Epoch J2000 Equinox J2000
|Right ascension||06h 43m 55.92626s|
|Declination||+25° 07′ 52.0515″|
|Apparent magnitude (V)||+3.06|
|Spectral type||G8 Ib|
|U−B color index||+1.46|
|B−V color index||+1.40|
|Radial velocity (Rv)||+8.09 ± 0.14 km/s|
|Proper motion (μ)||RA: –5.57 mas/yr
Dec.: −12.36 mas/yr
|Parallax (π)||3.86 ± 0.17 mas|
|Distance||840 ± 40 ly
(260 ± 10 pc)
|Mass||19.2 ± 0.1 M☉|
|Radius||140 ± 35 R☉|
|Surface gravity (log g)||0.88 ± 0.05 cgs|
|Temperature||4,662 ± 36 K|
|Metallicity [Fe/H]||0.15 ± 0.07 dex|
|Rotational velocity (v sin i)||8.7 ± 1.0 km/s|
|Age||8.3 ± 0.1 Myr|
Epsilon Geminorum (ε Gem, ε Geminorum) is a star in the constellation of Gemini. It has the traditional name Mebsuta (also Melboula or Melucta). Epsilon Geminorum is located on the outstretched right "leg" of the twin Castor. The apparent visual magnitude of +3.06 makes it one of the brighter stars in this constellation.
The distance to this star can be determined directly using parallax measurements, yielding a value of 840 light-years (260 parsecs), although this result has a relatively large margin of error of 40 ly (12 pc). Because Epsilon Geminorum is located near the ecliptic it can be occulted by the Moon or a planet. Just such an occultation took place on April 8, 1976 by Mars, which allowed the oblateness of the planet's outer atmosphere to be measured. Prior to that, the star was occulted by Mercury on June 10, 1940.
The spectrum of this star matches a stellar classification of G8 Ib, where the luminosity class of Ib indicates this is a lower luminosity supergiant star. Alternatively, it may be a star that has passed through the asymptotic giant branch stage and possesses a detached shell of dust. The estimated mass of this star is over 19 times the mass of the Sun, and it has expanded to a radius measured at around 105–175 times that of the Sun. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.
Epsilon Geminorum is radiating around 8,500 times the luminosity of the Sun from its outer atmosphere at an effective temperature of 4,662 K. It is this temperature that gives it the yellow-hued glow of a G-type star. A surface magnetic field with a strength of –0.14 ± 0.19 G has been detected on this star. This topologically complex field is most likely generated by a dynamo formed from the deep convection zone in the star's outer envelope.
The name Mebsuta has its roots in ancient Arabic, where it and the star Mekbuda (Zeta Geminorum) were the paws of a lion. Mebsuta (Mabsūṭah مبسوطة) comes from a phrase referring to the outstretched paw.
In Chinese, 井宿 (Jǐng Su), meaning Well (asterism), refers to an asterism consisting of ε Geminorum, μ Geminorum, ν Geminorum, γ Geminorum, ξ Geminorum, 36 Geminorum, ζ Geminorum and λ Geminorum. Consequently, ε Geminorum itself is known as 井宿五 (Jǐng Su wǔ, English: the Fifth Star of Well.)
- van Leeuwen, Floor (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752v1, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 Note: see VizieR catalogue I/311.
- Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics 352: 495–507, Bibcode:1999A&A...352..495M
- Nordgren, Tyler E. et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal 118 (6): 3032–3038, Bibcode:1999AJ....118.3032N, doi:10.1086/301114
- Johnson, H. L. et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory 4 (99): 99, Bibcode:1966CoLPL...4...99J
- De Medeiros, J. R. et al. (November 2002), "A catalog of rotational and radial velocities for evolved stars. II. Ib supergiant stars", Astronomy and Astrophysics 395: 97–98, Bibcode:2002A&A...395...97D, doi:10.1051/0004-6361:20021214
- Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x
- Wu, Yue et al. (January 2011), "Coudé-feed stellar spectral library - atmospheric parameters", Astronomy and Astrophysics 525: A71, arXiv:1009.1491, Bibcode:2011A&A...525A..71W, doi:10.1051/0004-6361/201015014
- Allen, Richard Hinckley (1899), Star-names and their meanings, G. E. Stechert, p. 235
- French, R. G.; Taylor, G. E. (March 1981), "Occultation of Epsilon Geminorum by Mars. IV - Oblateness of the Martian upper atmosphere", Icarus 45 (3): 577–585, Bibcode:1981Icar...45..577F, doi:10.1016/0019-1035(81)90023-3
- Lobel, A.; Dupree, A. K. (December 2000), "The Chromospheres of G-type Ib Supergiants", Bulletin of the American Astronomical Society 32: 1474, Bibcode:2000AAS...197.4415L
- Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04
- "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
- Grunhut, J. H. et al. (November 2010), "Systematic detection of magnetic fields in massive, late-type supergiants", Monthly Notices of the Royal Astronomical Society 408 (4): 2290–2297, arXiv:1006.5891, Bibcode:2010MNRAS.408.2290G, doi:10.1111/j.1365-2966.2010.17275.x
- (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
- (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.