W Cephei

Not to be confused with VV Cephei.

W Cephei is a spectroscopic binary and variable star located in the constellation Cepheus. It is thought to be a member of the Cep OB1 stellar association at about 8,000 light years.^[5][11] The supergiant primary star is one of the largest known stars and as well as one of the most luminous red supergiants.

W Cephei

A visual band light curve for W Cephei, adapted from Polyakova (2006)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Cepheus
Right ascension	22h 36m 27.56307s[2]
Declination	+58° 25′ 33.9554″[2]
Apparent magnitude (V)	6.83 - 9.20[3]
Characteristics
Evolutionary stage	Red supergiant + non-supergiant B class star
Spectral type	K0ep-M2ep Ia + B0/B1[3]
Variable type	SRc[3]
Astrometry
Radial velocity (Rv)	−44.64[4] km/s
Proper motion (μ)	RA: −3.176[2] mas/yr Dec.: −2.277[2] mas/yr
Distance	2,427[5] pc
Absolute magnitude (MV)	−7[6] + −3.5[7]
Orbit[8]
Period (P)	2,075 d
Eccentricity (e)	0.149
Inclination (i)	90°
Details
Radius	670+20 −10[2] R☉
Luminosity	292,000[9] L☉
Surface gravity (log g)	0.18[2] cgs
Temperature	3,681[9] – 4,400[10] K
Metallicity	0.0205[2]
Other designations
W Cep, BD+57°2568, HD 214369, HIP 111592, GSC 03995-00937, SAO 34614, PPM 40864, GC 31569, UCAC3 297-183471, IRAS 22345+5809, 2MASS J22362757+5825340, AAVSO 2232+57
Database references
SIMBAD	data

Discovery

W Cephei was catalogued as BD+57°2568 in the Bonner Durchmusterung published in 1903, and HD 214369 in the Henry Draper Catalogue. It was discovered to be a variable star by T. H. E. C. Espin, in 1885.^[12] It was described in 1896 as a red star varying from magnitude 7.3 to 8.3.^[13]

In 1925, W Cep was included in a listing of Be stars. It was recognised as a cool star with spectral type Mep.^[14] It was classified as K0ep Ia from a 1949 spectrum, but also recognised to have a small hot companion, plus an unusual infrared excess.^[15][16] Ultraviolet spectra allowed absorption lines from the companion to be studied and it was given a spectral type of B0-1.^[17]

System

The W Cephei system contains a luminous red supergiant star with a non-supergiant early B companion. The star has unusual emission lines including both permitted and forbidden Fe_II, produced by a circumstellar envelope containing dust and ionised gas.^[6] The two components have been resolved at 0.262″ using speckle interferometry.^[18] An orbital period of 2,090 days has been proposed.^[11]

Variability

W Cephei varies in brightness from 7th to 9th magnitude. The General Catalogue of Variable Stars lists it as a semiregular variable with a period of 370 days, but later attempts to find a period have shown only random variations.^[19][20] It has also been proposed that eclipses occur.^[21]