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] (3.43 - 5.1[3])
Spectral type M2e Ia[4]
U−B color index +2.42[2]
B−V color index +2.35[2]
Variable type SRc[3]
Radial velocity (Rv)+20.63[5] 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
Distanceapprox. 6,000 ly
(approx. 1,800 pc)
Absolute magnitude (MV)−7.63[6]
Mass19.2 ± 0.1[7] M
Radius1,260[8] (650[9]-1,420[6]R
Diameter1,260[8] (650[9]-1,420[6]D
Luminosity283,000[8] (60,000[9]-340,000[6]L
Surface gravity (log g)−0.63[8] cgs
Temperature3,750[8] (3,540[10]-3,789[11]K
Age10.0 ± 0.1[7] Myr
Other designations
Erakis, Herschel's Garnet Star, μ Cep, HD 206936, HR 8316, BD+58°2316, HIP 107259, SAO 33693
Database references

Mu Cephei (μ Cep, μ Cephei), also known as Herschel's Garnet Star, is a red hypergiant[12] star in the constellation Cepheus. It appears garnet red, which was fired observed by William Herschel (hence the star's name). Mu Cephei is 283,000 times more luminous than the Sun, with an absolute visible magnitude of around Mv = −7.6.


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.
Mu Cephei compared to the Sun.

A very luminous red supergiant, Mu Cephei is likely to be among the largest stars visible to the naked eye, and definitely one of the largest known. It has been described as a red hypergiant star. [13] Mu Cephei is a runaway star with a peculiar velocity of 80.7 ± 17.7 km/s.[7] Thus, the distance to Mu Cephei is not very 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 near the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390 ± 140 parsecs,[11] 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. Both Mu Cephei and Betelgeuse are red supergiants with bow shocks. Mu Cephei is over 1000 D in diameter, and were it placed in the center of the Solar System, it would reach past the orbit of Saturn.[8][6] Thus, Mu Cephei could fit more than a billion suns into its volume. Older studies frequently calculated lower luminosities and lower radii,[9] while the surface of Mu Cephei has an estimated temperature of 3,750 K.


  1. 1.0 1.1 1.2 1.3 1.4 Perryman, M. A. C. (April 1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics 323: L49–L52. Bibcode 1997A&A...323L..49P.
  2. 2.0 2.1 2.2 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. 3.0 3.1 Samus, N. N.Expression error: Unrecognized word "etal". (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S 1. Bibcode 2009yCat....102025S.
  4. Shenavrin, V. I.; Taranova, O. G.; Nadzhip, A. E. (2011). "Search for and study of hot circumstellar dust envelopes". Astronomy Reports 55: 31. Bibcode 2011ARep...55...31S. doi:10.1134/S1063772911010070.
  5. Famaey, B. (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.
  6. 6.0 6.1 6.2 6.3 Table 4 in Emily M. Levesque; Philip Massey; K. A. G. Olsen; Bertrand Plez; Eric Josselin; Andre Maeder; 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.
  7. 7.0 7.1 7.2 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.
  8. 8.0 8.1 8.2 8.3 8.4 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.
  9. 9.0 9.1 9.2 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.
  10. Meneses-Goytia, S.; Peletier, R. F.; Trager, S. C.; Falcón-Barroso, J.; Koleva, M.; Vazdekis, A. (2015). "Single stellar populations in the near-infrared. I. Preparation of the IRTF spectral stellar library". Astronomy & Astrophysics 582: A96. arXiv:1506.07184. Bibcode 2015A&A...582A..96M. doi:10.1051/0004-6361/201423837.
  11. 11.0 11.1 Perrin, G. (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.
  13. Shenoy, Dinesh; Humphreys, Roberta M; Terry Jay Jones; Marengo, Massimo; Gehrz, Robert D; Andrew Helton, L; Hoffmann, William F; Skemer, Andrew J et al. (2015). "Searching for Cool Dust in the Mid-to-Far Infrared: The Mass Loss Histories of the Hypergiants μ Cep, VY CMa, IRC+10420, and ρ Cas". The Astronomical Journal 151 (3): 51. arXiv:1512.01529. Bibcode 2016AJ....151...51S. doi:10.3847/0004-6256/151/3/51.

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