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 | |
| Database references | |
| SIMBAD | data |
Coordinates: 
21h 43m 30.46s, +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 Mv = −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.
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