Mu Cephei

Mu Cephei
Mu Cephei
Cepheus constellation map.svg
Locator Dot.gif

The red dot shows the location of Mu Cephei in Cepheus.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cepheus
Right ascension 21h 43m 30.4609s[1]
Declination +58° 46′ 48.166″[1]
Apparent magnitude (V) +4.08[2]
Spectral type M2Ia[3]
U−B color index +2.42[2]
B−V color index +2.35[2]
Variable type Mu Cephei variable
Radial velocity (Rv) +20.63[4] km/s
Proper motion (μ) RA: +5.24[1] mas/yr
Dec.: −2.88[1] mas/yr
Distance 573.3 ± 98.6[4] pc
Absolute magnitude (MV) −7.0
Mass 15[5] M
Radius 1650[5] R
Luminosity 6 × 104[5] L
Temperature 3690 ± 50 K[6] K
Other designations
Erakis, Herschel's Garnet Star, μ Cep, HD 206936, HR 8316, BD+58°2316, HIP 107259, SAO 33693.[7]
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 and is given the 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,"[8] and it is thus commonly known as Herschel's "Garnet Star".[9] Giuseppe Piazzi called it Garnet sidus in his catalogue.[10] 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.[11]

In 1848, English astronomer John Russell Hind discovered that it was variable. This variability was quickly confirmed by German astronomer Friedrich Wilhelm Argelander. Almost continual records of the star's variability have been maintained since 1881.[12]


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 distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.62 ± 0.52 milliarcseconds, which corresponds to an estimated distance of about 1,612 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390 ± 140 parsecs.[6] A 2005 maximum likelihood estimate of the distance gives a value of 573 ± 99 parsecs.[4]

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 Sagittarii, KY Cygni, V354 Cephei and VV Cephei) are believed to be larger. 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), the diameter of 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.

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.

The photosphere of Mu Cephei has an estimated temperature of 3,690 ± 50 K. It may be surrounded by a shell extending out to a distance at least equal to a 0.33 times the star's radius with a temperature of 2,055 ± 25 K. This outer shell appears to contain molecular gases such as CO, H2O and SiO.[6]

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).[5] Placed in the position of our Sun, its disk would span between 5.5 astronomical units (within Jupiter's orbital zone) and 11 astronomical units (beyond Saturn's orbit).[citation needed]

The star is surrounded by a spherical shell of ejected material that extends outward to an angular distance of 6″ with an expansion velocity of 10 km s−1. This indicates an age of about 2000–3000 years for the shell. Closer to the star, this material shows a pronounced asymmetry, which may be shaped as a torus. The star currently has a mass loss rate of a few times 10−7 solar masses per year.[13]

See also


  1. ^ a b c d Perryman, M. A. C.; et al. (April 1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics 323: L49–L52. Bibcode 1997A&A...323L..49P. 
  2. ^ a b c Nicolet, B. (October 1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy & Astrophysics, Supplement Series 34: =1–49. Bibcode 1978A&AS...34....1N. 
  3. ^ Morgan, W. W.; Keenan, P. C. (1973). "Spectral Classification". Annual Review of Astronomy and Astrophysics 11 (1): 29. Bibcode 1973ARA&A..11...29M. doi:10.1146/annurev.aa.11.090173.000333. 
  4. ^ a b c Famaey, B.; et al. (January 2005). "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters". Astronomy and Astrophysics 430 (1): 165–186. arXiv:astro-ph/0409579. Bibcode 2005A&A...430..165F. doi:10.1051/0004-6361:20041272. 
  5. ^ a b c d Tsuji, Takashi (2000). "Water in Emission in the Infrared Space Observatory Spectrum of the Early M Supergiant Star μ Cephei". The Astrophysical Journal Letters 540 (2): 99–102. arXiv:astro-ph/0008058. Bibcode 2000ApJ...540L..99T. doi:10.1086/312879. 
  6. ^ a b c 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 (1): 317–324. arXiv:astro-ph/0502415. Bibcode 2005A&A...436..317P. doi:10.1051/0004-6361:20042313. 
  7. ^ "V* mu. Cep -- Semi-regular pulsating Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-10-29. 
  8. ^ Herschel, W. (1783). Stars newly come to be visible. the Royal Astronomical Society of London. 257. 
  9. ^ Allen, R. H. (1899). Star-Names and Their Meanings. G. E. Stechert. p. 158. 
  10. ^ Piazzi, G., ed (1814). Praecipuarum Stellarum Inerrantium Positiones Mediae Ineunte Saeculo XIX: ex Observationibus Habitis in Specula Panormitana ab anno 1792 ad annum 1813. Palermo. p. 159. 
  11. ^ Laffitte, R., (2005). Héritages arabes: Des noms arabes pour les étoiles (2éme revue et corrigée ed.). Paris: Librairie Orientaliste Paul Geunthner / Les Cahiers de l'Orient. p. 156, note 267. 
  12. ^ Brelstaff, T.; Lloyd, C.; Markham, T.; McAdam, D. (June 1997). "The periods of MU Cephei". Journal of the British Astronomical Association 107 (3): 135–140. Bibcode 1997JBAA..107..135B. 
  13. ^ de Wit, W. J.; et al. (September 2008). "A Red Supergiant Nebula at 25 μm: Arcsecond-Scale Mass-Loss Asymmetries of μ Cephei". The Astrophysical Journal 685 (1): L75–L78. Bibcode 2008ApJ...685L..75D. doi:10.1086/592384. 

External links

<<< 3. VV Cephei 5. V838 Monocerotis >>>

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