Tau Geminorum

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Tau Geminorum
Diagram showing star positions and boundaries of the Gemini constellation and its surroundings
Cercle rouge 100%.svg
Location of τ Geminorum (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Gemini
Right ascension  07h 11m 08.37042s[1]
Declination +30° 14′ 42.5831″[1]
Apparent magnitude (V) +4.42[2]
Spectral type K2 III
U−B color index +1.41[2]
B−V color index +1.26[2]
Radial velocity (Rv)+21.83[3] km/s
Proper motion (μ) RA: -31.21[1] mas/yr
Dec.: -48.92[1] mas/yr
Parallax (π)10.16 ± 0.25[1] mas
Distance321 ± 8 ly
(98 ± 2 pc)
Absolute magnitude (MV)−0.55[4]
Mass1.98[5] M
Radius27[6] R
Luminosity224[6] L
Surface gravity (log g)2.22[5] cgs
Temperature4,528[5] K
Metallicity [Fe/H]+0.14[5] dex
Rotational velocity (v sin i)5.8[6] km/s
Other designations
τ Gem, 46 Gem, BD+30 1439, HD 54719, HIP 34693, HR 2697, SAO 59858
Database references

Tau Geminorum, Latinized from τ Geminorum, is a star in the northern zodiac constellation of Gemini. It has the apparent visual magnitude of +4.42,[2] making it visible to the naked eye under suitably good seeing conditions. This star is close enough to the Earth that its distance can be measured using the parallax technique, which yields a value of roughly 321 light-years (98 parsecs).[1]

Artist's illustration of the giant star Tau Geminorum (left) and its brown dwarf companion—the dark disk at right.[dubious ]

It is an evolved giant star of the spectral type K2 III. It has double[5] the mass of the Sun and has expanded to 27[6] times the Sun's radius. Tau Geminorum is radiating 224[6] as much radiation as the Sun from its expanded outer atmosphere at an effective temperature of 4,528 K,[5] giving it the characteristic orange-hued glow of a K-type star. It appears to be rotating slowly with a projected rotational velocity of 5.8 km s−1.[6]

This star has a brown dwarf companion Tau Geminorum b, whose mass is 18.1 Jupiter masses, and which was discovered in 2004 by Mitchell et al., who also discovered Nu Ophiuchi b at the same time. This brown dwarf takes 305 days or 0.84 years or 26.4 megaseconds to revolve around Tau Gem.[7]

The Tau Geminorum planetary system[7]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b ≥18.1 MJ ≥0.88 305 ?


  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:0708.1752, 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. ^ Famaey, B.; et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics, 430: 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272
  4. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  5. ^ a b c d e f Luck, R. Earle; Heiter, Ulrike (June 2007), "Giants in the Local Region", The Astronomical Journal, 133 (6): 2464–2486, Bibcode:2007AJ....133.2464L, doi:10.1086/513194
  6. ^ a b c d e f Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209
  7. ^ a b Mitchell, D. S.; et al. (2004), "Four Substellar Companions Found Around K Giant Stars", Bulletin of the American Astronomical Society, 35: 1234, Bibcode:2003AAS...203.1703M