Eta Hydrae

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.

A light curve for Eta Hydrae, plotted from TESS data^[12]

η Hydrae

Location of η Hydrae (circled)
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, ζ Hya, ρ Hya and σ Hya (Minchir).^[18]