Eta Pegasi
Binary star in the constellation Pegasus
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Eta Pegasi or η Pegasi, formally named Matar /ˈmeɪtɑːr/,[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 214 light-years (66 parsecs) from the Sun.[1]
 | |
| Observation data Epoch J2000.0 Equinox ICRS | |
|---|---|
| Constellation | Pegasus |
| Right ascension | 22h 43m 00.13743s[1] |
| Declination | +30° 13′ 16.4822″[1] |
| Apparent magnitude (V) | +2.95[2] |
| Characteristics | |
| Spectral type | G2 II + F0 V[3] |
| U−B color index | +0.57[2] |
| B−V color index | +0.86[2] |
| Astrometry | |
| Radial velocity (Rv) | +4.3[4] km/s |
| Proper motion (μ) | RA: +13.16[1] mas/yr Dec.: -25.67[1] mas/yr |
| Parallax (π) | 15.22±0.71 mas[1] |
| Distance | 214 ± 10 ly (66 ± 3 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 |
| Details | |
| η Peg A | |
| Mass | 3.51±0.13[7] M☉ |
| Radius | 24.51+1.11 −1.21[7] R☉ |
| Luminosity | 330.8±35.0[7] L☉ |
| Surface gravity (log g) | 2.40[8] cgs |
| Temperature | 4970±65[7] K |
| Metallicity [Fe/H] | +0.39[8] dex |
| Rotation | 818[5] |
| Rotational velocity (v sin i) | 1.4[6] km/s |
| Age | 0.27±0.03[7] Gyr |
| Other designations | |
| Matar, Eta Peg, η Peg, 44 Peg, BD+29°4741, FK5 857, HD 215182, HIP 112158, HR 8650, SAO 90734[9] | |
| Database references | |
| SIMBAD | data |
Nomenclature
η Pegasi (Latinised to Eta Pegasi) is the star's Bayer designation.
It bore the traditional name Matar, derived from the Arabic سعد المطر 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 η Pegasi A 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]
Namesake
USS Matar (AK-119) was[when?] a United States Navy Crater class cargo ship named after the star.[citation needed]
Properties
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] The secondary star is 3.56 magnitudes fainter that the primary star at 700 nm.[16] There are also 2 class G stars further away that may or may not be physically related to the main pair.[citation needed]