Delta Geminorum
Star in the constellation Gemini
From Wikipedia, the free encyclopedia
Delta Geminorum (δ Geminorum, abbreviated Delta Gem, δ Gem), formally named Wasat /ˈweɪsət/,[15][16] is a triple star system in the constellation of Gemini.
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Gemini |
| Right ascension | 07h 20m 07.37978s[1] |
| Declination | +21° 58′ 56.3377″[1] |
| Apparent magnitude (V) | +3.53[2] |
| Characteristics | |
| Evolutionary stage | main sequence[3] |
| Spectral type | F2VkF0mF0[4] |
| U−B color index | +0.04[2] |
| B−V color index | +0.34[2] |
| Astrometry | |
| Radial velocity (Rv) | +4.1[5] km/s |
| Proper motion (μ) | RA: –15.13[1] mas/yr Dec.: –9.79[1] mas/yr |
| Parallax (π) | 53.94±0.66 mas[1] |
| Distance | 60.5 ± 0.7 ly (18.5 ± 0.2 pc) |
| Absolute magnitude (MV) | 1.95[6] |
| Orbit[7] | |
| Primary | Aa |
| Name | Ab |
| Period (P) | 2,238.7 days |
| Eccentricity (e) | 0.3530 |
| Orbit[8] | |
| Primary | A |
| Name | B |
| Period (P) | ~550 yr |
| Semi-major axis (a) | 13.8±1.6″ |
| Eccentricity (e) | 0.98±0.30 |
| Inclination (i) | 75±10° |
| Longitude of the node (Ω) | 128±51° |
| Periastron epoch (T) | 2136±57 |
| Argument of periastron (ω) (secondary) | 266±32° |
| Details | |
| Mass | 1.57[9] M☉ |
| Radius | 2.06±0.25[10] R☉ |
| Luminosity | 9.88[11] L☉ |
| Temperature | 6,900[12] K |
| Metallicity [Fe/H] | –0.26[12] dex |
| Rotational velocity (v sin i) | 129.7[13] km/s |
| Age | 1.6[12] Gyr |
| Other designations | |
| Wasat, 55 Geminorum, BD+22°1645, FK5 279, Gl 271, HD 56986, HIP 35550, HR 2777, SAO 79294[14] | |
| Database references | |
| SIMBAD | data |
Nomenclature
δ Geminorum (Latinised to Delta Geminorum) is the system's Bayer designation.
It bore the traditional name Wasat, which derives from the Arabic word for "middle".[17][18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalogue and standardize proper names for stars. The WGSN approved the name Wasat for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[16]
In Chinese, 天樽 (Tiān Zūn), meaning Celestial Wine Cup, refers to an asterism consisting of Delta Geminorum, 57 Geminorum and Omega Geminorum.[20] Consequently, Delta Geminorum itself is known as 天樽二 (Tiān Zūn èr, English: the Second Star of Celestial Wine Cup.).[21] From this Chinese name, the name Ta Tsun has appeared.[22]
Properties
Delta Geminorum is a main-sequence star with the stellar classification F2VkF0mF0.[4] It is about 60.5 light-years (18.5 parsecs) distant.[1] This star has 1.57 times the mass of the Sun[9] and is rotating rapidly with a projected rotational velocity of 129.7 km s−1.[13] The estimated age is 1.6 billion years.[12]
It has an apparent visual magnitude of +3.53,[2] allowing it to be seen with the naked eye. It is 0.18 degree south of the ecliptic so it is occasionally occulted by the Sun, Moon and, rarely, by a planet; and is eclipsed by the sun from about 10-12 July.[24] Thus the star can be viewed the whole night, crossing the sky, in mid-January. The last occultation by a planet was by Saturn on June 30, 1857, and the next will be by Venus on August 12, 2420.[citation needed] In 1930, the dwarf planet Pluto was discovered about 0.5° to the east of this star by American astronomer Clyde Tombaugh.[25]
Delta Geminorum is a triple star system. The inner components form a spectroscopic binary with a period of 6.1 years (2,238.7 days) and an orbital eccentricity of 0.3530.[7] A cooler class K companion is not apparent to the naked eye but clearly visible in a small telescope. It orbits the inner pair with a period in the order of a hundred years, with a high but uncertain eccentricity of 0.98±0.30.[8] Although according to Evans et al. (1966) its radial velocity is away from the Sun,[5] it is actually approaching the Solar System. In about 1.1 million years, it will make its closest approach at a separation of roughly 6.7 ly (2.1 pc).[26]