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Eta2 Hydri is a star in the southern constellation of Hydrus. The name is Latinized from η2 Hydri and often catalogued as HD 11977. It is visible to the naked eye with an apparent visual magnitude of +4.68,[2] and is one of the least variable stars known. The distance to Eta2 Hydri is approximately 221 light-years based on parallax measurements, but it is drifting closer to the Sun with a radial velocity of −18 km/s.[6]
| 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−V color 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: +75.927 mas/yr[1] Dec.: +73.401 mas/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 | |
| Database references | |
| SIMBAD | data |
The stellar classification of Eta2 Hydri is G8IIIb,[4] which indicates this is a evolved giant star. It is located in the clump zone of the HR diagram, which means it has already ascended the red giant branch and is now generating energy by core helium fusion surrounded by a hydrogen fusing shell. Based on its mass, it was probably a class A star (similar to Vega or Fomalhaut) when it was on the main 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 as massive as the Sun.[3] The star has a leisurely rotation, with a period of no more than 230–270 days.[5] As of 2005, an extrasolar planet was confirmed to be orbiting the star.
In 2005, the giant planet Eta2 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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