}
: Check date values in: |date=
(
help)
{{
cite journal}}
: CS1 maint: date and year (
link) Cite error: The named reference "HERNANDEZ" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: date and year (
link) Cite error: The named reference "HAUBOIS" was defined multiple times with different content (see the
help page).
We derive a uniform-disk diameter of 42.05 ± 0.05 mas and a power-law-type limb-darkened disk diameter of 42.49 ± 0.06 mas and a limb-darkening parameter of (9.7 ± 0.5) × 10-2
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the Sun, researchers reported June 9 at an American Astronomical Society meeting and in the June 1 Astrophysical Journal Letters.Cite error: The named reference "COWEN" was defined multiple times with different content (see the help page).
{{
cite web}}
: Unknown parameter |month=
ignored (
help)
RAVI1
was invoked but never defined (see the
help page).{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "KAROVSKA1" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "KAROVSKA2" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "WILSON2" was defined multiple times with different content (see the
help page).
{{
cite web}}
: Check date values in: |date=
(
help)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite arXiv}}
: CS1 maint: multiple names: authors list (
link)
The mass of the star is unknown, but most investigators show a preference for a fairly large mass in the range of 10–20M☉
{{
cite journal}}
: CS1 maint: date and year (
link)
{{
cite web}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link) Cite error: The named reference "AAVSO" was defined multiple times with different content (see the
help page).
Earlier data had yielded a luminosity of 54,000L☉ with a radius of 108R☉
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
The image is reproduced in the Properties section below.
{{
cite web}}
: Unknown parameter |month=
ignored (
help)
Ridgway notes: 'Stellar mass loss is key to understanding the evolution of the universe from the earliest cosmological times to the current epoch, and of planet formation and the formation of life itself.'
{{
cite journal}}
: Unknown parameter |month=
ignored (
help); Unknown parameter |origin=
ignored (
help) Cite error: The named reference "SCHWARZSCHILD1975" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Explicit use of et al. in: |author=
(
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
DUPREE
was invoked but never defined (see the
help page).In the article, Lobel et al. equate 1 arcsecond to approximately 40 stellar radii, a calculation which in 2004 likely assumed a Hipparcos distance of 131 pc (430 ly) and a photospheric diameter of 0.0552" from Weiner et al.
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
Noriega in 1997 estimated the size to be 0.8 parsecs, having assumed the earlier distance estimate of 400ly. With a current distance estimate of 643ly, the bow shock would measure ~1.28 parsecs or over 4 ly
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link)
allen
was invoked but never defined (see the
help page).{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "MICHELSON" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "BUSCHER" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite book}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite web}}
: Unknown parameter |month=
ignored (
help) Cite error: The named reference "ESCIENCE" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Explicit use of et al. in: |author=
(
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite web}}
: Check date values in: |date=
(
help)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link) Cite error: The named reference "FREYTAG" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite web}}
: Unknown parameter |month=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Unknown parameter |month=
ignored (
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
Images of hotspots on the surface of Betelgeuse taken at visible and infra-red wavelengths using high resolution ground-based interferometersCite error: The named reference "YOUNG" was defined multiple times with different content (see the help page).
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link)
The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the sun, researchers reported June 9 at an American Astronomical Society meeting and in the June 1 Astrophysical Journal Letters.— Townes, C. H.; Wishnow, E. H.; Hale, D. D. S.; Walp, B. (2009). "A Systematic Change with Time in the Size of Betelgeuse". Astrophysical Journal Letters. 697 (2): L127. Bibcode: 2009ApJ...697L.127T. doi: 10.1088/0004-637X/697/2/L127. S2CID 121009337.
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
In the concluding sentence of the abstract, Deutsch notes that: This process may be important in the evolution of all massive stars that have exhausted their hydrogen—a reference to α Ori and other red giants experiencing mass loss.
Click on the "[show]" link in the box below.
Van Leeuwen distance
van Leeuwen 2007 distance of 152 ± 20 pc. [1]
Luminosity Calculations
Luminosity Calculations [2] EndLuminosityCalcs
The following calculations determine Betelgeuse's average radius in terms of solar units based on the following assumptions for angular diameter:
1) Weiner 2000 = 55.20 ± 0.50 mas (limb darkened)
2) Perrin 2004 = 43.33 ± 0.04 mas
The calculations begin with the formula for a star's angular diameter, as follows:
where
represents Betelgeuse's angular diameter in
arcseconds,
the Distance from Earth in
parsecs
Betelgeuse's diameter in
AU, and
Betelgeuse's Radius in AU. Therefore:
To convert the above into Solar units, the math is straightforward. Since 1 AU = 149,597,871 km and the mean diameter of the Sun = 1,392,000 km (hence a mean radius of 696,000 km), the calculation is as follows:
Luminosity Note
Nevertheless, since Betelgeuse is a pulsating variable star, there are times where the star's luminosity could theoretically exceed 200,000 L☉ [note 1]
Therefore:
Note: These calculations do not take into consideration any diminution caused by extinction, which in the case of Betelgeuse has been estimated at around 3.1%
The current debate revolves around which wavelength—the visible, near-infrared ( NIR) or mid-infrared ( MIR)—produces the most accurate angular measurement. [note 2] In 1996, Manfred Bester,working with the ISI in the mid-infrared, led a team at the Space Sciences Laboratory (SSL) at U.C. Berkeley to produce a solution showing Betelgeuse with a uniform disk of 56.6 ± 1.0 mas. In 2000, the SSL team produced another measure of 54.7 ± 0.3 mas, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared. [3] Also included was a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas. The Bester estimate equates to a radius of roughly 5.6 AU or 1,200 R☉, assuming the 2008 Harper distance of 197.0 ± 45 pc, [4] a figure roughly the size of the Jovian orbit of 5.5AU, published in 2009 in Astronomy Magazine and a year later in NASA's Astronomy Picture of the Day. [5]
Across the Atlantic, another team of astronomers working in the near-infrared and led by Guy Perrin of the Observatoire de Paris produced a 2004 document arguing that the more accurate photospheric measurement was 43.33 ± 0.04 mas. [6] The study also put forth an explanation as to why varying wavelengths from the visible to mid-infrared produce different diameters. The star is seen through a thick, warm extended atmosphere. At short wavelengths (the visible spectrum) the atmosphere scatters light thus slightly increasing the star's diameter. At near-infrared wavelengths ( K and L bands), the scattering is negligible, so the classical photosphere can be directly seen; in the mid-infrared the scattering increases once more causing the thermal emission of the warm atmosphere to increase the apparent diameter. [6] Comparing studies, however, is problematic since each wavelength produces a different view of the star, requiring different interpretations. [7]
Studies done in 2009 with the IOTA and VLTI brought strong support to Perrin's analysis yielding diameters ranging from 42.57 to 44.28 mas with comparatively insignificant margins of error. [9] [10] In 2011, Keiichi Ohnaka from the Max Planck Institute for Radio Astronomy produced a third estimate in the near-infrared corroborating Perrin's numbers, this time showing a limb-darkened disk diameter of 42.49 ± 0.06 mas. [11] Consequently, if we combine the smaller Hipparcos distance from van Leeuwen of 152 ± 20 pc with Perrin's angular measurement of 43.33 mas, a near-infrared photospheric estimate would equate to 3.4AU or roughly 730 R☉. [12]
Central to this discussion is another paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate. [13] [14] Unlike most papers heretofore published, this study encompassed a 15-year period at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in Betelgeuse's apparent size equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9 AU in 15 years. What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star's photosphere as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggested that if a cycle does exist, it is probably a few decades long. [13] Other possible explanations are photospheric protrusions due to convection or a star that is not spherical but rather asymmetric causing the appearance of expansion and contraction as the star rotates on its axis. [15]
In conclusion, the current debate between measurements in the mid-infrared, which suggest a possible expansion and contraction of the star, versus the relatively constant photosphere observed in the near-infrared is yet to be resolved. In a recent paper published in 2012, however, the Berkeley team reported that their measurements were "dominated by the behaviour of cool, optically thick material above the stellar photosphere", indicating that the apparent expansion and contraction may be due to activity in the star's outer shells and not the photosphere itself. This conclusion, if further corroborated, would suggest an average angular diameter for Betelgeuse closer to Perrin's estimate at 43.33 arcseconds, hence a stellar radius of about 3.4 AU (730 R☉), if we assume the shorter Hipparcos distance of 498±73ly in lieu of Harper's estimate at 643±146ly. [note 3] [16] The upcoming Gaia Mission will likely clarify many of the assumptions presently used in calculating the size of Betelgeuse's stellar disk, yielding a much wider consensus among astronomers.
