Beta Hydri

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Beta Hydri
Diagram showing star positions and boundaries of the Hydrus constellation and its surroundings
Cercle rouge 100%.svg

Location of β Hydri (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Hydrus
Right ascension 00h 25m 45.07036s[1]
Declination –77° 15′ 15.2860″[1]
Apparent magnitude (V) 2.80[2]
Spectral type G2 IV[3]
U−B color index +0.11[2]
B−V color index +0.62[2]
Radial velocity (Rv) +22.4[4] km/s
Proper motion (μ) RA: +2,219.54[1] mas/yr
Dec.: +324.09[1] mas/yr
Parallax (π) 134.07 ± 0.11[1] mas
Distance 24.33 ± 0.02 ly
(7.459 ± 0.006 pc)
Absolute magnitude (MV) 3.45 ± 0.01[5]
Mass 1.08 ± 0.03[3] M
Radius 1.809 ± 0.015[3] R
Luminosity 3.494 ± 0.087[3] L
Surface gravity (log g) 4.02 ± 0.04[5] cgs
Temperature 5,872 ± 44[3] K
Metallicity [Fe/H] −0.10 ± 0.07[3] dex
Rotational velocity (v sin i) 6.0[6] km/s
Age 6.40 ± 0.56[3] Gyr
Other designations
CD−77° 15, Gl 19, HD 2151, HIP 2021, HR 98, LHS 6, LTT 226, GCTP 69, SAO 255670, CPD–77° 16, FK5 11, LPM 22, LFT 43.
Database references

Beta Hydri (β Hyi, β Hydri) is a star in the southern circumpolar constellation of Hydrus. (Note that Hydrus is not the same as Hydra.) With an apparent visual magnitude of 2.8,[2] this is the brightest star in the constellation. Based upon parallax measurements the distance to this star is about 24.33 light-years (7.46 parsecs).[1]

This star has about 104% of the mass of the Sun and 181% of the Sun's radius, with more than three times the Sun's luminosity.[3] The spectrum of this star matches a stellar classification of G2 IV, with the luminosity class of 'IV' indicating this is a subgiant star. As such, it is a slightly more evolved star than the Sun, with the supply of hydrogen at its core becoming exhausted. It is one of the oldest stars in the solar neighborhood. This star bears some resemblance to what the Sun may look like in the far distant future, making it an object of interest to astronomers.[3]

A planetary system?[edit]

In 2002 Endl et al. inferred the possible presence of an unseen companion orbiting Beta Hydri as hinted by radial velocity linear trend with a periodicity exceeding 20 years. A substellar object with minimum mass of 4 Jupiter masses and orbital separation of roughly 8 AU could explain the observed trend.[7] If confirmed, it would be a true Jupiter-analogue, though 4 times more massive. So far no planetary/substellar object has been certainly detected.

These results were not confirmed in CES and HARPS measurements published on the arXiv in 2012. Instead the long-term radial velocity variations may be caused by the star's magnetic cycle.[8]

See also[edit]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J 
  3. ^ a b c d e f g h i Brandão, I. M.; et al. (March 2011), "Asteroseismic modelling of the solar-type subgiant star β Hydri", Astronomy & Astrophysics, 527: A37, arXiv:1012.3872Freely accessible, Bibcode:2011A&A...527A..37B, doi:10.1051/0004-6361/201015370 
  4. ^ Nordström, B.; et al. (May 2004), "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14 000 F and G dwarfs", Astronomy and Astrophysics, 418: 989–1019, arXiv:astro-ph/0405198Freely accessible, Bibcode:2004A&A...418..989N, doi:10.1051/0004-6361:20035959 
  5. ^ a b da Silva, L.; et al. (November 2006), "Basic physical parameters of a selected sample of evolved stars", Astronomy and Astrophysics, 458 (2): 609–623, arXiv:astro-ph/0608160Freely accessible, Bibcode:2006A&A...458..609D, doi:10.1051/0004-6361:20065105 
  6. ^ Pizzolato, N.; Maggio, A.; Sciortino, S. (September 2000), "Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases", Astronomy and Astrophysics, 361: 614–628, Bibcode:2000A&A...361..614P 
  7. ^ Endl; et al. (2002). "The planet search program at the ESO Coudé Echelle spectrometer. III. The complete Long Camera survey results". Astronomy and Astrophysics. 392: 671–690. arXiv:astro-ph/0207512Freely accessible. Bibcode:2002A&A...392..671E. doi:10.1051/0004-6361:20020937. 
  8. ^ M. Zechmeister; M. Kürster; M. Endl; G. Lo Curto; H. Hartman; H. Nilsson; T. Henning; A. P. Hatzes; W. D. Cochran (2012). "The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars". arXiv:1211.7263v1Freely accessible [astro-ph.EP]. 

External links[edit]