Eta Hydrae
Star in the constellation Hydra
From Wikipedia, the free encyclopedia
Eta Hydrae (η Hydrae) is a star in the equatorial constellation of Hydra. With an apparent visual magnitude of 4.3,[2] it is visible to the naked eye. However, it is the faintest of the five stars that form the "head" of the hydra.[11] Based upon an annual parallax shift of 5.56 mas,[1] it is located roughly 590 light years from the Sun.
  | |
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Hydra |
| Right ascension | 08h 43m 13.47499s[1] |
| Declination | +03° 23′ 55.1867″[1] |
| Apparent magnitude (V) | 4.294[2] |
| Characteristics | |
| Spectral type | B3 V[3] |
| U−B color index | −0.726[2] |
| B−V color index | −0.187[2] |
| Variable type | Candidate β Cep[4] |
| Astrometry | |
| Proper motion (μ) | RA: −19.39[1] mas/yr Dec.: −1.08[1] mas/yr |
| Parallax (π) | 5.56±0.24 mas[1] |
| Distance | 590 ± 30 ly (180 ± 8 pc) |
| Absolute magnitude (MV) | −1.48[5] |
| Details | |
| Mass | 7.0±0.1[6] M☉ |
| Radius | 3.9[7] R☉ |
| Luminosity (bolometric) | 2,680[8] L☉ |
| Surface gravity (log g) | 3.933[9] cgs |
| Temperature | 18,630±411[9] K |
| Rotational velocity (v sin i) | 101±5[9] km/s |
| Age | 31.6±3.9[6] Myr |
| Other designations | |
| η Hya, 7 Hydrae, BD+03°2039, FK5 2687, HD 74280, HIP 42799, HR 3454, SAO 117050[10] | |
| Database references | |
| SIMBAD | data |
This is a B-type main sequence star with a stellar classification of B3V.[3] It has been classified as a candidate Beta Cephei variable with a period of 2.2 days, although this designation was rejected by Stankov and Handler (2005).[4] The spectrum shows a slight underabundance of carbon, compared to the Sun.[13] The star is around 32[6] million years old and is spinning with a projected rotational velocity of 101[9] km/s. It has an estimated seven[6] times the mass of the Sun and nearly four[7] times the Sun's radius. Eta Hydrae radiates 2,680[8] times the solar luminosity from its outer atmosphere at an effective temperature of 18,630[9] K.
Name and etymology
This star, along with δ Hya (Lisan al Sudja), ε Hya, ζ Hya, ρ Hya and σ Hya (Minchir), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".[14] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Min al Azʽal or Minazal were the title for five stars:δ Hya as Minazal I, η Hya as Minazal II, ε Hya as Minazal III, ρ Hya as Minazal IV and ζ Hya as Minazal V (exclude σ Hya).[15]
In Chinese, 柳宿 (Liǔ Sù), meaning Willow, refers to an asterism consisting of η Hydrae, δ Hydra, σ Hydrae, ρ Hydrae, ε Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae[16] Consequently, η Hydrae itself is known as 柳宿三 (Liǔ Sù sān, English: the Third Star of Willow).[17]
The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, δ Hya (Lisan al Sudja), ε Hya (Ashlesha), ζ Hya, ρ Hya and σ Hya (Minchir).[18]