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.