Rho Hydrae

Not to be confused with R Hydrae.

Rho Hydrae, equally written ρ Hydrae, is a binary star^[9] in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.34.^[2] The distance to this system, based upon an annual parallax shift of 9.21 mas,^[1] is about 354 light years. At that distance, the visual magnitude is diminished by an interstellar extinction of 0.06 magnitudes, due to intervening dust.^[7]

Rho Hydrae

Location of ρ Hydrae (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	08h 48m 25.97057s[1]
Declination	+05° 50′ 16.1283″[1]
Apparent magnitude (V)	4.34[2]
Characteristics
Spectral type	A0 Vn[3]
U−B color index	−0.04[2]
B−V color index	−0.04[2]
Astrometry
Radial velocity (Rv)	+32.8[4] km/s
Proper motion (μ)	RA: −17.33[1] mas/yr Dec.: −29.41[1] mas/yr
Parallax (π)	9.21±0.21 mas[1]
Distance	354 ± 8 ly (109 ± 2 pc)
Absolute magnitude (MV)	−0.83[5]
Details[3]
Mass	3.24±0.05 M☉
Radius	2.0[6] R☉
Luminosity	242 L☉
Temperature	9,795 K
Rotational velocity (v sin i)	128 km/s
Age	350[7] Myr
Other designations
ρ Hya, 13 Hya, BD+06°2040, HD 75137, HIP 43234, HR 3492, SAO 117146[8]
Database references
SIMBAD	data

The primary component is an A-type main sequence star with a stellar classification of A0 Vn.^[3] It has around double^[6] the radius of the Sun and 3.2 times the Sun's mass. Rho Hydrae is around 350 million years old^[7] and has a high rate of spin, with a projected rotational velocity of 128 km/s. It radiates 242 times the solar luminosity from its outer atmosphere at an effective temperature of 9,795 K.^[3] The companion is a magnitude 11.9 star at an angular separation of 12.1 arc seconds along a position angle of 146°, as of 2000.^[10]

Name and etymology

This system appears among bright stars in a compact pentagon, resembling a quadrilateral due to the suggestive proximity (close arc distance) to Epsilon Hydrae (ε Hya). This shape in the Greco-Roman tradition, which draws on trading and navigation histories shared with nearby older-recorded astrologies is an asterism that represents the head of the water snake.^[11]

This light source, along with comparable strength (apparent magnitude) Epsilon, δ Hya (Lisan al Sudja), ζ Hya, η Hya, and σ Hya (Minchir), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".^[12] 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).^[13]

In Chinese, 柳宿 (Liǔ Sù), meaning Willow, refers to an asterism consisting of ρ Hydrae, δ Hydra, σ Hydrae, η Hydrae, ε Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae^[14] Consequently, ρ Hydrae itself is known as 柳宿四 (Liǔ Sù sì, English: the Fourth Star of Willow).^[15]

The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, δ Hya (Lisan al Sudja), ε Hya, η Hya, ζ Hya and σ Hya (Minchir).^[16]