Epsilon Pegasi

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Epsilon Pegasi
Diagram showing star positions and boundaries of the Pegasus constellation and its surroundings
Cercle rouge 100%.svg

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]
Spectral type K2 Ib[2]
U−B color index +1.722[3]
B−V color index +1.527[3]
Variable type LC[4]
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[1] mas
Distance 690 ± 20 ly
(211 ± 8 pc)
Absolute magnitude (MV) –4.142[6]
Mass 11.7 ± 0.8[7] M
Radius 185[8] R
Luminosity 3895[9] L
Luminosity (bolometric) 12,250[9] L
Surface gravity (log g) 1.01[6] cgs
Temperature 4,379[6] K
Metallicity [Fe/H] –0.04[6] dex
Rotational velocity (v sin i) 8[10] km/s
Age 20.0 ± 4.5[7] Myr
Other designations
Enif, Enf, Enir, Al Anf, Os Pegasi, Fom, 8 Peg, BD+09 4891, FK5 815, HD 206778, HIP 107315, HR 8308, SAO 127029.[11]
Database references

Epsilon Pegasi (ε Peg, ε Pegasi) is the brightest star in the northern constellation of Pegasus. It has the traditional name Enif (EE-nif). The name Enif is derived from the Arabic word for nose, due to its position as the muzzle of Pegasus. With an 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]

Other traditional names for the star include Fom al Feras, Latinised to Os Equi.[14] In Chinese, 危宿 (Wēi Sù), meaning Rooftop (asterism), refers to an asterism consisting of ε Pegasi, α Aquarii and θ Peg.[15] Consequently, ε Pegasi itself is known as 危宿三 (Wēi Sù sān, English: the Third Star of Rooftop.)[16]


This is an evolved star that has entered the supergiant stage, as indicated by the stellar classification of K2 Ib.[2] It is estimated to be 12[7] times the Sun's mass. The measured angular diameter of this star, after correction for limb darkening, is 8.17 ± 0.09 mas.[17] At the estimated distance of this star, this yields an enormous physical size of about 185 times the radius of the Sun.[8] From this expanded envelope, it is radiating roughly 12,250[9] times the luminosity of the Sun at an effective temperature of 4,337 K.[6] This temperature is cooler than the Sun, giving it the orange-hued glow of a K-type star.[18]

Enif probably only has a few million years left to go, although it is unknown whether it will explode in a supernova or die off as a rare neon-oxygen white dwarf, due to its mass straddling the dividing line between stars destined to explode or not. Enif has been observed to brighten radically upon a few occasions, giving rise to the theory that it (and possibly other supergiants) erupt in massive flares that dwarf those of our own Sun.[19] It is a type LC slow irregular variable star that varies from +0.7 to +3.5 in magnitude.[4] 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.[9] It has a relatively high peculiar velocity of 21.6 km s−1.[7]

See also[edit]


  1. ^ a b c d e f g 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 
  2. ^ a b 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 
  3. ^ a b c Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars 8: 59, Bibcode:1984SAAOC...8...59C 
  4. ^ a b "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 
  6. ^ a b c d e 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 
  7. ^ a b c d 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. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3 ed.), Birkhäuser, ISBN 3-540-29692-1 . The radius (R*) is given by:
    \begin{align} 2\cdot R_*
 & = \frac{(10^{-3}\cdot 211\cdot 8.17)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\
 & \approx 370.8\cdot R_{\bigodot}
  9. ^ a b c d 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: 563–579, Bibcode:1987MNRAS.226..563S, doi:10.1093/mnras/226.3.563 
  10. ^ Bernacca, P. L.; Perinotto, M. (1970). "A catalogue of stellar rotational velocities". Contributi Osservatorio Astronomico di Padova in Asiago 239 (1). Bibcode:1970CoAsi.239....1B. 
  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, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5 
  14. ^ 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: 429–38. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429. 
  15. ^ (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.
  16. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
  17. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039 
  18. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  19. ^ Kaler, James B., "ENIF (Epsilon Pegasi)", Stars (University of Illinois), retrieved 2012-02-08 

Coordinates: Sky map 21h 44m 11.158s, +09° 52′ 30.04″