THETA MUS (Theta Muscae). The list of the modern constellations originally had two flies, Musca Australis, the Southern Fly, and Musca Borealis, the Northern Fly, which may have been meant to be bees, not flies, the history a bit of a mess. The northern one, on Aries' back, was in later times thankfully removed, but the southern one (now just "Musca") still buzzes between Crux and the southern pole. But be not annoyed by it, as Musca is highlighted by a grand star, sixth magnitude (5.51) Theta Muscae, a stellar system that contains the second brightest "Wolf-Rayet" star in the sky (following Regor, Gamma-2 Velorum). Wolf-Rayet (WR) stars, named after their 19th century discoverers Charles Wolf and Georges Rayet, are highly evolved massive supergiants that began their lives as hot class O stars, have lost most of their outer envelopes, and now display their nuclear-enriched innards. There are two kinds, those dominated by either carbon (WC) or nitrogen (WN). Both are rich in helium, while hydrogen is essentially absent, the stellar envelopes stripped away. Both also have strong broad spectral emissions that tell of powerful winds that make analysis difficult, as the winds tend to hide the stars within them, stars that are as hot or hotter than the O stars (in the tens of thousands of Kelvins) that birthed them and have luminosities in the hundreds of thousands of Suns. As the stars lose mass and strip themselves down, the WN versions probably evolve to WC. WR stars typically have masses around 20 to 40 or so times that of the Sun and are the evolved products of stars up to 100 or more solar masses (like Eta Carinae), hence are some of the absolute jewels of the sky. Many are surrounded by bright, ring-shaped ejecta.

Like Gamma-2 Vel, the WR star in the Theta Muscae system is of the carbon variety. A "companion," eighth magnitude Theta Muscae B five seconds of arc away, is just in the line of sight, so we need not bother with it. Beyond that, Theta Mus has long been a puzzle. The distance, beyond the range of parallax, is essentially unknown, though 7500 light years is commonly adopted. The class is formally listed as "B0 supergiant (maybe O9.5) plus WC5." Doppler shifts in the spectrum of the WC star show it to be orbiting a companion in 19.14 days, but it's not the O supergiant, which has a stationary spectrum. Interferometer observations later showed them to be 46 milliseconds, or about 100 AU, apart. The WC star must then be in orbit about something else, probably an O6 or O7 dwarf separated from the Wolf-Rayet star by the order of half an AU, making Theta Muscae a triple system, whose luminosity must approach that of a million Suns. Masses are unknown, but that of the lightest of them, the purported O dwarf, must at least be in the low tens of Suns. Both the WR star and the supergiant blow powerful winds that collide to produce X-rays. All three stars are above the supernova limit of 8-10 Suns, the distant future seeing one after another destroy themselves (or even their companions). (Thanks particularly to Sugawara, Y. Tsuboi, and Y Maeda, Astronomy and Astrophysics, vol. 490, p. 259, 2008.)

Written by Jim Kaler 6/14/13. Return to STARS.