ALPHA MUS (Alpha Muscae). Musca, once the "Southern Fly" and, with the official departure of Musca Borealis (the Northern Fly), now just "the Fly," closely circles the South Celestial Pole. Much too far south to have an ancient proper name, the constellation's third magnitude (but almost second, 2.69) luminary is simply Alpha Muscae. This magnificent hot class B (B2) subgiant/giant shines from a goodly distance of 305 light years at a luminosity (with a 13 percent correction for interstellar dust absorption) of 4520 time that of the Sun from blue-white surface with a fairly well determined temperature of 21,900 Kelvin (so hot that most of the radiation emerges in the ultraviolet where we cannot see it). The combination of temperature and luminosity give a radius 4.7 times that of the Sun, and these in conjunction with the theory of stellar structure and evolution tell of an 8 solar mass star that is roughly midway through its 32 million year hydrogen-fusing dwarf lifetime (the "subgiant" classification clearly inappropriate, subgiants stars that have given up their core hydrogen fusion). Like most class B stars, it is a fast rotator, spinning with an equatorial velocity of at least 114 kilometers per second, which gives it a rotation period of less than two days. Also like many stars in its class, Alpha Mus is a "Beta Cephei" variable, subtly pulsating in brightness by about one percent over a 2.2 hour period. About half a minute of arc away is a purported 13th magnitude companion, which could also easily be a line-of-sight coincidence. If the companionship is real, the neighbor has the luminosity of a K8 dwarf. The two would be separated by at least 2600 Astronomical Units and take at least 45,000 years to make a full orbit. Given this separation, from Alpha Muscae proper, the companion would appear about as bright as four Venuses, while the companion would be illuminated with the visual light of 100 full Moons. While this duplicity may not be real, Alpha Mus does keep company with others, as it is a part of the loose (unbound) "Centaurus-Crux" association of O and B stars, all of which were born more or less at the same time from a massive interstellar cloud.
Written by Jim Kaler. Return to STARS.