  | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Hydrus |
| Right ascension | 01h 54m 56.131s[1] |
| Declination | −67° 38′ 50.29″[1] |
| Apparent magnitude (V) | +4.68[2] |
| Characteristics | |
| Evolutionary stage | Red clump[3] |
| Spectral type | G8IIIb[4] |
| B−Vcolor index | 0.931±0.036[2] |
| Variable type | Constant[5] |
| Astrometry | |
| Radial velocity (Rv) | −17.75±0.15[6] km/s |
| Proper motion (μ) | RA: +75.927mas/yr[1] Dec.: +73.401mas/yr[1] |
| Parallax (π) | 14.7444±0.0878 mas[1] |
| Distance | 221 ± 1 ly (67.8 ± 0.4 pc) |
| Absolute magnitude (MV) | 0.57±0.07[7] |
| Details[3] | |
| Mass | 2.08±0.04 M☉ |
| Radius | 11.17±0.07 R☉ |
| Luminosity | 65.92±0.86 L☉ |
| Surface gravity (log g) | 2.71±0.27 cgs |
| Temperature | 4,932±20 K |
| Metallicity[Fe/H] | −0.24±0.05 dex |
| Rotational velocity (v sin i) | 2.4±1.0[5] km/s |
| Age | 0.87±0.05 Gyr |
| Other designations | |
| η2 Hyi,CPD−68 101,HD 11977,HIP 8928,HR 570,SAO 248460[8] | |
| Database references | |
| SIMBAD | data |
| Exoplanet Archive | data |
Eta2 Hydri is astar in the southernconstellation ofHydrus. The name isLatinized from η2 Hydri and often catalogued asHD 11977. It is visible to the naked eye with anapparent visual magnitude of +4.68,[2] and is one of the leastvariable stars known. The distance to Eta2 Hydri is approximately 221 light-years based onparallax measurements, but it is drifting closer to theSun with aradial velocity of −18 km/s.[6]
Thestellar classification of Eta2 Hydri is G8IIIb,[4] which indicates this is aevolvedgiant star. It is located in theclump zone of theHR diagram, which means it has already ascended the red giant branch and is now generating energy bycorehelium fusion surrounded by ahydrogen fusing shell. Based on its mass, it was probably aclass A star (similar toVega orFomalhaut) when it was on themain sequence.[5] It is estimated to be around 870 million years old and has expanded to 11 times the Sun's diameter, though is only around two times asmassive as the Sun.[3] The star has a leisurely rotation, with a period of no more than 230–270 days.[5] As of 2005, anextrasolar planet was confirmed to be orbiting the star.
In 2005, the giant planetEta2 Hydri b was found in orbit around Eta2 Hydri by measuring radial velocity variations.[5] This object has at least 6.3[9] times the mass of Jupiter and is orbiting with a period of 1.95 years. It is the first giant planet to be found in orbit around an intermediate-mass giant star.[5]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | >6.29±0.07[9] MJ | 1.89±0.11[9] | 711±8 | 0.40±0.07 | — | — |
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