# Epsilon Pegasi

Observation data Epoch J2000      Equinox J2000 Constellation Location of ε Peg (circled) Pegasus 21h 44m 11.15614s[1] +09° 52′ 30.0311″[1] 2.399[1] K2 Ib[2] +1.722[3] +1.527[3] 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 Enif, Enf, Enir, Al Anf, Os Pegasi, Fom,[citation needed] 8 Peg, BD+09° 4891, FK5 815, HD 206778, HIP 107315, HR 8308, SAO 127029.[11] SIMBAD data

Epsilon Pegasi (ε Pegasi, abbreviated Epsilon Peg, ε Peg), also named Enif (EE-nif),[12] is the brightest star in the northern constellation 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,[13][14] 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)[15] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[16] included a table of the first two batches of names approved by the WGSN; which included Enif for this star.

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, Epsilon Pegasi itself is known as 危宿三 (Wēi Sù sān, English: the Third Star of Rooftop.)[19]

## Properties

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.[20] 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.[21]

Enif probably only has a few million years left to live, 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 the Sun.[22] 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]

## 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
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. 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:
{\displaystyle {\begin{aligned}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 }\end{aligned}}}
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 (3): 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. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
13. ^ 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
14. ^ Perryman, Michael (2010), "The Making of History's Greatest Star Map", 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
15. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
16. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). 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. ^ 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
21. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
22. ^ Kaler, James B., "ENIF (Epsilon Pegasi)", Stars, University of Illinois, retrieved 2012-02-08