Epsilon Geminorum

From Wikipedia, the free encyclopedia
Jump to: navigation, search
ε Geminorum
Diagram showing star positions and boundaries of the Gemini constellation and its surroundings
Cercle rouge 100%.svg

Location of ε Geminorum (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Right ascension 06h 43m 55.92626s[1]
Declination +25° 07′ 52.0515″[1]
Apparent magnitude (V) +3.06[2]
Spectral type G8 Ib[3]
U−B color index +1.46[4]
B−V color index +1.40[4]
Radial velocity (Rv) +8.09 ± 0.14[5] km/s
Proper motion (μ) RA: –5.57[1] mas/yr
Dec.: −12.36[1] mas/yr
Parallax (π) 3.86 ± 0.17[1] mas
Distance 840 ± 40 ly
(260 ± 10 pc)
Mass 19.2 ± 0.1[6] M
Radius 140 ± 35[3] R
Luminosity 8,500[2] L
Surface gravity (log g) 0.88 ± 0.05[7] cgs
Temperature 4,662 ± 36[7] K
Metallicity [Fe/H] 0.15 ± 0.07[7] dex
Rotational velocity (v sin i) 8.7 ± 1.0[5] km/s
Age 8.3 ± 0.1[6] Myr
Other designations
27 Geminorum, FK5 254, HD 48329, HIP 32246, HR 2473, SAO 78682.

Epsilon Geminorum (ε Gem, ε Geminorum) is a star in the constellation of Gemini. It has the traditional name Mebsuta (also Melboula or Melucta).[8] Epsilon Geminorum is located on the outstretched right "leg" of the twin Castor. The apparent visual magnitude of +3.06[2] 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).[1] 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.[9] Prior to that, the star was occulted by Mercury on June 10, 1940. On September 3, 2015, Epsilon Geminorum is expected to be occulted by the asteroid Iphigenia.[10]


The spectrum of this star matches a stellar classification of G8 Ib,[3] 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.[11] The estimated mass of this star is over 19[6] times the mass of the Sun, and it has expanded to a radius measured at around 105–175 times that of the Sun.[3] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[12]

Epsilon Geminorum is radiating around 8,500[2] times the luminosity of the Sun from its outer atmosphere at an effective temperature of 4,662 K.[7] It is this temperature that gives it the yellow-hued glow of a G-type star.[13] 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.[14]

In culture[edit]

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.[8]

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.[15] Consequently, ε Geminorum itself is known as 井宿五 (Jǐng Su wǔ, English: the Fifth Star of Well.)[16]

USS Melucta (AK-131) was a United States Navy Crater class cargo ship named after the star.


  1. ^ a b c d e f 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.
  2. ^ a b c d Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics 352: 495–507, Bibcode:1999A&A...352..495M 
  3. ^ a b c d 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 
  4. ^ a b 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 
  5. ^ a b 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 
  6. ^ a b c 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 
  7. ^ a b c d 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 
  8. ^ a b Allen, Richard Hinckley (1899), Star-names and their meanings, G. E. Stechert, p. 235 
  9. ^ 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 
  10. ^ Steve's Asteroid Occultations 
  11. ^ 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 
  12. ^ 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 
  13. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  14. ^ 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 
  15. ^ (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.
  16. ^ (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.