Once considered as having the largest angular diameter of any star in the sky after the Sun, Betelgeuse lost that distinction in 1997 when a group of astronomers measured R Doradus with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse. [17]
In 1985, Margarita Karovska, in conjunction with other astrophysicists at the CfA, announced the discovery of two close companions orbiting Betelgeuse. Analysis of polarization data from 1968 through 1983 indicated a close companion with a periodic orbit of about 2.1 years. Using speckle interferometry, the team concluded that the closer of the two companions was located at 0.06 ± 0.01 " (~9 AU) from the main star with a position angle (PA) of 273 degrees, an orbit that would potentially place it within the star's chromosphere. The more distant companion was estimated at 0.51 ± 0.01 " (~77 AU) with a PA of 278 degrees. [18] [19]
In the years that followed no confirmation of Karovska's discovery was ever published. Rather in 1992, a team of collaborators from the Cavendish Astrophysics Group published a paper noting that the brightness features on the surface of Betelgeuse appear to be "too bright to be associated with a passage of the suggested companions in front of the red giant." They also noticed that these features were fainter at 710 nanometers compared to 700 by a factor of 1.8, indicating that such features would have to reside within the molecular atmosphere of the star. [20]
That same year, Karovska published a new paper reconfirming her team's exegesis, but also noting that there was a meaningful correlation between the calculated position angles of the orbiting companion and the reported asymmetries, suggesting a possible connection between the two. [21] Since then, researchers have turned their attention to analyzing the intricate dynamics of the star's extended atmosphere and little else has been published on the possibility of orbiting companions, although as Xavier Haubois and his team reiterate in 2009, the possibility of a close companion contributing to the overall flux has never been fully ruled out. [10] Dommanget's double star catalog (CCDM) lists at least four adjacent stars, all within three arcminutes of this stellar giant, yet aside from apparent magnitudes and position angles, little else is known. [22] As the decade unfolds and new technologies are brought to unraveling the star's enigmatic past, conclusive evidence will likely emerge of any potential star system. Given the planned capabilities of the upcoming Gaia mission, a confirmation could occur any time after the mission's scheduled launch in August 2013. [23]
Because the original discovery of a companion star in 1985 by Karovska was never confirmed, there is no direct method of measuring Betelgeuse's mass. [24] A mass estimate is only possible using theoretical modeling, a situation which has produced mass estimates ranging from 5—30 M☉ in the last decade. [25] [26] The current estimate that is widely accepted is 17.5±2.5M⊙, based on a photospheric measurement of 5.6AU or 1,200R⊙. However, a novel method of determining the supergiant's mass was proposed in 2011 by Hilding Neilson et al. arguing for a stellar mass of 11.6M⊙, based on observations of the star’s intensity profile from narrow H-band interferometry and using a photospheric measurement of 4.3AU or 995R⊙. How the debate is resolved is still open to question—at least until a companion star is identified allowing for a direct calculation of the star's mass.
These uncertainties also mean that no consensus has yet emerged regarding the star's mass. Estimates range from 5 to 30 solar masses (M☉) with most investigators showing a preference for a relatively large mass ranging from 10 to 20M☉. One model reports a mass at the lower end of the scale at 14M☉, although a mass ranging from 18 to 20 is more commonplace. [25] [27]
Across the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement. [6] "A consistent scenario to explain the observations of this star from the visible to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slightly increasing its diameter. The scatter becomes negligible above 1.3 μm. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." The argument has yet to receive widespread support among astronomers. [7]
Across the Atlantic, another team of astronomers led by Guy Perrin of the
Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement.
[6] The study also puts forth an explanation as to why varying wavelengths from the visible to mid-infrared produce different diameters. The star is seen through a thick, warm extended atmosphere. At short wavelengths (the visible spectrum) the atmosphere scatters light thus slightly increasing its diameter. At infrared wavelengths (
K and L bands), the scattering is negligible, so the classical photosphere can be directly seen; in the mid-infrared the scattering increases once more causing the thermal emission of the warm atmosphere to increase the apparent diameter.
[6] Comparing studies at different wavelengths, however, is problematic, since each wavelength produces a different view of the star, requiring different interpretations.
[7]
I reworked paragraph pursuant to observations
To compute these density ratios, the first step is to calculate the mass and volume of each component from which density calculations can be made. Given the density of the Sun, Betelgeuse, and the Earth's atmosphere, dividing gives the appropriate ratios.
Calculations for the Sun:
Calculations for Betelgeuse:
Calculations for the Earth's atmosphere:
Average Density Ratios:
Conclusion:
The following calculations determine Betelgeuse's average radius in terms of solar units based on the following assumptions for angular diameter:
1) Weiner 2000 = 55.20 ± 0.50 mas (limb darkened)
2) Perrin 2004 = 43.33 ± 0.04 mas
The calculations begin with the formula for a star's angular diameter, as follows:
where
represents Betelgeuse's angular diameter in
arcseconds,
the Distance from Earth in
parsecs
Betelgeuse's diameter in
AU, and
Betelgeuse's Radius in AU. Therefore:
To convert the above into Solar units, the math is straightforward. Since 1 AU = 149,597,871 km and the mean diameter of the Sun = 1,392,000 km (hence a mean radius of 696,000 km), the calculation is as follows:
Luminosity is generally understood as a measurement of brightness. Each discipline, however, defines the term differently, depending on what is being measured.
In astronomy, luminosity measures the total amount of energy emitted by a star or other astronomical object in joules per second, which are watts. Watts are the unit of power, and so just as a light bulb is measured in watts, so too is the Sun, the latter having a total power output of 3.846×1026 W. It is this number which constitutes the basic metric used in astronomy and is known as 1 solar luminosity, the symbol for which is . Radiant power, however, is not the only way to conceptualize brightness, so other metrics are also used. The most common is apparent magnitude, which is the perceived brightness of an object from an observer on Earth at visible wavelengths. Other metrics are absolute magnitude, which is an object's intrinsic brightness at visible wavelengths, irrespective of distance, while bolometric magnitude is the total power output across all wavelengths.
The field of optical photometry uses a different set of distinctions, the main ones being luminance and illuminance. Astronomical photometry, by contrast, is concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation. In the field of computer graphics the concept of luminosity is different altogether and is typically used as a synonym for the concept of lightness, otherwise known as the value or tone component of a color.
In astronomy, luminosity is the amount of electromagnetic energy a body radiates per unit of time. It is measured in two forms: apparent (visible light only) and bolometric (total radiant energy). A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term "luminosity" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities. A star also radiates neutrinos, which carry off some energy, about 2% in case of our sun, producing a stellar wind and contributing to the star's total luminosity.[ citation needed]
A star's luminosity is determined primarily by two stellar characteristics: size and temperature. The former is typically represented in terms of solar radii, , while the latter is represented in degrees Kelvin. To determine a star's radius, however, two other metrics are needed: the star's angular diameter and its distance from Earth, usually calculated using parallax measurements. A third metric is the degree of interstellar extinction that is present, a condition that usually arises because of gas and dust present in the interstellar medium (ISM), the Earth's atmosphere, and circumstellar matter. Consequently, one of astronomy's central challenges in determining a star's luminosity is to derive accurate measurements for each of these components, without which an accurate luminosity figure remains elusive.
In the current system of stellar classification, stars are grouped according to temperature, with the very young and energetic Class O stars boasting temperatures in excess of 30,000 K while the older Class M stars exhibit temperatures less than 3,700K—a vast difference that has a huge impact on the star's luminosity. As a result, the most luminous stars in the galaxy are usually the youngest; as stars evolve, their stellar wind causes them to lose mass, a phenomenon which varies greatly with each class of stars but which nevertheless causes a diminution in the star's luminosity over its lifetime. In the Hertzsprung-Russel diagram, the vast majority of stars are found along the main sequence with blue Class 0 stars found at the top left of the chart, given their high luminosity, while red Class M stars fall to the right. The reason certain stars like Deneb and Betelgeuse are found off the main sequence is because of their extremely high luminosity resulting from their enormous size. A star like Deneb, for instance, has a radius that is 203, yielding a mass of 19 and luminosity of 196,000, which means that this blue-white supergiant radiates one hundred and ninety-six thousand times as much energy as the Sun. [30] By contrast, the much cooler Betelgeuse has a luminosity of approximately 140,000, a figure which is only possible because it is considerably larger than Deneb; with a radius of 995, Betelgeuse is about 15 times its size. The most brilliant star ever discovered is a Wolf-Rayet star known as R136a1, spotted in the Large Magellanic Cloud in 2010. At 265 it is one of the most massive and most luminous stars yet identified, boasting a total bolometric luminosity 8,700,000 times that of the Sun.
