Epsilon Pegasi

Epsilon Pegasi (Latinised from ε Pegasi, abbreviated Epsilon Peg, ε Peg), formally named Enif /ˈiːnɪf/, is the brightest star in the northern constellation of Pegasus.

ε Pegasi
Location of ε Pegasi (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Pegasus
Right ascension 21h 44m 11.15614s[1]
Declination +09° 52′ 30.0311″[1]
Apparent magnitude (V) 2.399[1] (0.7 - 3.5)
Characteristics
Spectral type K2 Ib-II[2]
U−B color index +1.722[3]
B−V color index +1.527[3]
Variable type LC[4]
Astrometry
Radial velocity (Rv)3.39 ± 0.06[5] km/s
Proper motion (μ) RA: +26.92[1] mas/yr
Dec.: +0.4[1] mas/yr
Parallax (π)4.73 ± 0.17 mas[1]
Distance690 ± 20 ly
(211 ± 8 pc)
Absolute magnitude (MV)–4.142[6]
Details
Mass7.07 - 7.45[7] M☉
Radius211[7] R☉
Luminosity3,895[8] L☉
Luminosity (bolometric)9,716 - 9,898[7] L☉
Surface gravity (log g)1.01[6] cgs
Temperature3,963 - 3,965[7] K
Metallicity [Fe/H]–0.04[6] dex
Rotational velocity (v sin i)8[9] km/s
Age20.0 ± 4.5[10] Myr
Other designations
Enif, 8 Pegasi, BD+09°4891, FK5 815, HD 206778, HIP 107315, HR 8308, SAO 127029[11]
Database references
SIMBADdata

With an average apparent visual magnitude of 2.4,[3] this is a second-magnitude star that is readily visible to the naked eye. The distance to this star can be estimated using parallax measurements from the Hipparcos astrometry satellite,[12][13] yielding a value of around 690 light-years (210 parsecs).[1]

Nomenclature

ε Pegasi (Latinised to Epsilon Pegasi) is the star's Bayer designation.

It bore the traditional name Enif derived from the Arabic word for 'nose', due to its position as the muzzle of Pegasus. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Enif for this star.[16]

Other traditional names for the star include Fom al Feras, Latinised to Os Equi.[17] In Chinese, 危宿 (Wēi Sù), meaning Rooftop (asterism), refers to an asterism consisting of Epsilon Pegasi, Alpha Aquarii and Theta Pegasi.[18] Consequently, the Chinese name for Epsilon Pegasi itself is 危宿三 (Wēi Sù sān, English: the Third Star of Rooftop.)[19]

Physical characteristics

Epsilon Pegasi is a red supergiant star, as indicated by the stellar classification of K2 Ib.[20] It is estimated to be seven times the Sun's mass. The measured angular diameter of this star, after correction for limb darkening, is 8.17±0.09 mas.[21] At the estimated distance of this star, this yields an enormous physical size of about 211 times the radius of the Sun. From this expanded envelope, it is radiating roughly 9,800 times the luminosity of the Sun at an effective temperature of 3,965 K. This temperature is cooler than the Sun, giving it the orange-hued glow of a K-type star.[22]

Epsilon Pegasi is a slow irregular variable star that usually has a brightness between magnitudes 2.37 and 2.45. However, it was once observed very briefly at magnitude 0.7, giving rise to the theory that it (and possibly other supergiants) erupt in massive flares that dwarf those of the Sun.[23][24] It has also been observed as faint as magnitude 3.5.[23]

The spectrum shows an overabundance of the elements strontium and barium, which may be the result of the s-process of nucleosynthesis in the outer atmosphere of the star.[8] It has a relatively high peculiar velocity of 21.6 km/s.[10]

Evolution

Epsilon Pegasi has exhausted its core hydrogen and expanded away from the main sequence. It is almost certainly on the horizontal branch fusing helium in its core.[7] It is sufficiently massive that it may die in a core-collapse supernova, or if it lose more mass in the supergiant phase in its evolution, it may shed its outer layers and leave behind an unusual high mass oxygen–neon white dwarf near the Chandrasekhar limit.[24]

Pulfrich effect

Epsilon Pegasi is a fine example to observe the Pulfrich effect. This optical phenomenon is described on page 1372 of Burnham's Celestial Handbook. According to John Herschel: The apparent pendulum-like oscillation of a small star in the same vertical as the large one, when the telescope is swung from side to side.[25]

See also

References
  1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
  2. Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  3. Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars, 8: 59, Bibcode:1984SAAOC...8...59C
  4. "eps Peg", General Catalogue of Variable Stars, Centre de Données astronomiques de Strasbourg, retrieved 2010-01-05
  5. 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, S2CID 17804304
  6. Soubiran, C.; et al. (2008), "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants", Astronomy and Astrophysics, 480 (1): 91–101, arXiv:0712.1370, Bibcode:2008A&A...480...91S, doi:10.1051/0004-6361:20078788, S2CID 16602121
  7. Stock, S.; Reffert, S.; Quirrenbach, A. (2018). "VizieR Online Data Catalog: Stellar parameters of 372 giant stars (Stock+, 2018)". VizieR On-line Data Catalog. 361. Bibcode:2018yCat..36160033S.
  8. Smith, Verne V.; Lambert, David L. (June 1987), "Are the red supergiants Epsilon Peg and 12 PUP victims of mild s-processing?", Monthly Notices of the Royal Astronomical Society, 226 (3): 563–579, Bibcode:1987MNRAS.226..563S, doi:10.1093/mnras/226.3.563
  9. Bernacca, P. L.; Perinotto, M. (1970). "A catalogue of stellar rotational velocities". Contributi Osservatorio Astronomico di Padova in Asiago. 239 (1): 1. Bibcode:1970CoAsi.239....1B.
  10. 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, S2CID 118629873
  11. "V* eps Peg -- Pulsating variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2010-01-05
  12. Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52, Bibcode:1997A&A...323L..49P
  13. Perryman, Michael (2010), The Making of History's Greatest Star Map, Astronomers’ Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
  14. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  15. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  16. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  17. Knobel, Edward B. (1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of". Monthly Notices of the Royal Astronomical Society. 55 (8): 429–38. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  18. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  19. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2011-01-30 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  20. 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
  21. Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
  22. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-18, retrieved 2012-01-16
  23. "Epsilon Pegasi", Variable Star Index, retrieved 2022-03-07
  24. Kaler, James B., "ENIF (Epsilon Pegasi)", Stars, University of Illinois, retrieved 2012-02-08
  25. Burnham, Robert (1978), Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, Dover books on astronomy and astrophysics, vol. 3, Courier Corporation, p. 1372, ISBN 978-0486236735
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