EW Lacertae

Location of EW Lacertae (circled)
Observation data Epoch J2000.0       Equinox J2000.0
Constellation	Lacerta
Right ascension	22h 57m 04.50206s [1]
Declination	+48° 41′ 2.6456″ [1]
Apparent magnitude (V)	+5.22 – +5.48 [2]
Characteristics
Spectral type	B4IIIpe [3]
Variable type	γ Cas [2]
Astrometry
Proper motion (μ)	RA: 9.298±0.062 [1]  mas/ yr Dec.: −5.409±0.064 [1]  mas/ yr
Parallax (π)	3.4800 ± 0.0706  mas [1]
Distance	940 ± 20  ly (287 ± 6  pc)
Absolute magnitude (MV)	−1.65 [4]
Details
Mass	5.9 [1]  M☉
Radius	7.0 [1]  R☉
Luminosity	2,084 [1]  L☉
Surface gravity (log g)	3.54 [1]  cgs
Temperature	15,230 [1]  K
Metallicity [Fe/H]	+0.51 [1]  dex
Rotational velocity (v sin i)	340 [5] km/s
Age	40 [6]  Myr
Other designations
HD 217050, BD+47 3985, HIP 113327, HR 8731, SAO 52526, [7] Boss 5918 [8]
Database references
SIMBAD	data

EW Lacertae, also known as HD 217050 and HR 8731, is a star about 940 light years from the Earth, in the constellation Lacerta. ^[1] It is a 5th magnitude star, so it will be faintly visible to the naked eye of an observer located far from city lights. It is a Gamma Cassiopeiae variable, varying in brightness from magnitude 5.22 to 5.48, over a period of about 8.7 hours. ^[2] The star's variable spectrum, which shows changes on timescales ranging from hours to decades, has been monitored for more than a century. ^{[9] [10]}

The spectral class of EW Lacertae has been given as B4IIIpe, ^[3] a hot giant star showing emission lines. Other publications have given types between B1 and B5, a luminosity class of III (giant) or IV ( subgiant), and noted various spectral peculiarities related to being a shell star. ^[11] Models published in Gaia Data Release 3 place the star towards the end of its main sequence life. ^[1]

Although spectrograms of EW Lacertae, then known as Boss 5918 or BD+47°3985, had been acquired as early as 1887, ^[9] the existence of an envelope surrounding EW Lacertae was first noticed in a spectrogram taken in 1913. ^[10] Edwin Frost noted that the star's spectrum was variable, in 1919. ^[8] In 1943, Ralph Baldwin reported that EW Lacertae had a shell spectrum. ^[13] The shell spectrum had disappeared in the years 1918 - 1921, but reappeared in 1922. ^[14] Spectra taken in 1925, 1926 and 1928 again showed no features associated with a shell, but the shell features in the spectrum were very clear by the end of 1940. ^[9]

Observations in the early 1950s at the Lick Observatory by Merle Walker revealed that EW Lacertae was a variable star, ^[15] and it was given its variable star designation in the General Catalogue of Variable Stars. ^[16]

The complex variations seen in the spectrum of EW Lacertae may be caused by a disk of gas surrounding the star, seen nearly edge-on by an observer on the Earth, which occasionally has temporary density enhancements which persist for years. ^[17]

References