To conceptualize the range of magnitudes in our own galaxy, the
Milky Way, the smallest star to be identified has about 8% of the Sun’s mass and glows feebly at absolute magnitude +19. Compared to the Sun, which has an absolute of +4.8, this faint star is 14 magnitudes or 400,000 times dimmer than our Sun. Our galaxy's most massive stars begin their lives with masses of roughly 100 times solar, radiating at upwards of absolute magnitude –8, over 160,000 times the solar luminosity. The total range of stellar luminosities, then, occupies a range of 27 magnitudes, or a factor of 60 billion!
The most common distinctions encountered are apparent magnitude, which is the measurement of a celestial body's visible light as seen by an observer from Earth. By contrast, absolute Magnitude is a visible light measurement irrespective of distance and so measures a luminous body's intrinsic brightness. Finally, bolometric magnitude measures light at all wavelengths in the electromagnetic spectrum. Every star will thus have three different measurements. Betelgeuse has an median apparent magnitude of 0.42, but given a distance of 197 parsecs has an absolute magnitude of -6.02. Being a cool, red star, its bolometric luminosity is -7.99, which means the visible light that reaches our eye is only 6% of the star's total radiant energy. If the human eye could see light at all wavelengths, Betelgeuse would show up as the brightest star in the sky.
The word "luminosity" may also refer to spectral luminosity, measured either in W/Hz or W/nm.
In the late phase of stellar evolution, massive stars like Betelgeuse exhibit high rates of mass loss, possibly as much as 1M☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux. [31] All stars exhibit mass loss. Rates vary from about 10−14 to 10−4 yr −1 depending on spectral type, luminosity class, rotation rate, companion proximity, and evolutionary stage. [32] Exactly how this mass loss occurs, however, has been a mystery confronting astronomers for decades. When Schwarzschild first proposed his theory of monster convection cells, he argued it was the likely cause in red supergiants. Prior attempts to explain mass loss in terms of solar wind theory had proven unsuccessful as they led to a contradiction with observations involving circumstellar shells. [33] Other theories that have been advanced include magnetic activity, global pulsations and shock structures as well as stellar rotation. [34]
As a result of work done by Pierre Kervella and his team at the Paris observatory in 2009, astronomers may be close to solving this mystery. Kervella noticed a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions. [35] [31] The plume's presence, in fact, implies that the spherical symmetry of its photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation. [35] Probing deeper still with ESO's AMBER, Keiichi Ohnaka observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella. [31] [36]
In addition to the photosphere, six other components of Betelgeuse's atmosphere have now been identified. Extending outward, one encounters a molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide (CO). There is also evidence of coronal plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence. [25] Some of these elements are known to be asymmetric while others overlap. [10]
At about .45 stellar radii (~2–3 AU) above the photosphere, the closest membrane appears to be the molecular layer known as the MOLsphere. Studies show it to be composed of water-vapor and carbon monoxide with an effective temperature of about 1500 ± 500K. [10] [37] Water-vapor had been originally detected in the supergiant's spectrum back in the 1960s with the two Stratoscope projects but had been ignored for decades. Recent studies suggest that the MOLsphere may also contain SiO and Al2O3—molecules which could explain the formation of dust particles.
Between two and seven stellar radii (~10–40 AU), astronomers have identified another region known as an asymmetric gaseous envelope composed of elemental abundances C, N and O. [10] [38] The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense atmosphere with a "remarkably complex structure". [39] Observations show the atmosphere to be boiling with a temperature of 3,450 ± 850K—similar to the temperature recorded on the star's surface but much lower than surrounding gas in the same region. [39] [40] The VLA images also showed this lower-temperature gas progressively decreasing in temperature as it extends outward—the existence of which, although unexpected, turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere." [39] This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing C N and extending at least six photospheric radii in the southwest direction of the star, is believed to exist. [10]
The chromosphere, as mentioned earlier, was resolved in 1996 at about 2.2 times the optical disk (~10 AU) at ultraviolet wavelengths and is reported to have a temperature no higher than 5,500K. [41] [10] The image was taken with the Faint Object Camera on board the Hubble Space Telescope and also revealed a bright area in the southwest quadrant of the disk. However, in 2004 observations with the STIS, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of 197 pc, the size of the chromosphere could be 200 AU [42] The CfA team led by Alex Lobel concluded that the spatially resolved STIS spectra directly demonstrate the co-existence of warm chromospheric plasma with cool gas in Betelgeuse's circumstellar dust envelope. [42]
The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton and colleagues, who noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, they concluded that the red supergiant emits most of its excess beyond 12 stellar radii or roughly the distance of the Kuiper belt at 50 to 60 AU, depending on the assumed stellar radius. [10] [45] Since then, there have been numerous studies done of this dust envelope at varying wavelengths yielding decidedly different results. More recent studies have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, or 100 to 200 AU. [46] [47] What these studies point out is that the dust environment surrounding Betelgeuse is anything but static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. A few years later, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in just one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots. [46] The 1984 report of a giant asymmetric dust shell located 1 pc (206,265 AU) from the star has not been corroborated in recent studies, although another report published the same year said that three dust shells were found extending four light years from one side of the decaying star, suggesting that, like a snake, Betelgeuse sheds its outer layers as it journeys across the heavens. [48] [49]
Although the exact size of the two outer CO shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0 arcseconds with the other expanding as far as 7.0 arcseconds. [50] Using the Jovian orbit of 5.5 AU as the "average" radius for this gargantuan star, the inner shell would extend roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer one as far as 250 stellar radii (~1400 AU). With the heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of the Solar System.
Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~6.3 AU per year) creating a bow shock. [51] The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first imaged. The cometary structure is estimated to be at least 1 parsec, assuming a current distance of 643 light years. [52]
Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young—less than 30,000 years old—suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. [53] In their 2012 paper, Mohammed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks." [54] [55] Moreover, if future research bears out this hypothesis, Betelgeuse may well prove to have traveled close to 200,000 AU as a red supergiant scattering as much as 3 along its trajectory.
Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as ὑπόκιρρος. It was described by a translator of Ulugh Beg's Zij-i Sultani as rubedinem "ruddiness". [56]
The variation in Betelgeuse's brightness was first described in 1836 by Sir John Herschel, when he published his observations in Outlines of Astronomy; he noted an increase in activity from 1836–1840, followed by a subsequent reduction. In 1849, he noted a shorter cycle of variability which peaked in 1852. Later observers recorded unusually high maxima with an interval of several years, but only small variations from 1957 to 1967. The records of the American Association of Variable Star Observers (AAVSO) show a maximum apparent magnitude (brightness) of 0.2 in the years 1933 and 1942, with a minimum fainter than magnitude 1.2 in both 1927 and 1941. [28] [29] This variability in brightness may explain why Johann Bayer designated the star alpha as it may have rivalled the usually brighter Rigel (beta). [57]
In 1919, Albert Michelson and Francis Pease mounted a 6-metre (20 ft) interferometer on the front of the 2.5 metre (100 inch) telescope at Mount Wilson Observatory. Helped by John Anderson, in December 1920 the trio measured the angular diameter of α Orionis as 0.047 "—a figure which resulted in a diameter of 3.84 × 108 km (240 million miles or 2.58 AU) based on the then-current parallax value of 0.018 ". [58] However, there was known uncertainty owing to limb darkening and measurement errors—a central theme which would be the focus of scientific inquiry for a whole century. Beginning with this first measurement at visible wavelengths, researchers have conducted multiple experiments ranging from the ultraviolet to the far infrared, with each producing different results fueling much debate.
The 1950s and '60s saw several important developments, the two Stratoscope projects and the 1958 publication of Structure and Evolution of the Stars, both the work of Martin Schwarzschild. [59] The book taught a generation of astrophysicists how to use emerging computer technology to create stellar models while the Stratoscope projects, by taking instrumented balloons above the Earth's turbulance, produced some of the finest images of solar granules and sunspots ever seen, thus confirming the existence of convection in the solar atmosphere. Both developments would prove to have a significant impact on our understanding of the structure of red supergiants like Betelgeuse.
