Μ Cephei

Mu Cephei

Location of μ Cep (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cepheus
Right ascension	21h 43m 30.4609s
Declination	+58° 46′ 48.166″
Apparent magnitude (V)	+4.08(3.43 - 5.1)
Characteristics
Spectral type	M2e Ia
U−B color index	+2.42
B−V color index	+2.35
Variable type	SRc
Astrometry
Radial velocity (Rv)	+20.63 km/s
Proper motion (μ)	RA: +5.24 mas/yr Dec.: −2.88 mas/yr
Parallax (π)	0.55 ± 0.2 mas
Distance	approx. 6,000 ly (approx. 1,800 pc)
Absolute magnitude (MV)	−7.63
Details
Mass	19.2 ± 0.1 M☉
Radius	1,260 R☉
Luminosity	283,000 L☉
Surface gravity (log g)	−0.63 cgs
Temperature	3,750 K
Age	10.0 ± 0.1 Myr
Other designations
Erakis, Herschel's Garnet Star, μ Cep, HD 206936, HR 8316, BD+58°2316, HIP 107259, SAO 33693
Database references
SIMBAD	data

Coordinates: 21^h 43^m 30.46^s, +58° 46′ 48.2″ Mu Cephei (μ Cep, μ Cephei), also known as Herschel's Garnet Star, is a red supergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. Since 1943, the spectrum of this star has served as the M2 Ia standard by which other stars are classified.

Mu Cephei is visually nearly 100,000 times brighter than the Sun, with an absolute visible magnitude of M_v = −7.6. Summing radiation at all wavelengths gives a luminosity of around 280,000 L_☉ (bolometric magnitude −8.8), making it one of the most luminous red supergiants in the Milky Way. It is also one of the largest stars so far discovered.

The deep red color of Mu Cephei was noted by William Herschel, who described it as "a very fine deep garnet colour, such as the periodical star ο Ceti". It is thus commonly known as Herschel's "Garnet Star". Mu Cephei was called Garnet sidus by Giuseppe Piazzi in his catalogue. An alternative name, Erakis, used in Antonín Bečvář's star catalogue, is probably due to confusion with Mu Draconis, which was previously called al-Rāqis [arˈraːqis] in Arabic.