Mu Cephei

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Mu Cephei
Cepheus constellation map.svg
Red circle.svg

Location of μ Cep (circled)
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 M1I[3]
U−B color index +2.42[2]
B−V color index +2.35[2]
Variable type SRc
Radial velocity (Rv) +20.63[4] km/s
Proper motion (μ) RA: +5.24[1] mas/yr
Dec.: −2.88[1] mas/yr
Parallax (π) 0.55 ± 0.2 [1] mas
Distance approx. 6,000 ly
(approx. 1,800 pc)
Absolute magnitude (MV) −7.63[3]
Mass 19.2 ± 0.1[5] M
Radius 650[6]–1,420[3] R
Luminosity 3.7 × 105[3] L
Surface gravity (log g) -0.63[7] cgs
Temperature 3690 ± 50[8] K
Age 10.0 ± 0.1[5] Myr
Other designations
Erakis, Herschel's Garnet Star, μ Cep, HD 206936, HR 8316, BD+58°2316, HIP 107259, SAO 33693
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 M2 Ia. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[9]


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".[10] It is thus commonly known as Herschel's "Garnet Star".[11] Mu Cephei was called Garnet sidus by Giuseppe Piazzi in his catalogue.[12] 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.[13]

In 1848, English astronomer John Russell Hind discovered that Mu Cephei 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.[14]


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 the largest star visible to the naked eye, and one of the largest known. It is best seen from the northern hemisphere from August to January.

This is a runaway star with a peculiar velocity of 80.7 ± 17.7 km/s.[5] The distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.55 ± 0.20 milliarcseconds, which corresponds to an estimated distance of 1,333–2,857 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,[8] so it is clear that Mu Cephei is either a much larger star than Betelgeuse or much closer (and smaller and less luminous) than expected.

The star is approximately 1000 times larger than our Sun's solar radius, and were it placed in the Sun's position, its radius would reach between the orbit of Jupiter and Saturn. Mu Cephei could fit almost 1 billion Suns into its volume.

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 visually nearly 100,000 times brighter than the Sun, with an absolute visible magnitude of Mv = −7.6. 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 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.[8]

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).[6] Placed in the position of our Sun, its disk would span between 6 astronomical units (within Jupiter's orbital zone) and 12 astronomical units (beyond Saturn's orbit).

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.[15]

See also[edit]


  1. ^ a b c d e 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. ^ a b c d Table 4 in Emily M. Levesque, Philip Massey, K. A. G. Olsen, Bertrand Plez, Eric Josselin, Andre Maeder, and Georges Meynet (August 2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought". The Astrophysical Journal 628 (2): 973–985. arXiv:astro-ph/0504337. Bibcode:2005ApJ...628..973L. doi:10.1086/430901. 
  4. ^ 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 Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x. 
  6. ^ a b 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. 
  7. ^ Josselin, E.; Plez, B. (2007). "Atmospheric dynamics and the mass loss process in red supergiant stars". Astronomy and Astrophysics 469 (2): 671–680. arXiv:0705.0266. Bibcode:2007A&A...469..671J. doi:10.1051/0004-6361:20066353. 
  8. ^ 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. 
  9. ^ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode:1993AAS...183.1710G 
  10. ^ Herschel, W. (1783). Stars newly come to be visible. Philosophical Transactions (the Royal Astronomical Society of London). p. 257. 
  11. ^ Allen, R. H. (1899). Star-Names and Their Meanings. G. E. Stechert. p. 158. 
  12. ^ Piazzi, G., eds. (1814). Praecipuarum Stellarum Inerrantium Positiones Mediae Ineunte Saeculo XIX: ex Observationibus Habitis in Specula Panormitana ab anno 1792 ad annum 1813. Palermo. p. 159. 
  13. ^ 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. 
  14. ^ 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. 
  15. ^ 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. arXiv:0808.1341. Bibcode:2008ApJ...685L..75D. doi:10.1086/592384. 

External links[edit]

  • "V* mu. Cep". SIMBAD. Retrieved 15 December 2013. 
  • "GARNET STAR (Mu Cephei)". Jim Kaler: Stars. Retrieved 15 December 2013. 
  • "Mu Cephei". AAVSO: Variable Star of the Season Archive. Retrieved 15 December 2013. 
  • "IC 1396". Matt Ben Daniel: Starmatt Astrophotography. Retrieved 15 December 2013. 
  • "Garnet Star". Webprojects: Big and Giant Stars. Retrieved 15 December 2013. 
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