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]
Characteristics
Spectral type G8 Ib[3]
U−B color index +1.46[4]
B−V color index +1.40[4]
Astrometry
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]
Details
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 Gem, ε Geminorum) is a star in the constellation of Gemini. It has the traditional name Mebsuta (also Melboula or Melucta).[9] 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.[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]

Properties

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.[12] 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.[13]

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.[14] 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.[15]

In culture

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.[9]

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.[16] Consequently, ε Geminorum itself is known as 井宿五 (Jǐng Su wǔ, English: the Fifth Star of Well.)[17]

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

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Lua error in package.lua at line 80: module 'strict' not found. Note: see VizieR catalogue I/311.
  2. ↑ 2.0 2.1 2.2 2.3 Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 3.2 3.3 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ 4.0 4.1 Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ 5.0 5.1 Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ 7.0 7.1 7.2 Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ 8.0 8.1 8.2 8.3 Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ 9.0 9.1 Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ (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.
  17. ↑ (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.
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