Eta Pegasi

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Eta Pegasi
Diagram showing star positions and boundaries of the Pegasus constellation and its surroundings
Cercle rouge 100%.svg
Location of η Pegasi (circled) near the center
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Pegasus
Right ascension  22h 43m 00.13743s[1]
Declination +30° 13′ 16.4822″[1]
Apparent magnitude (V) +2.95[2]
Spectral type G2 II + F0 V[3]
U−B color index +0.57[2]
B−V color index +0.86[2]
Radial velocity (Rv)+4.3[4] km/s
Proper motion (μ) RA: –100.06[1] mas/yr
Dec.: +15.46[1] mas/yr
Parallax (π)19.51 ± 0.18[1] mas
Distance167 ± 2 ly
(51.3 ± 0.5 pc)
Absolute magnitude (MV)–1.18[5]
Period (P)813 days
Eccentricity (e)0.183
Periastron epoch (T)2452025 HJD
Argument of periastron (ω)
Semi-amplitude (K1)
14.37 km/s
η Peg A
Mass3.51±0.13[7] M
[7] R
Luminosity330.8±35.0[7] L
Surface gravity (log g)2.40[8] cgs
Temperature4970±65[7] K
Metallicity [Fe/H]+0.39[8] dex
Rotational velocity (v sin i)1.4[6] km/s
Age0.27±0.03[7] Gyr
Other designations
Matar, 44 Peg, FK5 857, HD 215182, HIP 112158, HR 8650, SAO 90734, BD+29° 4741[9]
Database references

Eta Pegasi (η Pegasi, abbreviated Eta Peg, η Peg), formally named Matar /ˈmtɑːr/,[10][11] is a binary star in the constellation of Pegasus. The apparent visual magnitude of this star is +2.95,[2] making it the fifth-brightest member of Pegasus. Based upon parallax measurements, the distance to this star is about 167 light-years (51 parsecs) from the Sun.[1]


η Pegasi (Latinised to Eta Pegasi) is the star's Bayer designation.

It bore the traditional name Matar, derived from the Arabic سعد المطر Al Saʽd al Maṭar, meaning lucky star of rain.[12] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalogue and standardize proper names for stars; the WGSN approved the name Matar for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[11]

In Chinese, 離宮 (Lì Gōng), meaning Resting Palace, refers to an asterism consisting η Pegasi, λ Pegasi, μ Pegasi, ο Pegasi, τ Pegasi and ν Pegasi.[14] Consequently, η Pegasi itself is known as 離宮四 (Lì Gōng sì), "the Fourth Star of Resting Palace".[15]


USS Matar (AK-119) was a United States Navy Crater class cargo ship named after the star.


The Eta Pegasi system consists of a pair of stars in a binary orbit with a period of 813 days and an eccentricity of 0.183.[6] The primary component is a bright giant star with a stellar classification of G2 II[3] and about three and a half times the mass of the Sun;[7] the interferometry-measured angular diameter of this star, after correcting for limb darkening, is 3.471 ± 0.027 mas,[7] which, at its estimated distance, equates to a physical radius of more than 24 times the radius of the Sun.[7] It is radiating 331[7] times the luminosity of the Sun from its expanded outer envelope at an effective temperature of 4,970 K;[7] the rotation rate of the star slowed as it expanded, so it has a projected rotational velocity of 1.7 km s−1 with an estimated rotation period of 818 days.[5]

The secondary component is an F-type main sequence star with a classification of F0 V.[3] There are also 2 class G stars further away that may or may not be physically related to the main pair.


  1. ^ a b c d e f 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 c d Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99): 99. Bibcode:1966CoLPL...4...99J.
  3. ^ a b c Parsons, Sidney B.; Ake, Thomas B. (November 1998), "Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V; the Entire IUE Sample", The Astrophysical Journal Supplement Series, 119 (1): 83–104, Bibcode:1998ApJS..119...83P, doi:10.1086/313152
  4. ^ Wilson, Ralph Elmer (1953). General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. Bibcode:1953GCRV..C......0W.
  5. ^ a b c Pizzolato, N.; Maggio, A.; Sciortino, S. (September 2000), "Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases", Astronomy and Astrophysics, 361: 614–628, Bibcode:2000A&A...361..614P
  6. ^ a b c Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209
  7. ^ a b c d e f g h i j Baines, Ellyn K.; et al. (2018). "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer". The Astronomical Journal. 155. 30. arXiv:1712.08109. Bibcode:2018AJ....155...30B. doi:10.3847/1538-3881/aa9d8b.
  8. ^ a b Luck, R. Earle; Wepfer, Gordon G. (November 1995), "Chemical Abundances for F and G Luminosity Class II Stars", Astronomical Journal, 110: 2425, Bibcode:1995AJ....110.2425L, doi:10.1086/117702
  9. ^ "eta Peg". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-03-03.
  10. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  11. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. ^ Davis, George R., Jr. (1944), "The pronunciations, derivations, and meanings of a selected list of star names", Popular Astronomy, 52: 8, Bibcode:1944PA.....52....8D
  13. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  14. ^ ‹See Tfd›(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.
  15. ^ ‹See Tfd›(in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.