BETA MUS (Beta Muscae). Were it in the north, Beta Muscae, if not famed, would certainly be among the better known stars. It is, however, in the deep south, more than two-thirds of the way from the celestial equator to the South Celestial Pole and within the small obscure modern constellation Musca, the Fly (once Musca Australis in response to Mucsa Borealis near Aries, which was gladly dumped and no longer buzzes among the formal celestial figures). Fairly bright at mid-third magnitude (2.97), Beta Mus is topped in its setting only by Alpha Muscae, which is about half a magnitude brighter. Beta Muscae, however, fools a bit, as it is not one star, but instead consists of two closely similar blue jewels that is reminiscent of nearby Alpha Crucis. Classed as a B2.5 dwarf, the star -- 341 light years away (give or take 10) -- is actually made of the fourth magnitude (3.52) class B2 dwarf Beta Mus A and somewhat dimmer but also fourth magnitude (3.98) Beta B, a class B3 dwarf. The two, just a second of arc or less apart, orbit in 194.3 years at a calculated average separation of 101 Astronomical Units (but see below), and have yet to make a full turn since first observation. They were last farthest apart in 1954 and are now again closing in on each other.
Beta Mus The 194-year orbit of the blue double Beta Muscae (in which the fainter of the two is graphed as if it is going about the brighter rather than about the center of mass) has been only partially mapped (the scale in seconds of arc). Moreover, the closeness of the pair, just a second of arc or so apart, makes observation itself quite difficult, as can be seen in the scatter of the measures. The average separation from the best-fit orbit is 101 Astronomical Units, but from the masses of the stars and the period (see below) is probably more like 80. The orbit will be improved with time and continued observation. The primary star is offset from the focus of the apparent ellipse because of the 37 degree tilt of the orbit to the plane of the sky. (From the Sixth Catalog of Orbits of Visual Binary Stars, W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.)
With respective temperatures of 22,500 and 18,500 Kelvin estimated from their classes (to account for a lot of ultraviolet light), and after a 10 percent correction for dimming by interstellar dust, the two respectively shine with the light of 2750 and 1200 Suns, giving both of them radii of about 3.5 times solar. With a projected rotation speed of 160 kilometers per second (typical of hot B stars), Beta Mus A turns in under a day. Theory assigns masses of 8 and 6 solar masses, making Beta A perilously close to the limit above which stars explode. Odds are though that they will expire as double white dwarf, each with the rough mass of the Sun. Very young, both have a long way to go -- respectively 30 to 60 million years -- before beginning to expire. With a modestly high space velocity of 47 kilometers per second (thrice normal), Beta Muscae (part of the Lower Centaurus-Crux association) is considered a "runaway star" that may have been ejected from some prior binary connection.

But there are problems. A singular temperature measure of 18,200 Kelvin is not in keeping with those derived from class. If applied to Beta Mus A, it would bring the luminosity down to 1800 Suns and the mass to 6.5 Suns. More important, Kepler's Laws applied to the orbit give a total system mass of 28 times solar, double that derived from temperature and luminosity, which is vastly too high for their classes, the orbit clearly in error, no surprise given the rather short observation period. Using the period and the 14-solar mass sum, the mean orbital radius reduces to 81 AU, a high eccentricity taking them between 128 and 32 AU apart. Clearly the star deserves far more attention than it has been given.
Written by Jim Kaler 6/22/10. Return to STARS.