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.