Mu Cephei

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**Mu Cephei**
Observation data Epoch J2000.0
Constellation	Cepheus
Right ascension	21^h 43^m 30.46^s
Declination	+58° 46' 48.2"
Apparent magnitude (V)	+4.04
Spectral Characteristics
Spectral type	M2Ia
U-B color index	2.42
B-V color index	2.26
Variable type	Mu Cephei variable
Astrometry
Radial velocity (R_v)	+19.3 km/s
Proper motion (μ)	RA: 5.24 mas/yr Dec.: −2.88 mas/yr
Parallax (π)	0.62 ± 0.52 mas
Distance	approx. 5000 ly (approx. 2000 pc)
Absolute magnitude (M_V)	−7.0
Physical Characteristics
Mass	25 M_☉
Radius	1,420 R_☉
Luminosity	350,000(bolometric) L_☉
Temperature	3,700 K
Metallicity	?
Rotation	?
Age	? years
Other designations
Erakis, Herschel's Garnet Star, HD 206936, HR 8316, BD+58°2316, and HIP 107259
Database references
SIMBAD	data

Mu Cephei (μ Cep / μ Cephei), also known as Erakis or Herschel's Garnet Star is a star in the constellation Cepheus. It was named the Garnet Star by Sir William Herschel, since it appears garnet red due to its spectral class of M2Ia.

Mu Cephei is a variable star and the prototype of the class of the Mu Cephei variables. Its apparent brightness varies without recognizable pattern between +3.62 mag and +5 mag in a period of 2 to 2.5 years. Mu Cephei is 38,000 times brighter than the Sun, with its absolute visible magnitude of M_v = −7.0. Its absolute visible brightness coupled with its infrared radiation and its interstellar extinction gives a luminosity of around 350,000 solar luminosities, making it one of the most luminous stars known. It is approximately 3,000 light years away from the Sun.

Mu Cephei is now in the dying stage of stars. It has begun to fuse helium into carbon, which is the second step after it ceases to fuse hydrogen into helium. A normal star fuses hydrogen into helium, and the helium-carbon cycle shows that the star is about to, or has already gone supernova. When a star goes supernova, it is destroyed, leaving behind a vast hydrogen cloud, centered on, in the situation of Erakis, a black hole.

A very luminous red supergiant, one of the largest and brightest stars visible not only to the naked eye but in the entire Galaxy. It is approximately 1,420 times larger than our sun's solar radius, and if it were placed in the Sun's position, its radius would reach between the orbits of Jupiter and Saturn. Mu Cephei could fit over 1 billion suns into its body. Only five stars (VY Canis Majoris, KW Sagitarii, KY Cygni, V354 Cephei and VV Cephei) are believed to be larger than it.

Mu Cephei is visible with the naked eye, and one does not need a telescope to see how red it is. It is best seen from the Northern hemisphere and it can be seen from August to January.