Rho Hydrae
Binary star in the constellation Hydra
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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][3]
 | |
| 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 | |
| Evolutionary stage | subgiant[3] |
| 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 | 5.04[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 has an stellar classification of A0 Vn, suggesting that it is an A-type main sequence star,[3] but stellar evolution models indicate it has left the main sequence and is thus a subgiant star.[3] It has 5.04 the radius of the Sun[6] 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 (Ashlesha), η Hya, ζ Hya and σ Hya (Minchir).[16]