Epsilon Geminorum

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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)
Absolute magnitude (MV) –3.90[6]
Mass 19.2 ± 0.1[7] M
Radius 140 ± 35[3] R
Luminosity 8,500[2] L
Surface gravity (log g) 0.88 ± 0.05[8] cgs
Temperature 4,662 ± 36[8] K
Metallicity [Fe/H] 0.15 ± 0.07[8] dex
Rotational velocity (v sin i) 8.7 ± 1.0[5] km/s
Age 8.3 ± 0.1[7] Myr
Other designations
27 Geminorum, FK5 254, HD 48329, HIP 32246, HR 2473, SAO 78682.

Epsilon Geminorum (ε Geminorum, abbreviated Epsilon Gem, ε Gem), also named Mebsuta,[9] is a star in the constellation of Gemini and 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.[10] 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.[11]


ε Geminorum (Latinised to Epsilon Geminorum) is the star's Bayer designation. It bore the traditional names Mebsuta, Melboula or Melucta. 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.[12] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[14] included a table of the first two batches of names approved by the WGSN; which included Mebsuta for this star.

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]


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.[17] The estimated mass of this star is over 19[7] 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.[18]

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.[8] It is this temperature that gives it the yellow-hued glow of a G-type star.[19] 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.[20]

In culture[edit]

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:0708.1752v1Freely accessible, 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. ^ Park, Sunkyung; Kang, Wonseok; Lee, Jeong-Eun; Lee, Sang-Gak (2013). "Wilson-Bappu Effect: Extended to Surface Gravity". The Astronomical Journal. 146 (4): 73. arXiv:1307.0592Freely accessible. Bibcode:2013AJ....146...73P. doi:10.1088/0004-6256/146/4/73. 
  7. ^ 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:1007.4883Freely accessible, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x 
  8. ^ a b c d Wu, Yue; et al. (January 2011), "Coudé-feed stellar spectral library - atmospheric parameters", Astronomy and Astrophysics, 525: A71, arXiv:1009.1491Freely accessible, Bibcode:2011A&A...525A..71W, doi:10.1051/0004-6361/201015014 
  9. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  10. ^ 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 
  11. ^ Steve's Asteroid Occultations 
  12. ^ Allen, Richard Hinckley (1899), Star-names and their meanings, G. E. Stechert, p. 235 
  13. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  14. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016. 
  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 Archived January 30, 2011, at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.
  17. ^ 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 
  18. ^ 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 
  19. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16 
  20. ^ 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:1006.5891Freely accessible, Bibcode:2010MNRAS.408.2290G, doi:10.1111/j.1365-2966.2010.17275.x