# Eta Pegasi

Observation data Characteristics Epoch J2000.0      Equinox J2000.0 Location of η Pegasi (circled) near the center 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 Distance 167 ± 2 ly (51.3 ± 0.5 pc) Absolute magnitude (MV) –1.18[5] Orbit[6] Period (P) 813 days Eccentricity (e) 0.183 Periastron epoch (T) 2452025 HJD Argument of periastron (ω) (secondary) 344.7° Semi-amplitude (K1) (primary) 14.37 km/s Mass 3.82 ± 0.52[7] M☉ Radius 18[8] R☉ Luminosity 247[7] L☉ Surface gravity (log g) 2.40[9] cgs Temperature 5,450[7] K Metallicity [Fe/H] +0.39[9] dex Rotation 818[5] Rotational velocity (v sin i) 1.4[6] km/s Matar, 44 Peg, FK5 857, HD 215182, HIP 112158, HR 8650, SAO 90734.[10]

Eta Pegasi (η Peg) is a binary star in the constellation Pegasus. It has the traditional name Matar, which comes from the Arabic Al Saʽd al Maṭar (سعد المطر) meaning lucky star of rain.[11] The apparent visual magnitude of this star is +2.95,[2] making this the fifth brightest member of Pegasus. Based upon parallax measurements, the distance to this star is about 167 light-years (51 parsecs).[1]

This 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 four times the mass of the Sun.[5] The interferometry-measured angular diameter of this star, after correcting for limb darkening, is 3.26 ± 0.07 mas,[12] which, at its estimated distance, equates to a physical radius of nearly 18 times the radius of the Sun.[8] It is radiating 247[7] times the luminosity of the Sun from its expanded outer envelope at an effective temperature of 5,450 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.

## Namesakes

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

## 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 c d Johnson, H. L. et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (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:1953QB901.W495.....
5. ^ a b c d 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. Hohle, M. M.; Neuhäuser, R.; Schutz, B. F. (April 2010), "Masses and luminosities of O- and B-type stars and red supergiants", Astronomische Nachrichten 331 (4): 349, arXiv:1003.2335, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355
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{(51.3\cdot 3.26\cdot 10^{-3})\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 36\cdot R_{\bigodot} \end{align}
9. ^ 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
10. ^ "MATAR -- Star in double system", SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), retrieved 2012-03-03
11. ^ 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
12. ^ Richichi; 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