In the late 1980s and early 1990s, Betelgeuse became a regular target for aperture masking interferometry visible-light and infrared imaging, which revealed a number of bright spots on the star's surface, thought to result from convection. [61] [62] These were the first optical and infrared images of the disk of a star other than our Sun, and generally showed one or more bright patches—indicating the location of hotspots in the star's photosphere. [63]
In 1995, the Hubble Space Telescope's Faint Object Camera captured an ultraviolet image of comparable resolution—this was the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star. The image was taken at ultraviolet wavelengths since ground-based instruments cannot produce images in the ultraviolet with the same precision as Hubble. Like earlier images, this ultraviolet image also contained a bright patch, indicating a hotter region of about 2,000 Kelvin, in this case on the southwestern portion of the star's surface. [64] Subsequent utraviolet spectra taken with the GHRS suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a position angle from celestial North of about 55°. [65]
Recent ground-based measurements of the disk of Betelgeuse in the mid- infrared, at 11.15 μm, gave an angular diameter of 54.7 ± 0.3 milliarcseconds (mas) in November 1999, slightly smaller than the typical visible-light angular diameter. This measurement assumes a uniform disk model and ignores any possible contribution from hotspots (which are less noticeable in the mid-infrared.) Allowing for the limb darkening effect, whereby the intensity of a star's image diminishes near the edge, gives an increase of approximately 1%, to 55.2 ± 0.5 milliarcseconds (mas). It is difficult to define the precise diameter of Betelgeuse as the photosphere has no "edge"—instead the gas making up the photosphere gets gradually thinner with distance from the star. [3]
On June 9, 2009, Nobel Laureate Charles Townes announced that the star has shrunk 15% since 1993 with an increasing rate. He presented evidence that UC Berkeley's Infrared Spatial Interferometer (ISI) atop Mt. Wilson Observatory had observed 15 consecutive years of stellar contraction. The average speed at which the radius of the star has been shrinking during this period is around 1,000 km/hr. [note 4] Despite Betelgeuse's diminished size, Townes and his colleague, Edward Wishnow, pointed out that the star's visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers (AAVSO), had shown no significant dimming over that same time frame. According to the university, Betelgeuse's diameter is about 5.5 AU, and the star's radius has shrunk by a distance equal to half an astronomical unit, or about the orbit of Venus. [7] [66] Townes concluded it was unclear whether the shrinkage is part of a periodic process or not; if it is, then the cycle is likely to be decades long. [67]
In July 2009, images released by European Southern Observatory, taken by the ground based Very Large Telescope, gave a more detailed view of the surface of the star. [31] In the picture a plume of gas six times its photospheric diameter is seen extending from the star. [35] This is comparable to the distance between the Sun and Neptune.
A series of spectropolarimetric observations, obtained in 2010 with the Bernard Lyot Telescope at Pic du Midi Observatory, revealed the presence of a weak magnetic field at the surface of Betelgeuse, suggesting that the giant convective motions of supergiant stars are able to trigger the onset of a small-scale dynamo. [68]
(already exported)
(already exported)
The uncertainty of Betelgeuse's distance has puzzled astronomers for centuries, and has made reliable estimates difficult for many other stellar parameters such as luminosity. When combined with the angular diameter of the star, one can estimate the radius and its effective temperature; luminosity combined with an understanding of isotopic abundances provides an estimate of the stellar age and mass. [69] In 1920, when the first interferometric studies were performed, the assumed parallax was 0.180 arcseconds. That equates to a distance of 56 pc or roughly 180 ly. Since then, there has been an ongoing inquiry as to the actual distance of this mysterious star with estimated distances as high as 900 light years being proposed. [69]
Before the publication of the Hipparcos Catalogue (1997), there were two respected publications with up-to-date parallax data on Betelgeuse. The first was the Yale University Observatory (1991) with a published parallax of π = 9.8 ± 4.7 milliarcseconds, yielding a distance of roughly 102 pc or 330 ly. [70] The second was the Hipparcos Input Catalogue (1993) with a trigonometric parallax of π = 5 ± 4 mas, a distance of 200 pc or 650 ly—almost twice the Yale estimate. [71] With such uncertainty, researchers were adopting a wide range of distance estimates, a phenomenon which fueled much debate—not only in terms of the star's distance, but also in terms of its many other implications. [69]
The long-awaited results from the Hipparcos mission were finally released in 1997. Instead of resolving the issue, a new parallax figure was published of π = 7.63 ± 1.64 mas, which equated to a distance of 131 pc or roughly 430 ly. [72] Because stars like Betelgeuse vary in brightness, they raise specific problems in quantifying their distance. [73] As a result, the large cosmic error in the Hipparcos solution may be of stellar origin, perhaps related to movements of the photocenter, of order 3.4 mas, in the Hipparcos photometric Hp band. [69] [74]
Where a breakthrough in this debate appears to have come is with the advances in radio astronomy. New high spatial resolution, multiwavelength, NRAO Very Large Array (VLA) radio positions of Betelgeuse have produced a more precise estimate, which combined with the recent Hipparcos data furnished a new astrometric solution: π = 5.07 ± 1.10 mas, a tighter error factor yielding a distance of 197 +/- 45 pc or 643 +/- 146 ly. [69]
In 2012, the
European Space Agency's (ESA) upcoming
Gaia mission will explore the detailed physical properties of each star observed, revealing luminosity, temperature, gravity and composition. Gaia will achieve this by repeatedly measuring the positions of all objects down to magnitude 20, and those brighter than magnitude 15, to an accuracy of 24
microarcseconds—akin to measuring the diameter of a human hair from 1000 km away. On-board detection equipment will ensure that variable stars like Betelgeuse will all be detected and catalogued to this faint limit, thus addressing most of the limitations of the earlier Hipparcos mission. The nearest stars, in fact, will have their distances measured to the extraordinary accuracy of 0.001%. Even stars near the Galactic centre, some 30 000 light-years away, will have their distances measured to within an accuracy of 20%.
[75]
Distance is not the only mystery that has bewitched astronomers. Known as a semiregular pulsating variable, researchers have offered different hypotheses to explain α Ori's volatile choreography. Our current understanding of stellar structure suggests that the outer layers of this supergiant gradually expand and then contract causing the surface area ( chromosphere) to alternately increase and decrease, the temperature to rise and fall — thus eliciting the measured cadence in the star's brightness between its dimmest magnitude of 1.2, seen as early as 1927, and its brightest of 0.2, seen in 1933 and 1942. A red supergiant like Betelgeuse will pulsate like this because its stellar atmosphere is inherently unstable. As the star contracts, it absorbs more and more of the energy that passes through it, causing the atmosphere to heat up and expand. Conversely, as the star expands, its atmosphere becomes less dense allowing the energy to escape and the atmosphere to cool, thus initiating a new contraction phase. [28] What's been particularly challenging in this regard has been apprehending the irregularity of the star's pulsations and modeling its periodicity, as it appears there are several cycles interlaced.
*Paragraph Purpose: Explain periodicity Next Step: Need to pull together all the different periodicity estimates from different articles.
Let us first recall that Alpha Orionis varies in both visual brightness and radial velocity on time scales of a few weeks ormonths. As shown in classical papers, the short term fluctuations modulate a regular, cyclic variation with a period of nearly six years. Stebbins (1931) monitored the brightness of Betelgeuse photoelectrically more or less continuously, weather and seasons permitting, from 1917 to 1931. He concluded that "there is no law or order in the rapid changes of Betelgeuse" and used a drastic smoothing process to derive a mean cure that was best fitted by a period of 5.40 days, although the period of 5.781 days, which had been obtained by Spencer Jones (1928) from radial-velocity data, represented the brightness data nearly as well. The observations from Stebbins and Sanford indicate a possible sub-period of about 100 days in the brightness variations. [76] It is noteworthy that on at least 3 occasions during the past 60 years, unusually large and relatively rapid decreases in the radial velocity of Alpha Ori were followed by suddendecreases of about half a magnitude in visual brightness. Such events seem to have a tendency to occur just after the time of the minimum in the radial velocity connected with the star's pulsations. As the pulsating star reaches maximum velocity of expansion and begins to decelerate there may be instabilities set up in the atmosphere which trigger mass ejection adn the formation of dust grains. The correlation between the observed angular diamter of Betelgeuse and the opacity of the TiO (Balega et al. 1982) indicates that the scale height of the region in which the Fraunhofer spectrum is formed is on the order of one stellar radius. The extrem widths of the line in the Alpha Ori and other supergians suggest that the levitation of the atmosphere may be caused by nonthermal gas pressure, but no matter what holds up the atmosphere, its coupling to the pulsating star cannot be very tight and there will be a tendency for the atmosphere to continue accelerating and to over shoot the minimum, as may have been observed. A survey of variations in the radial-velocity and visual brightness of the star Betelgeuse (alpha-Orionis) over the last six decades is presented. On the basis of a comparison of the results of several observations, it is suggested that major disturbances in Betelgeuse's atmosphere are likely to occur in the year or two following the minimum in the six-year velocity curve. A coordinated observing program is proposed to take place during and after the next minimum, which is predicted to take place in early 1984. The desirable observations include multicolor photometry (particularly in the infrared), polarization measurements, and spectroscopy.
