MEBSUTA (Epsilon Geminorum). The name, strikingly similar to and easily confused with Mekbuda (the Zeta star of Gemini), "Mebsuta," also in Gemini, has a similar origin. In ancient Arabic lore the two stars were the paws of a lion (related to, but not exactly, Leo). Mekbuda comes from a phrase meaning the lion's folded paw, while Mebsuta refers to the outstretched paw. The names originally referred to other stars, and were applied to the two in Gemini in more modern times. Mid-third magnitude (2.98), as fifth brightest star, Mebsuta received Gemini's Epsilon designation, but not by much logic, as it is exceeded slightly by Tejat (Mu Geminorum) and rather handily beats out Wasat (Delta). Mebsuta is one of the sky's rarer stars, a cool class G supergiant. In the early days of stellar studies, astronomers thought that stars were born hot and then cooled and dimmed as they died. In fact (at least for a time), they do just the opposite, brightening and swelling (though indeed cooling at their surfaces) when internal hydrogen fusion, which powers ordinary stars, ceases. Hot stars were therefore erroneously called "early," cool stars "late." Mebsuta is a prime example of a "late G" (G8) supergiant. And it lives up to its class. From its rather large distance of 900 light years, the star is found to shine with a luminosity 7600 times that of the Sun from a surface with a temperature of 4360 Kelvin, notably cooler than solar. These numbers give a diameter 150 times that of the Sun, or about the size of the orbit of Venus. Mebsuta is one of the few supergiants that lie along the path of the Moon. By timing how long it takes the moving Moon to cover, or occult, a star, we can measure the star's angular diameter. Such measurement, combined with another method that involves interference of light waves from the star, gives exactly the same diameter, showing the star's various parameters are correct. Mebsuta's luminosity and temperature suggest that its mass lies between 7 and 9 times that of the Sun and that it is now in a rather advanced state of aging, most likely fusing helium into carbon in its deep core. Stars like the Sun will become white dwarfs (like Sirius B) of about 6/10 of a solar mass. Mebsuta will push the envelope, and when it dies will weigh in just short of the limit at white dwarfs cannot exist, more massive stars exploding as supernovae. Mebsuta has a ninth magnitude but distant "companion" about 2 minutes of arc away, long thought to be just a line-of- sight coincidence. Its nature (an evolved class K star) suggests a distance exactly equal to that of Mebsuta. However, given Mebsuta's age of at most 50 million years, no true lower mass companion, one born at the same time, could possibly have evolved. The "companion" is probably just passing through Mebsuta's neighborhood, enjoying the show put on by its brilliant, but temporary, acquaintance.
Written by Jim Kaler 3/31/00. Return to STARS.