Mu Cephei (Latinized fromμ Cephei, abbreviatedMu Cep orμ Cep), also known as theGarnet Star, is ared supergiant star in theconstellationCepheus. It appearsgarnet red and is located at the edge of theIC 1396 nebula. It is a 4thmagnitude star easily visible to thenaked eye under good observing conditions. Since 1943, thespectrum of this star has served as a spectral standard by which other stars are classified.
Mu Cephei is more than 100,000 times brighter than the Sun, with anabsolute visual magnitude of −7.6. It is also one of thelargest known stars with a radius around or over 1,000 times that of the sun (R☉), and were it placed in the Sun's position it would engulf the orbit ofMars andJupiter.
In 1848, English astronomerJohn Russell Hind discovered that Mu Cephei was variable. This variability was quickly confirmed by German astronomerFriedrich Wilhelm Argelander. Almost continual records of the star's variability have been maintained since 1881.[22]
The angular diameter of μ Cephei has been measuredinterferometrically. One of the most recent measurements gives a diameter of18.672±0.435 mas at800 μm, modelled as alimb-darkened disk20.584±0.480 mas across.[23] However, this later turned out to be the surrounding molecular layer and not the actual star, as the star has an angular diameter of 14.11 ± 0.6 mas.[24] μ Cephei was used as one of the original "dagger stars", those with well-defined spectra that could be used for the classification of other stars, for MK spectral classifications. In 1943 it was the standard star for M2 Ia, updated in 1980 to be the standard star for the new type M2- Ia.[4][25]
Mu Cephei (circled) as can be seen in binoculars. The bright star on the right isAlderamin (Alpha Cephei).Mu Cephei and surrounding nebulosity, imaged atH-alpha and OIII wavelengths (north is towards top left)
The distance to Mu Cephei is not very well known. TheHipparcos satellite was used to measure a parallax of0.55±0.20 mas, which corresponds to an estimated distance of1,800 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison withBetelgeuse gives an estimate of390±140 parsecs.[26]
Calculation of the distance from the measured angular diameter, surface brightness, and calculated luminosity leads to641 pc.[10] Averaging the distances of nearby luminous stars with similar reddening and reliableGaia Data Release 2 parallaxes gives a distance of940 pc.[11]
Mu Cephei is surrounded by a shell extending out to a distance at least equal to 0.33 times the star's radius with a temperature of2,055±25 K. This outer shell appears to contain molecular gases such asCO,H2O, andSiO.[26]Infrared observations suggest the presence of a wide ring of dust and water with an inner radius about twice that of the star itself, extending to about four times the radius of the star.[27][28]
The star is surrounded by a spherical shell of ejected material that extends outward to an angular distance of6″ with an expansion velocity of10 km s−1. This indicates an age of about 2,000–3,000 years for the shell. Closer to the star, this material shows a pronounced asymmetry, which may be shaped as atorus.
Mu Cephei is avariable star and the prototype of the obsolete class of theMu Cephei variables. It is now considered to be asemiregular variable of type SRc. Its apparent brightness varies erratically between magnitude 3.4 and 5.1. Many different periods have been reported, but they are consistently near 860 days or 4,400 days.[29]
Thebolometric luminosity, summed over all wavelengths, is calculated from integrating thespectral energy distribution (SED) to be 269,000 L☉, making μ Cephei one of themost luminous red supergiants in the Milky Way.[11] Itseffective temperature of3,750 K, determined from colour index relations, implies a radius of 1,259 R☉.[14] Other recent publications give similar effective temperatures. Calculation of the luminosity from a visual and infrared colour relation give 340,000 L☉ and a corresponding radius of 1,420 R☉.[12] An estimate made based on its angular diameter and an assumed distance of2,400 light years gives it a radius of 1,650 R☉,[31] however the angular diameter used later turned out to be the diameter of the molecular layer around the star.[24]
The radius has been estimated to be 830 R☉ in 2010 based on the star's effective temperature of3660 K and the 111,200 L☉ luminosity estimate.[32]
A 2019 paper measurement based on the641+148 −144 pc distance gives the star a lower luminosity below 140,000 L☉ and a correspondingly lower radius of972±228 R☉, and as well as a lower temperature of3,551±136 K. These parameters are all consistent with those estimated for Betelgeuse.[10]
The initial mass of Mu Cephei has been estimated from its position relative to theoreticalstellar evolutionary tracks to be between 15 M☉ and 25 M☉.[10][14] The star currently has a mass loss rate of(4.9±1.0)×10−7M☉ per year.[10]
Mu Cephei is nearing death. It has begun tofusehelium intocarbon, whereas amain sequence star fuseshydrogen into helium. When a supergiant star has converted elements in its core to iron, the core collapses to produce asupernova and the star is destroyed, leaving behind a vast gaseous cloud and a small, dense remnant. For a star as massive as Mu Cephei the remnant is likely to be ablack hole. The most massivered supergiants will evolve back toblue supergiants,Luminous blue variables, orWolf-Rayet stars before their cores collapse, and Mu Cephei appears to be massive enough for this to happen. A post-red supergiant will produce a type IIn or type II-b supernova, while a Wolf Rayet star will produce a type Ib or Ic supernova.[33]
^abcNicolet, 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.
^abSamus, N. N.; Durlevich, O. V.; et al. (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: B/gcvs.Bibcode:2009yCat....102025S.
^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.
^abBrelstaff, 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.
^Mozurkewich, D.; Armstrong, J. T.; Hindsley, R. B.; Quirrenbach, A.; Hummel, C. A.; Hutter, D. J.; Johnston, K. J.; Hajian, A. R.; Elias, Nicholas M.; Buscher, D. F.; Simon, R. S. (2003). "Angular Diameters of Stars from the Mark III Optical Interferometer".The Astronomical Journal.126 (5): 2502.Bibcode:2003AJ....126.2502M.doi:10.1086/378596.S2CID67789347.
^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
^De Beck, E.; Decin, L.; de Koter, A.; Justtanont, K.; Verhoelst, T.; Kemper, F.; Menten, K. M. M. (November 2010). "Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae".Astronomy & Astrophysics.523: A18.arXiv:1008.1083.Bibcode:2010A&A...523A..18D.doi:10.1051/0004-6361/200913771.ISSN0004-6361.S2CID16131273.
^Meynet, G.; Chomienne, V.; Ekström, S.; Georgy, C.; Granada, A.; Groh, J.; Maeder, A.; Eggenberger, P.; Levesque, E.; Massey, P. (2015). "Impact of mass-loss on the evolution and pre-supernova properties of red supergiants".Astronomy & Astrophysics.575 (60): A60.arXiv:1410.8721.Bibcode:2015A&A...575A..60M.doi:10.1051/0004-6361/201424671.S2CID38736311.
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