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Mu Cephei

Coordinates: Sky map 21h 43m 30.46s, +58° 46′ 48.2″
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This is an old revision of this page, as edited by Lithopsian (talk | contribs) at 15:48, 20 May 2012 (Corrected typo in radius from 1650 to 650, noted that text comparisons are now in error). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Mu Cephei
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]
Characteristics
Spectral type M2Ia[3]
U−B color index +2.42[2]
B−V color index +2.35[2]
Variable type Mu Cephei variable
Astrometry
Radial velocity (Rv)+20.63[4] km/s
Proper motion (μ) RA: +5.24[1] mas/yr
Dec.: −2.88[1] mas/yr
Distance573.3 ± 98.6[4] pc
Absolute magnitude (MV)−7.0
Details
Mass15[5] M☉
Radius650[5]-1,420[6] R☉
Luminosity6 × 104[5] L☉
Temperature3690 ± 50 K[7] K
Other designations
Erakis, Herschel's Garnet Star, μ Cep, HD 206936, HR 8316, BD+58°2316, HIP 107259, SAO 33693.[8]
Database references
SIMBADdata

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]

History

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] and it is thus commonly known as Herschel's "Garnet Star".[11] Giuseppe Piazzi called it Garnet sidus 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 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.[14]

Properties

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.[7] 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 three known stars (VY Canis Majoris, VV Cephei and V838 Monocerotis) 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).[foot 1]

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.[7]

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.[15]

Footnotes

  1. ^ The comparisons in this paragraph are based on a radius of 1,650 which was apparently a typographic error and should have been 650. Some other research estimates a radius over 1,000 but this paragraph should probably be rewritten or removed.

See also

References

  1. ^ a b c d Perryman, M. A. C. (1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics. 323: L49–L52. Bibcode:1997A&A...323L..49P. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  2. ^ a b c Nicolet, B. (1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy & Astrophysics, Supplement Series. 34: =1–49. Bibcode:1978A&AS...34....1N. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: extra punctuation (link)
  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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ a b c Famaey, B. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  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. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ a b c 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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ "V* mu. Cep -- Semi-regular pulsating Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-10-29.
  9. ^ Garrison, R. F. (1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society, 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04 {{citation}}: Unknown parameter |month= ignored (help)
  10. ^ Herschel, W. (1783). Stars newly come to be visible. the Royal Astronomical Society of London. p. 257. {{cite book}}: |journal= ignored (help)
  11. ^ Allen, R. H. (1899). Star-Names and Their Meanings. G. E. Stechert. p. 158.
  12. ^ Praecipuarum Stellarum Inerrantium Positiones Mediae Ineunte Saeculo XIX: ex Observationibus Habitis in Specula Panormitana ab anno 1792 ad annum 1813. Palermo. 1814. p. 159. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
  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.{{cite book}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  14. ^ Brelstaff, T.; Lloyd, C.; Markham, T.; McAdam, D. (1997). "The periods of MU Cephei". Journal of the British Astronomical Association. 107 (3): 135–140. Bibcode:1997JBAA..107..135B. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  15. ^ de Wit, W. J. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
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