Eta2 Hydri

For other stars with this Bayer designation, see η Hydri.
Eta2 Hydri
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 Horizontal branch
Spectral type G8IIIb[3]
B−V color index 0.931±0.036[2]
Variable type Constant[4]
Astrometry
Radial velocity (Rv)−17.75±0.15[5] 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]
Distance221 ± 1 ly
(67.8 ± 0.4 pc)
Absolute magnitude (MV)0.57±0.07[6]
Details[4]
Mass1.91±0.21 M☉
Radius10.09±0.32 R☉
Luminosity68.96±0.86[5] L☉
Surface gravity (log g)2.66±0.10[6] cgs
Temperature4,970±70 K
Metallicity [Fe/H]−0.21±0.10 dex
Rotational velocity (v sin i)2.4±1.0 km/s
Age1.30±0.48[6] Gyr
Other designations
η2 Hyi, CPD−68 101, HD 11977, HIP 8928, HR 570, SAO 248460[7]
Database references
SIMBADdata

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.[5]

The stellar classification of Eta2 Hydri is G8IIIb,[3] 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.[4] It is estimated to be around 1.3[6] billion years old and has expanded to 10 times the Sun's diameter, though is only around 1.9 times as massive as the Sun. The star has a leisurely rotation, with a period of no more than 230–270 days.[4] As of 2005, an extrasolar planet was confirmed to be orbiting the star.

  1. ^ a b c d Cite error: The named reference GaiaDR3 was invoked but never defined (see the help page).
  2. ^ a b c Cite error: The named reference Anderson_Francis_2012 was invoked but never defined (see the help page).
  3. ^ a b Cite error: The named reference Keenan_McNeil_1989 was invoked but never defined (see the help page).
  4. ^ a b c d Cite error: The named reference Setiawan was invoked but never defined (see the help page).
  5. ^ a b c Cite error: The named reference GaiaDR2 was invoked but never defined (see the help page).
  6. ^ a b c d Cite error: The named reference aaa458_2_609 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference SIMBAD was invoked but never defined (see the help page).