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.