EPS MUS (Epsilon Muscae). Musca, the Fly,
a small, compact collection of half a dozen of so stars seen to the
south of Crux, the Southern Cross, is
clearly prominent enough to have its own designation as a
constellation, but why the astronomer Abbe Nicholas de Lacaille
(1713-1762) came up with such an annoying creature is beyond
knowing. The top two, Alpha and Beta
Muscae, are obvious third magnitude stars, while the next three
(more or less in order) lie at fourth magnitude, including our
Epsilon (magnitude 4.11). Along with somewhat brighter Lambda,
this set of six provides a lovely color
contrast. While Alpha, Beta, and Gamma are all hot, blue class B
stars at about the same distance (about 300 light years, and
related to one another through the loose Centaurus-Crux OB association), Delta is a much closer
orange class K star. Epsilon, a cool, red, class M (M5) dying
giant carries the color contrast further. Coincidentally at about
the same distance as Alpha, Beta, and Gamma Muscae, Epsilon
(measured at 302 light years with a 4 percent error) is not at all
related, as it is moving at much higher speed (a whopping 100
kilometers per second compared with the Sun, five to six times
normal), and is just passing through the vicinity of its
constellation-mates as well as through our part of the Galaxy (the
star a visitor from beyond the Galaxy's thin disk that holds the
Sun). Cool, about 3400 Kelvin (though there is no actual
measurement), Epsilon glows with the luminosity of 1800 to 2300
Suns (depending on how one assesses the invisible infrared
radiation). From these data, we derive a radius about 130 times
solar (0.6 Astronomical Units, some 80 percent the size of the
orbit of Venus), making it a true giant indeed, and an initial mass
(before the mass loss expected from such a giant) of roughly 1.5 to
2 times solar. Epsilon Muscae is a "semi-regular" variable star
that wanders between magnitudes 4.0 and 4.3 over an interval of 40
to 45 days. That, the cool temperature, and the great radius, tell
that the star is of advanced
age, and -- like Mira in Cetus -- is brightening and expanding with
a dead carbon-oxygen core. Some 1.5 to 3 billion years old, Eps
Mus does not have much time left before it loses its outer envelope
through a powerful wind and exposes its core as a hot white dwarf.