As discussed in classical papers by Stebbins and Sanford in the 1930s, there are short term variations of roughly 150 to 300 days that modulate a regular cyclic variation with a period of roughly 5.7 years. In 2000, astronomers announced the detection of statistically significant cycles in "longwave" (1900-3200 Å) ultraviolet light radiated by the star at 0.531 years (just over six months) and 3.46 years Rinehart et al., 2000). [77]
In fact, the supergiant consistently displays irregular photometric, polarimetric and spectroscopic variations, which points to the occurrence of complex activity on the star's surface and in its extended atmosphere. [61] In marked contrast to most giant stars that are typically long-period variables with reasonably regular periods, red giants and supergiants like Betelgeuse and Antares are generally semi-regular or irregular with pulsating characteristics. In a landmark paper published in 1975, Martin Schwarzschild attributed these brightess fluctuations to the changing granulation pattern formed by a few giant convection cells covering the surface of these stars. [33] [78] For the Sun, these convection cells, otherwise known as solar granules, represent the foremost mode of heat transfer — hence those convective elements which dominate the brightness variations in the solar photosphere. [33] The typical diameter for a solar granule is about 2,000 km (yielding a surface area roughly the size of India), with an average depth of 700 km. With a surface of roughly 6 trillion square kilometers, there are about 2 million of such granules lying on the Sun's photosphere. Beneath these granules, it is conjectured that there are 5-10 thousand supergranules, the average diameter of which is 30,000 km with a depth of about 10,000 km. [79] By contrast, stars like Betelgeuse, Schwardschild argues, may have only a dozen monster granules with diameters of 180,000,000 km or more dominating the surface of the star with depths of about 60,000,000 kilometers, which, because of the very low temperatures and extremely low density found in red giant envelopes, result in convective inefficiency. Consequently, if only one-third of these convective cells are visible to us at any one time, the time variations in their observable light may well be reflected in the brightness variations of the integrated light of the star. [33]
Schwarzschild's hypothesis of gigantic convection cells dominating the surface of red giants and supergiants seems to have stuck with the astronomical community. In 1995, the Hubble Space Telescope captured the first direct picture of the supergiant's surface. The image revealed an extended chromosphere of roughly twice the star's angular diameter with a mysterious hot spot located in the southwest quadrant of the disk that dominated the total ultraviolet flux. The hot spot appeared to be hotter than the surrounding chromosphere by at least 2,000K. "Such a major single feature is distinctly different from scattered smaller regions of activity typically found on the Sun", authors Andrea Dupree and Ronald Gilliland reported, "although the strong ultraviolet flux enhancement is characteristic of stellar magnetic activity. This inhomogeneity may be caused by a large scale convection cell or result from global pulsations and shock structures that heat the chromosphere." [80] [28] Two years later in 1997, astronomers observed intricate assymetries in the brightness distribution of the star, revealing at least three bright spots on the stellar disk. [81] And then in the year 2000, another team of astronomers led by Alex Lobel of the Harvard–Smithsonian Center for Astrophysics (CfA) noted that Betelgeuse exhibits raging storms of hot and cold gas in its turbulent atmosphere, a potential cause being asymmetric pulsations in the star's chromosphere. The team surmised that huge areas of the star's photosphere will vigorously bulge out in different directions at times, ejecting long plumes of warm gas into the cold dust envelope. Another explanation that was also given was the occurrence of shockwaves caused by warm gas traversing cooler regions of the star. [82] [77] The team investigated the atmosphere of Betelgeuse, over a period of five years between 1998 and 2003 with the STIS instrument aboard Hubble. They found that the bubbling action of the chromosphere tosses gas out one side of the star, while it falls inward at the other side, similar to the slow-motion churning of a lava lamp.
*Paragraph Purpose: Latest findings that explain variability Next Step: Read Dupree and other recent articles; integrate
A Harvard-Smithsonian Center for Astrophysics Press Release announcing the first direct image of a star (Betelgeuse), suggests that this event holds new implications for stellar research. Researchers said that this new picture of Betelgeuse suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. "What we see on Betelgeuse is completely different from what occurs on the surface of the Sun," said Dupree, a senior scientist at the CfA. "Instead of lots of little sunspots, we find an enormous bright area more than 2,000 K degrees hotter than the surrounding surface of the star." Until follow-up observations of Betelgeuse are completed, however, astronomers won't know the mystery spot's true nature--or how it formed. [28]
Dupree explains that the spot might change position over time. Dupree and colleagues using the International Ultraviolet Explorer found a 420-day period, during which the star oscillates, or "rings" like a bell. The oscillations, thought to be caused by turbulence below the surface of the star, might cause changes in the bright spot. Future oscillations might spur other bright spots on different regions of the star's surface, causing it to wink on and off like blinking lights on a Christmas tree, said Dupree. Alternatively, the spot might move systematically across the star's surface, which would imply that the star has magnetic fields strong enough to hold the bright spot's hot gas in position. Either scenario would lead astronomers to re-think completely current ideas of how stars evolve. "We hope this work will also pave the way for a generation of space interferometers," said Dupree. Such instruments would greatly improve the resolution of structures on stellar surfaces. [28]
The major question this hot spot raised was whether it was linked to oscillations previously detected in the giant star, or whether it somehow moves systematically across the star's surface under the grip of powerful magnetic fields. [28]
As a variable star, Betelgeuse has the sub-classification "SRC": supergiants with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days. If we assume a distance of 197 pc (± 640 ly), the peak absolute magnitude of Betelgeuse would be -6.27, while its minimum would be -5.27, and its mean -6.05.
A third challenge that has confronted astronomers has been measuring this behemoth's angular diameter. As noted earlier, Betelgeuse became the first star to have its diameter measured on December 13, 1920 by means of a 20-foot beam interferometer. [58] Though interferometry was still very much in its infancy, the experiment proved to be a success and the angular diameter was found to average .047 arcseconds. What was noteworthy were the astronomers' insights on limb darkening. In addition to a measurement error of 10%, Michelson and Pease estimated the actual size of the star to be about 17% larger because of the diminishing intensity of light around the edges—hence an angular diameter as large as .055 ". [58] [13] Since that time, there have been many other studies conducted with angles ranging anywhere from .042 to .069 arcseconds. [3] [83] [84] If we simply take that data and combine it with historical distance estimates of 180 to 815 ly, the projected diameter of the stellar disk could be anywhere from 2.4 to 17.8 AU, hence radii of 1.2 to 8.9 AU respectively. [note 2] That's a wide margin—hence one of the reasons Betelgeuse has been such a mystery. Using the Solar System as a yardstick, the orbit of Mars is approximately 1.5 AU, Ceres in the asteroid belt 2.7 AU, Jupiter 5.5 AU—hence a photosphere which, depending on Betelgeuse's actual distance from Earth, could well extend beyond the orbit of Jupiter but not quite as far as Saturn at 9.5 AU.
The precise diameter has been hard to define for several reasons:
To overcome these challenges, researchers have employed a number of solutions. Astronomical interferometry was first imagined by Hippolyte Fizeau in 1868. [86] He proposed the observation of stars through two apertures to obtain interferences that would provide information on the spatial intensity distribution of the source. Since then, the science of interferometry has evolved considerably where multiple-aperture interferometers are now used consisting of a large number of images superimposed on each other. These "speckled" images are then synthesized using Fourier analysis—a method which has been used for a wide array of astronomical objects including the study of binary stars, quasars, asteroids and galactic nuclei. [87] Space observatories like Hipparcos, Hubble and Spitzer have produced significant breakthroughs and recently another instrument, the Astronomical Multi-BEam Recombiner (AMBER), is yielding new insights. As part of the VLTI, AMBER is capable of combining the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond spatial resolution. Also by combining three baselines instead of two, which customary with conventional interferometry, AMBER enables astronomers to compute the closure phase—an important element in astronomical imaging. [85] [9]
The current debate centers around which wavelength—the visible, near-infrared ( NIR) or mid-infrared ( MIR)—produces the most accurate angular measurement. [note 2] The solution that has been most widely adopted, it appears, is the one performed with the ISI in the mid-infrared by astronomers from the Space Sciences Laboratory at U.C. Berkeley. In the epoch year 2000, the group, under the leadership of John Weiner, published a paper showing Betelgeuse as having a uniform disk of 54.7 ± 0.3 mas. [3] The paper also included a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas—a figure which equates to a radius of roughly 5.5 AU (1,180 times solar), assuming a distance of 197.0 ± 45 pc. [note 3] Nevertheless, given the angular error factor of ± 0.5 mas combined with a parallax error of ± 45 pc found in Harper's numbers, the photosphere's radius could actually be as small as 4.2 AU or as large as 6.9 AU.
