Epsilon Geminorum

Epsilon Geminorum or ε Geminorum, formally named Mebsuta, is a star in the constellation of Gemini, 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 is determined at 860 ly.

Nomenclature
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 an ancient Arabian view where it and the star Mekbuda (Zeta Geminorum) were the paws of a huge lion extending over many present-day constellations. Mebsuta ('Mabsūṭah' مبسوطة) comes from a phrase referring to the outstretched paw. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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. Consequently, ε Geminorum itself is known as 井宿五 (Jǐng Su wǔ, the Fifth Star of Well.)

Properties
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 5.3 times the mass of the Sun, and, at an age of 100 million years, it has expanded to a 130 times the Sun's size. 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 9,640 times the luminosity of the Sun from its outer atmosphere at an effective temperature of $8.09$. It is this temperature that gives it the yellow-hued glow of a G-type star. A surface magnetic field with a strength of $861$ 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.

It is located around 860 light-years from Earth based on a photogeometric distance from Bailer-Jones et al (2021). At this distance, the apparent magnitude is diminished by 0.92 magnitudes due to interstellar extinction between Earth and the star.

Occultations
Epsilon Geminorum lies near the ecliptic, so it can be occulted by the Moon or a planet. Such an occultation took place on April 8, 1976 by Mars, which allowed the oblateness of the planet's outer atmosphere to be measured. Epsilon Geminorum was occulted by Mercury on June 10, 1940, and on September 3, 2015 it was occulted by the asteroid 112 Iphigenia.

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