MU-1 SCO (Mu-1 Scorpii). In the crowded Milky Way the stars can fool you. In middle of the curve of Scorpius, the Scorpion, are two apparent naked-eye double stars, Zeta Sco and Mu Sco, both of which are made just of line-of-sight coincidences. The real fooler is Mu. For decades it was thought that the two in central Scorpius, Mu-1 (the western of the pair) and Mu-2, were a real physical (though wide) double, as they seemed to share the same motion. Modern distances, however, show that they are not joined. Mu-2 is a fourth magnitude (3.57) class B (B2) subgiant that lies 515 light years away. Brighter third magnitude (3.08) Mu-1, also a hot class B star (combination B1.5 and B6 dwarf), is notably farther, 820 light years. Moreover, the motions are now seen not to match well either. Mu-1 and Mu-2 are connected only in that both are part of the massive, but unbound, Scorpius-Centaurus Association (and the "Upper Sco" subgroup). Mu-1, however, as given away by its combination spectrum, really is double, though one whose components are so close as to be entirely inseparable through the telescope. Its duplicity has been known since 1896 through spectroscopic analysis and from the later discovery that the close pair partially eclipse each other, leading to a drop of 0.3 or so magnitudes every orbital period of 1.440269 days (when the fainter partially blocks the light of the brighter). The two are remarkably close, a mere 14.89 solar radii apart, which leads to a summed mass of 21 times that of the Sun. The individuals are a bit hard to separate out, however. From its class, the hotter (and brighter) should have a temperature of 26,500 Kelvin, while the cooler should be at 15,000 Kelvin. Photometric analysis of the eclipses and allowance for a lot of ultraviolet light and a bit of interstellar dust absorption yields respective luminosities of 28,000 and 16,000 times that of the Sun, which in turn give masses of 13 and 8 solar, just in accord with the sum of masses from the orbit. Direct analysis of the temperatures, however, suggest the brighter to be cooler (21,200 Kelvin) and the fainter to be somewhat hotter (16,200, closer to class B3), giving a lower "primary" mass of 11 solar. What makes Mu-1 special is that it is a "mass transfer" binary. The lower mass star fills its "zero-gravity" tidal surface, the surface around the star at which a particle could flow to either component. It is distorted into a tear-drop shape, and is losing its matter to the higher mass star. The mass transfer has turned the system on its head. Like Algol in Perseus, the lower mass star USED to be the higher mass one, which had the fate to evolve first and to expand to the point that it set the stage for its own near-destruction. When the pair was born, what is now the 8 solar mass star had a mass near 14 solar, while what is now the higher mass star was closer to 9 or 10. Over the past 10 million years, the two have switched places for dominance. Mass loss will continue until the current "gainer" begins to expand and return to favor to the current "loser." Just where the stars will wind up is not clear. The current mass loser will probably become an odd white dwarf, while the system may for a time become some kind of "cataclysmic variable" in which even more erratic mass loss causes it to vary erratically. One member may someday even explode. Thanks to Jerry Diekmann for directing me to this star.
Written by Jim Kaler. Return to STARS.