Crossing the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43:33 ± 0:04 mas was a more accurate photospheric measurement. "A consistent scenario to explain the observations of this star from the visible to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slighty increasing its diameter. The scatter becomes negligible above 1.3 μm. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." It's a compelling argument but one which has yet to receive widespread support among astronomers. [7]
More recent studies done in the near-infrared with the IOTA and VLTI have brought strong support to Perrin's analysis yielding diameters that range from 42.57 to 44.28 mas with impressively tight error factors of no more than 0.04 mas. [9] [10] Of pivotal importance in this discussion, however, is a second paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate. [13] [14] Unlike most papers heretofore published, this seminal paper represented a systematic study of the star over a 15-year horizon at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in angular separation equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9 AU. [note 4] What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggests that if a cycle does exist, it is likely a few decades long. [13] Consequently, until a full cycle of data has been gathered, we will not know whether the 1993 figure of 56.0 mas represents the maximum extension of the star or its mean, or whether the 2008 figure of 47.0 in fact represents a minimum. It will likely take another 15 years or longer (2025 C.E.) before we know with any certainty, meaning that the Jovian orbit of 5.5 AU will likely serve as the star's "average" diameter for some time. [5]
Once considered as having the largest angular diameter of any star in the sky after the Sun, in 1997 Betelgeuse lost that distinction when a group of astronomers measured R Doradus with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse. [17] _____________END OF SECTION______________________________________________________________
Because of the size and proximity of this star it has the third largest angular diameter as viewed from Earth, being smaller than only the Sun and R Doradus. The angular size of Betelgeuse was one of the first to be measured with an astronomical interferometer and the apparent diameter was found to be variable. Between 1993 and 2009, the star's diameter has contracted by over 15 percent. [88] The precise diameter is hard to define since optical emissions decrease very gradually with radius from the center of Betelgeuse and the color of these emissions also vary with radius.
If we assume a compromise distance of 570 light years, the star's diameter would be about 950 to 1,000 times that of the Sun. If the Sun were the size of a beach ball, then Betelgeuse would be as large as a professional sports stadium. Although only 20 times more massive than the Sun, [27] this star could be hundreds of millions times greater in volume.
Betelgeuse is not only one of the most eminent stars in the heavens, but also one of the most luminous. In the SIMBAD astronomical database the star's spectral class is listed as M2Iab. The "ab" suffix is derived from the Yerkes spectral classification system signifying that Betelgeuse is an intermediate luminous supergiant, less luminous than other supergiants like Deneb. However given some of the recent findings, it may be that this classification is outdated, as Betelgeuse, it appears, is actually much brighter than Deneb and other stars in its class.
If we assume an average radius of 5.5 AU and a distance of 197 pc, Betelgeuse has a luminosity in excess of 180,000 Suns at maximum. When the star contracts as it has since 1993, its luminosity diminishes to about 130,000 Suns. Either way, that amount of electromagnetic energy certainly dwarfs Deneb's output of about 50,000 Suns. [note 1] However, with most of the star's radiant energy occurring in the infrared and huge amounts of it being absorbed by circumstellar matter in the star's outer shell, we simply don't experience the star's total luminosity.
Betelgeuse is a cool star, typical of red supergiants, with a surface temperature of about 3,500 degrees Kelvin. It's also a slow rotator, with the most recent velocity recorded at 5 km/s. At 5.5 AU, it takes the star roughly 32.3 years to turn on its axis — extremely slow when you compare it to a fast rotator like Pleione in the Pleiades star cluster which turns on its axis once every 11.8 hours.
Source Data
The kinematics of Betelgeuse are intriguing yet not easily explained. The age of a Type M supergiant with an initial mass of 20 times solar is roughly 10 million years. [89] [69] Given its current space motion, a projection back in time would take Betelgeuse around 290 parsecs farther from the galactic plane where there is no star formation region — an implausible scenario. Although the space motion for the 25 Ori group has yet to be measured, α Ori’s projected path does not appear to intersect with it either. Also, formation close to the far younger Orion Nebula Cluster (ONC, also known as Ori OB1d) is doubtful. VLBA astrometry yields a distance to the ONC between 389 and 414 parsecs. Consequently, it is likely that Betelgeuse has not always had its current motion through space and has changed course at one time or another. [69]
The most likely star-formation scenario for Betelgeuse is that it's a runaway star from the Orion OB1 Association. Originally a member of a high-mass multiple system within Ori OB1a, which includes the late type O and B stars in Orion's belt — Alnitak, Alnilam and Mintaka — Betelgeuse was probably formed about 10-12 million years ago from the molecular clouds observed in Orion but has evolved rapidly due to its unusually high mass. [69]
Like many of the stars of Orion where massive young stars with over 10 times the Sun's mass can be found in abundance, Betelgeuse will use its fuel quickly and not live very long. On the Hertzsprung-Russell diagram, Betelgeuse has moved off the main sequence and has swelled and cooled to become the red supergiant that it known as. Although this titanic star may have only been in existence for ten million years, unlike its OB cousins born about the same time, Betelgeuse has probably exhausted the hydrogen in its core causing it to contract under the force of gravity into a hotter and denser state. As a result, it has begun to fuse helium into carbon and oxygen producing enough radiation to unfurl its outer envelopes of hydrogen and helium. Its extreme luminosity is being generated by a mass so large that the star will eventually fuse higher elements through neon, magnesium, sodium, and silicon all the way to iron, at which point it will likely collapse and explode as a supernova [27] [77]
As an early M-type supergiant, Betelgeuse is one of the largest and most luminous stars of its class. A radius of 5.5 AU is roughly 1,180 times the radius of the Sun — a sphere so huge that it could contain over 2 quadrillion Earths (2.15 × 1015) or more than 1.6 billion (1.65 × 109) Suns. That's the equivalent of Betelgeuse being a giant football coliseum like Wembley Stadium in London with the Earth a tiny pearl, 1 millimeter in diameter, orbiting a Sun the size of a mango. [note 5] Of particular interest in this respect is the impact of a 15% reduction in the star's radius as reported. That equates to a shortening of the star's radius from about 5.5 AU to 4.6 AU, and a diminution in the star's photospheric volume of approximately 41% or 680 million Suns. [note 6]
Not only is the photosphere enormous, but the star is surrounded by a circumstellar gaseous envelope that extends well over a trillion kilometers from the star. As a result, it takes over two months for light to escape its own shell. In the outer reaches of the photosphere, the density is extremely low. In volume, Betelgeuse exceeds the Sun by a factor of about 1.6 billion Suns. Yet the actual mass of the star is no more than 18-19 solar masses, since the star is estimated to have lost 1-2 solar masses since its birth. [27] Consequently, the average density of this stellar mystery is less than one-sextillionth (1.116 × 10−23) the density of our Sun. If we compare such star matter to the density of ordinary air at sea level, the ratio is roughly 1.286 × 10−5 — a density so tenuous, one would have to get above the noctilucent clouds in the Earth's mesosphere to experience it. [note 7] Such star matter is so ethereal that Betelgeuse has often been called a "red-hot vacuum". [29] [28]
In the late phase of stellar evolution, massive stars like Betelgeuse exhibit high rates of mass loss, possibly as much as 1M☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux. [31] All stars exhibit mass loss. Rates vary from about 10−14 to 10−4 yr −1 depending on spectral type, luminosity class, rotation rate, companion proximity, and evolutionary stage. [32] Exactly how this mass loss occurs, however, has been a mystery confronting astronomers for decades. When Schwarzschild first proposed his theory of monster convection cells, he argued it was the likely cause in red supergiants. Prior attempts to explain mass loss in terms of solar wind theory had proven unsuccessful as they led to a contradiction with observations involving circumstellar shells. [33] Other theories that have been advanced include magnetic activity, global pulsations and shock structures as well as stellar rotation. [34]
As a result of work done by Pierre Kervella and his team at the Paris observatory in 2009, astronomers may be close to solving this mystery. Kervella noticed a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions. [35] [31] The plume's presence, in fact, implies that the spherical symmetry of its photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation. [35] Probing deeper still with ESO's AMBER, Keiichi Ohnaka observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella. [31] [36]
Evidence of circumstellar shells surrounding M supergiants was first proposed by Walter Adams and Elizabeth MacCormack in 1935 when they observed anomalies in the spectral signature of such stars and concluded that the likely cause was an expanding gaseous envelope. [90] [91] In 1955, Armin Deutsch noticed in the Rasalgethi system that spectroscopic peculiarities were mysteriously occurring in the G star companion, α2 Her, from which he concluded that the whole system had to be enveloped by a circumstellar shell composed of matter being ejected by the main star, M supergiant α1 Her, and extending to at least 170 stellar radii. [90] [92] In the mid 1970s, Andrew Bernat undertook a detailed analysis of four circumstellar shells, Betelgeuse, Antares, Rasalgethi and Mu Cephei, concluding that red stars dominate mass return to the Galaxy. [90]
In addition to the photosphere, six other components of Betelgeuse's atmosphere have now been identified. Extending outward, one encounters a molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide (CO). There is also evidence of coronal plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence. [25] Some of these elements are known to be asymmetric while others overlap. [10]
At about .45 stellar radii (~2–3 AU) above the photosphere, the closest membrane appears to be the molecular layer known as the MOLsphere. Studies show it to be composed of water-vapor and carbon monoxide with an effective temperature of about 1500 ± 500K. [10] [37] Water-vapor had been originally detected in the supergiant's spectrum back in the 1960s with the two Stratoscope projects but had been ignored for decades. Recent studies suggest that the MOLsphere may also contain SiO and Al2O3—molecules which could explain the formation of dust particles.
