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.
Written by Jim Kaler. Return to STARS.