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Mu Cephei
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cepheus
Right ascension 21h 43m 30.46s
Declination +58° 46′ 48.2″
Apparent magnitude (V) +4.04
Spectral type M2Ia
U-B color index 2.42
B-V color index 2.26
Variable type Mu Cephei variable
Radial velocity (Rv) +19.3 km/s
Proper motion (μ) RA: 5.24 mas/yr
Dec.: −2.88 mas/yr
Absolute magnitude (MV) −7.0
Mass 15[1] M
Radius 1650[1] R
Luminosity 60,000[1] L
Temperature 3,600 K
Metallicity ?
Rotation ?
Age ? years
Other designations
Erakis, Herschel's Garnet Star, μ Cep, HD 206936, HR 8316, BD+58°2316, HIP 107259
Database references

Coordinates: Sky map 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 is one of the largest and most luminous stars known in the Milky Way. It appears garnet red due to its spectral class of M2Ia.



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,"[2] and it is thus commonly known as Herschel's "Garnet Star".[3] Giuseppe Piazzi called it Garnet Sidus in his catalogue.[4]

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.[citation needed]


Relative sizes of the planets in the Solar System and several well known stars, including Mu Cephei.
1. Mercury < Mars < Venus < Earth
2. Earth < Neptune < Uranus < Saturn < Jupiter
3. Jupiter < Wolf 359 < Sun < Sirius
4. Sirius < Pollux < Arcturus < Aldebaran
5. Aldebaran < Rigel < Antares < Betelgeuse
6. Betelgeuse < Mu Cephei < VV Cephei A < VY Canis Majoris.

A very luminous red supergiant, Mu Cephei is one of the largest stars visible to the naked eye, and in the entire Galaxy. It is best seen from the Northern hemisphere from August to January.

The star is approximately 1,650 times larger than our sun's solar radius, and were it placed in the Sun's position, its radius would reach between the orbits of Jupiter and Saturn. Mu Cephei could fit almost 4.5 billion suns into its volume. Only five known stars (VY Canis Majoris, KW Sagitarii, KY Cygni, V354 Cephei and VV Cephei) are believed to be larger than it. It is so large that it could fit 6.4 quadrillion Earths in it. If Earth were a golf ball (about 1.7 in/4.3 cm), Mu Cephei would be greater than the length of two Golden Gate Bridges laid end-to-end (about 3.4 mi./5.5 km).

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 magnitude +3.62 and +5 in a period of 2 to 2.5 years. Mu Cephei is 38,000 times brighter than the Sun, with an absolute visible magnitude of Mv = −7.0. Combining its absolute visible brightness, its infrared radiation and correcting for its interstellar extinction gives a luminosity of around 350,000 solar luminosities (bolometric magnitude about -9.1), making it one of the most luminous stars known. Its distance is not very well known. Parallax measurements or distance estimates in the scientific literature give values between 390 and 1,600 parsecs (1,300 and 5,200 ly).[5]

Mu Cephei is nearing death. It has begun to fuse helium into carbon, whereas a main sequence star fuses hydrogen into helium. The helium-carbon cycle shows that Mu Cephei is in the last phase of its life and may explode as a supernova 'soon' in astronomical terms, although this might not be for some millions of years.[citation needed] When a supergiant star becomes a supernova it is destroyed, leaving behind a vast gaseous cloud and a small, dense remnant, which for a star as massive as Mu Cephei may be a black hole. Mu Cephei is currently an unstable star, showing irregular variations in light output, temperature and size.

Emissions from the star suggest the presence of a wide ring of dust and water with outer radius four times that of the star (i.e 2,600 Solar radii) and inner boundary twice the radius of the star (1,300 Solar radii)[1]. If replaced to our Sun, such disk would span between 5.5 Astronomical Units (within Jupiter's orbital zone) and 11 Astronomical Units (beyond Saturn's orbit).

See also


  1. ^ a b c d Tsuji (2000). "Water in Emission in the Infrared Space Observatory Spectrum of the Early M Supergiant Star μ Cephei". Astronomy and Astrophysics 540 (2): 99–102. Bibcode2000ApJ...540L..99T. 
  2. ^ Herschel W., "Stars newly come to be visible," Philosophical Transactions, the Royal Astronomical Society of London, 1783, p.257.
  3. ^ Allen R. H., Star-Names and Their Meanings, G. . Stechert, 1899, p.158.
  4. ^ Piazzi, G., Palermo Catalogue, 1814.
  5. ^ Perrin, G.; et al. (2005). "Study of molecular layers in the atmosphere of the supergiant star µ Cep by interferometry in the K band". Astronomy & Astrophysics 436: 317–324. doi:10.1051/0004-6361:20042313. 

External links

<<< 3. VV Cephei 5. V354 Cephei >>>


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