Between two and seven stellar radii (~10–40 AU), astronomers have identified another region known as an asymmetric gaseous envelope composed of elemental abundances C, N and O. [10] [38] The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense atmosphere with a "remarkably complex structure". [39] Observations show the atmosphere to be boiling with a temperature of 3,450 ± 850K—similar to the temperature recorded on the star's surface but much lower than surrounding gas in the same region. [39] [40] The VLA images also showed this lower-temperature gas progressively decreasing in temperature as it extends outward—the existence of which, although unexpected, turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere." [39] This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing C N and extending at least six photospheric radii in the southwest direction of the star, is believed to exist. [10]
The chromosphere, as mentioned earlier, was resolved in 1996 at about 2.2 times the optical disk (~10 AU) at ultraviolet wavelengths and is reported to have a temperature no higher than 5,500K. [41] [10] The image was taken with the Faint Object Camera on board the Hubble Space Telescope and also revealed a bright area in the southwest quadrant of the disk. However, in 2004 observations with the STIS, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of 197 pc, the size of the chromosphere could be 200 AU [42] The CfA team led by Alex Lobel concluded that the spatially resolved STIS spectra directly demonstrate the co-existence of warm chromospheric plasma with cool gas in Betelgeuse's circumstellar dust envelope. [42]
The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton and colleagues, who noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, they concluded that the red supergiant emits most of its excess beyond 12 stellar radii or roughly the distance of the Kuiper belt at 50 to 60 AU, depending on the assumed stellar radius. [10] [45] Since then, there have been numerous studies done of this dust envelope at varying wavelengths yielding decidedly different results. More recent studies have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, or 100 to 200 AU. [46] [47] What these studies point out is that the dust environment surrounding Betelgeuse is anything but static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. A few years later, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in just one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots. [46] The 1984 report of a giant asymmetric dust shell located 1 pc (206,265 AU) from the star has not been corroborated in recent studies, although another report published the same year said that three dust shells were found extending four light years from one side of the decaying star, suggesting that, like a snake, Betelgeuse sheds its outer layers as it journeys across the heavens. [48] [49]
Although the exact size of the two outer CO shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0 arcseconds with the other expanding as far as 7.0 arcseconds. [50] Using the Jovian orbit of 5.5 AU as the "average" radius for this gargantuan star, the inner shell would extend roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer one as far as 250 stellar radii (~1400 AU). With the heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of the Solar System.
Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~6.3 AU per year) creating a bow shock. [51] The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first imaged. The cometary structure is estimated to be at least 1 parsec, assuming a current distance of 643 light years. [52]
Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young—less than 30,000 years old—suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. [53] In their 2012 paper, Mohammed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks." [54] [55] Moreover, if future research bears out this hypothesis, Betelgeuse may well prove to have traveled close to 200,000 AU as a red supergiant scattering as much as 3 along its trajectory.
______________________________ END OF SUB-SECTION______________________________________
1.7 The spots observed on the surface of Betelgeuse
Thanks to the pupil masking technique, Wilson et al. (1992,1997) and Buscher et al. (1990) have detected the presence of spots on the surface of Betelgeuse that represent up to 15-20% of the observed flux in the visible. The most likely origin of these asymmetries is a signature of the convection phenomenon. However, the hypotheses of inhomogeneities in the molecular layers or the presence of a transiting companion are not ruled out (Buscher et al. 1990). These observed spots are few, 2 or 3, and their characteristrics change with time (Wilson et al. 1992). Confirming previous results, Young et al. (2000) observed spots in the visible but show a fully centro-symmetric picture of Betelgeuse in the near infrared (at 1.29 m). Lately Tatebe et al. (2007) also report the observation of an asymmetry at 11.15 m located on the southern edge of the disk of Betelgeuse. In order to deepen our knowledge on RSGs, it is critical to observe spots at high angular resolution. Many unknowns remain such as their size, location, chemical composition, dynamical properties, lifetime and origin.
Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~1 billion km per year) creating a bow shock. The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first successfully imaged. The cometary structure is estimated to be 3 light-years wide.
Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young, less than 30,000 years old, suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. In their 2012 paper, Mohammed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, recent evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks."
Need to handle all this speculation that Betelgeuse is going to blow up soon. Sky & Telescope says "anytime" in the next million years. In stellar time, that's soon.
The future fate of Betelgeuse depends on its mass; as it probably contains more than 15 solar masses, it will continue to burn and fuse elements until its core is iron, at which point Betelgeuse will explode as a type II supernova. During this event the core will collapse, leaving behind a neutron star remnant some 20 km in diameter. [57]
Betelgeuse is already old for its size class and will explode relatively soon compared to its age of several million years. [93] At the current distance of Betelgeuse from the Earth, such a supernova explosion would be the brightest recorded; outshining the Moon in the night sky and becoming easily visible in broad daylight. [93] Professor J. Craig Wheeler of The University of Texas at Austin predicts the supernova will emit 1053 ergs of neutrinos, which will pass through the star's hydrogen envelope in around an hour, then reach the solar system several centuries later. Since its rotational axis is not pointed toward the Earth, Betelgeuse's supernova is unlikely to send a gamma ray burst in the direction of Earth large enough to damage Earth's ecosystems. [94] The flash of ultraviolet radiation from the explosion will be weaker than the ultraviolet output of the Sun.
The supernova would brighten to an apparent magnitude of –12 over a two-week period, then remain at that intensity for two or three months before rapidly dimming. The year following the explosion, radioactive decay of cobalt to iron will dominate emission from the supernova remnant, and the resulting gamma rays will be blocked by the expanding envelope of hydrogen. If the neutron star remnant became a pulsar, then it might produce gamma rays for thousands of years. [95]
In 1985, Margarita Karovska, in conjunction with other astrophysicists at the Harvard–Smithsonian Center for Astrophysics, announced the discovery of two close companions orbiting Betelgeuse. Analysis of polarization data from 1968 through 1983 indicated that Alpha Ori had a close companion with a periodic orbit of about 2.1 years. The team realized that the observed polarization could be caused by a systemic asymmetry created by the close companion orbiting Alpha Ori inside its extended dust envelope. Using speckle interferometry, the team concluded that the closer of the two companions was located at 0.06 ± 0.01 arcseconds from the main star with a position angle of 273 degrees. The more distant companion was estimated at 0.51 ± 0.01 " with a PA of 278 degrees. The magnitude differences with respect to the primary, measured at 656.3 (Hα) and 656.8 nm (red continuum), were 3.4 and 3.0 for the close component and 4.6 and 4.3 for the distant component. [18] [19]
In the years that followed, different teams of astronomers began to monitor the data in the hope of obtaining additional confirmation. In 1987, Andrea Dupree made the following observation: "Periastron of the recently discovered close optical companion to Alpha Ori is predicted to be 1986.7; detection of atmospheric disturbances similar to those found subsequent to the last periastron (~ 1984.6) would give strong support to the presence of a companion." [96] However it appears that such detection never materialized. Rather, in 1990, David F. Buscher, John E. Baldwin and a team of collaborators from the Cavendish Astrophysics Group made a number of high-resolution images of the supergiant at wavelengths of 633, 700, and 710 nm using the nonredundant masking method. At all these wavelengths, they remarked, there was unambiguous evidence for an asymmetric feature on the surface of the star, which contributed 10-15 percent of the star's total observed flux. Their conclusion was that such a phenomenon could be caused by a close companion passing in front of the stellar disk, differential photospheric brightening due to the effects of stellar rotation or the more likely scenario of "large-scale convection in the stellar atmosphere" as suggested by Schwarzschild. [61]
Two years later in 1992, the Cavendish colleagues published another paper, this time under the helm of Richard. W. Wilson, noting that the brightness features on the surface of Betelgeuse appear to be "too bright to be associated with a passage of the suggested companions in front of the red giant." They also noticed that these features were fainter at the 710 than at 700 nm, by a factor of 1.8, indicating that they would have to reside within the molecular atmosphere of the star. [20]
That same year, Karovska published a new paper reconfirming her and her colleagues' interpretation of the data, but also noting that "the correlation between the calculated position angles of the companion and the measured position angles of the asymmetries suggests that there is a possible connection between the asymmetries and the companion. The asymmetry in the images of α Ori could be caused by the unresolved companion, by tidal distortion of the supergiant's atmosphere, or possibly by an unresolved bright spot on the stellar surface facing the companion. To determine the nature of the companion (which presently remains a puzzle), it is crucial to obtain further speckle observations using large aperture telescopes, coordinated with other ground-based observations and the observations from space." [21]
Since then, researchers turned their attention to analyzing the intricate dynamics of the star's extended atmosphere and little else has been published on the possibility of orbiting companions. As the decade unfolds and new technologies are brought to unraveling the star's enigmatic past, we will likely see conclusive evidence, one way or another, of any potential star system. Given the planned capabilities of the upcoming Gaia mission, a confirmation could occur any time after the mission's scheduled launch in December 2012.
Article | Year1 | Telescope | # | Spectrum | λ ( μm) | ∅ ( mas)2 | Radii3 @ 197±45 pc |
Notes |
---|---|---|---|---|---|---|---|---|
Michelson | 1920 | Mt-Wilson | 1 | Visible | 0.575 | 47.0 ± 4.7 | 3.2 - 6.3 AU | Limb darkened +17% = 55.0 |
Bonneau | 1972 | Palomar | 8 | Visible | 0.422-0.719 | 52.0 - 69.0 | 3.6 - 9.2 AU | Strong correlation of ∅ with λ |
Balega | 1978 | ESO | 3 | Visible | 0.405-0.715 | 45.0 - 67.0 | 3.1 - 8.6 AU | No correlation of ∅ with λ |
1979 | SAO | 4 | Visible | 0.575-0.773 | 50.0 - 62.0 | 3.5 - 8.0 AU | ||
Buscher | 1989 | WHT | 4 | Visible | 0.633-0.710 | 54.0 - 61.0 | 4.0 - 7.9 AU | Discovered asymmetries/hotspots |
Wilson | 1991 | WHT | 4 | Visible | 0.546-0.710 | 49.0 - 57.0 | 3.5 - 7.1 AU | Confirmation of hotspots |
Tuthill | 1993 | WHT | 8 | Visible | 0.633-0.710 | 43.5 - 54.2 | 3.2 - 7.0 AU | Study of hotspots on 3 stars |
1992 | WHT | 1 | NIR | 0.902 | 42.6 ± 0:03 | 3.0 - 5.6 AU | ||
Weiner | 1999 | ISI | 2 | MIR ( N Band) | 11.150 | 54.7 ± 0.3 | 4.1 - 6.7 AU | Limb darkened = 55.2 ± 0.5 |
Perrin | 1997 | IOTA | 7 | NIR ( K Band) | 2.200 | 43:33 ± 0:04 | 3.3 - 5.2 AU | K&L Band,11.5μm data contrast |
Haubois | 2005 | IOTA | 6 | NIR ( H Band) | 1.650 | 44.28 ± 0.15‡ | 3.4 - 5.4 AU | Rosseland diameter 45.03 ± 0.12 |
Hernandez | 2006 | VLTI | 2 | NIR ( K Band) | 2.099-2.198 | 42:57 ± 0:02 | 3.2 - 5.2 AU | High precision AMBER results. |
Ohnaka | 2008 | VLTI | 3 | NIR ( K Band) | 2.280-2.310 | 43.19 ± 0.03 | 3.3 - 5.2 AU | Limb darkened 43.56 ± 0.06 |
Townes | 1993 | ISI | 17 | MIR ( N Band) | 11.150 | 56.00 ± 1.00 | 4.2 - 6.8 AU | Systematic study involving 17 measurements at the same wavelength from 1993-2009 |
2008 | ISI | MIR ( N Band) | 11.150 | 47.00 ± 2.00 | 3.6 - 5.7 AU | |||
2009 | ISI | MIR ( N Band) | 11.150 | 48.00 ± 1.00 | 3.6 - 5.8 AU | |||
Harper | 2004 | VLA | Also noteworthy, Harper et al in the conclusion of their paper make the following remark: "In a sense, the derived distance of 200 pc is a balance between the 131 pc (425 ly) Hipparcos distance and the radio which tends towards 250 pc (815 ly)"—hence establishing ± 815 ly as the outside distance for the star. |
1The final year of observations, unless otherwise noted. 2Uniform disk measurement, unless otherwise noted. 3Radii calculations use the same methodology as outlined in Note #2 below ‡Limb darkened measurement.
{{
cite journal}}
: Unknown parameter |duplicate-journal=
ignored (
help)CS1 maint: date and year (
link)
{{
cite journal}}
: Explicit use of et al. in: |author=
(
help)CS1 maint: multiple names: authors list (
link) Cite error: The named reference "WEINER" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
The measurements cannot be compared anyway, because the star's size depends on the wavelength of light used to measure it, Townes said. This is because the tenuous gas in the outer regions of the star emits light as well as absorbs it, which makes it difficult to determine the edge of the star.
{{
cite web}}
: Unknown parameter |month=
ignored (
help) Cite error: The named reference "UC BERKELEY" was defined multiple times with different content (see the
help page).
{{
cite web}}
: Check date values in: |date=
(
help)
{{
cite journal}}
: CS1 maint: date and year (
link) Cite error: The named reference "HERNANDEZ" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: date and year (
link) Cite error: The named reference "HAUBOIS" was defined multiple times with different content (see the
help page).
We derive a uniform-disk diameter of 42.05 ± 0.05 mas and a power-law-type limb-darkened disk diameter of 42.49 ± 0.06 mas and a limb-darkening parameter of (9.7 ± 0.5) × 10-2
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the Sun, researchers reported June 9 at an American Astronomical Society meeting and in the June 1 Astrophysical Journal Letters.Cite error: The named reference "COWEN" was defined multiple times with different content (see the help page).
{{
cite web}}
: Unknown parameter |month=
ignored (
help)
RAVI1
was invoked but never defined (see the
help page).{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "KAROVSKA1" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "KAROVSKA2" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "WILSON2" was defined multiple times with different content (see the
help page).
{{
cite web}}
: Check date values in: |date=
(
help)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite arXiv}}
: CS1 maint: multiple names: authors list (
link)
The mass of the star is unknown, but most investigators show a preference for a fairly large mass in the range of 10–20M☉
{{
cite journal}}
: CS1 maint: date and year (
link)
{{
cite web}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link) Cite error: The named reference "AAVSO" was defined multiple times with different content (see the
help page).
Earlier data had yielded a luminosity of 54,000L☉ with a radius of 108R☉
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
The image is reproduced in the Properties section below.
{{
cite web}}
: Unknown parameter |month=
ignored (
help)
Ridgway notes: 'Stellar mass loss is key to understanding the evolution of the universe from the earliest cosmological times to the current epoch, and of planet formation and the formation of life itself.'
{{
cite journal}}
: Unknown parameter |month=
ignored (
help); Unknown parameter |origin=
ignored (
help) Cite error: The named reference "SCHWARZSCHILD1975" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Explicit use of et al. in: |author=
(
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
DUPREE
was invoked but never defined (see the
help page).In the article, Lobel et al. equate 1 arcsecond to approximately 40 stellar radii, a calculation which in 2004 likely assumed a Hipparcos distance of 131 pc (430 ly) and a photospheric diameter of 0.0552" from Weiner et al.
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
Noriega in 1997 estimated the size to be 0.8 parsecs, having assumed the earlier distance estimate of 400ly. With a current distance estimate of 643ly, the bow shock would measure ~1.28 parsecs or over 4 ly
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: date and year (
link)
allen
was invoked but never defined (see the
help page).{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "MICHELSON" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link) Cite error: The named reference "BUSCHER" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite book}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite web}}
: Unknown parameter |month=
ignored (
help) Cite error: The named reference "ESCIENCE" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Explicit use of et al. in: |author=
(
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite web}}
: Check date values in: |date=
(
help)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link) Cite error: The named reference "FREYTAG" was defined multiple times with different content (see the
help page).
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
{{
cite web}}
: Unknown parameter |month=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link) CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Check date values in: |date=
(
help); Unknown parameter |month=
ignored (
help); Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
Images of hotspots on the surface of Betelgeuse taken at visible and infra-red wavelengths using high resolution ground-based interferometersCite error: The named reference "YOUNG" was defined multiple times with different content (see the help page).
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: date and year (
link)
The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the sun, researchers reported June 9 at an American Astronomical Society meeting and in the June 1 Astrophysical Journal Letters.— Townes, C. H.; Wishnow, E. H.; Hale, D. D. S.; Walp, B. (2009). "A Systematic Change with Time in the Size of Betelgeuse". Astrophysical Journal Letters. 697 (2): L127. Bibcode: 2009ApJ...697L.127T. doi: 10.1088/0004-637X/697/2/L127. S2CID 121009337.
{{
cite journal}}
: Unknown parameter |origin=
ignored (
help)CS1 maint: multiple names: authors list (
link)
{{
cite journal}}
: CS1 maint: multiple names: authors list (
link)
In the concluding sentence of the abstract, Deutsch notes that: This process may be important in the evolution of all massive stars that have exhausted their hydrogen—a reference to α Ori and other red giants experiencing mass loss.