MU CRU (Mu Crucis). Crux, the Southern
Cross, is so dominated by the four stars that make it (Acrux, Mimosa, Gacrux, and Delta
Cru) that we tend to pay little attention to the others that
inhabit the constellation. Look then to a delightful wide double
star, Mu Crucis, made of fourth magnitude (4.03) Mu-1 to the west,
fifth magnitude (5.17) Mu-2 to the east, the pair separated by 35
seconds of arc. Unresolved to the naked eye, combined they shine
at magnitude 3.7. While too far apart to exhibit orbital motion,
they are at the same distance (averaging 370 light years away) and
are moving through space together, so that there is no question
about their togetherness. Both are hot class B stars, Mu-1 classed
a B2 subgiant-dwarf, Mu-2 a B5 dwarf. Mu-1 is the simpler of the
pair. Allowing for a large amount of ultraviolet light from its
22,800-Kelvin surface, it shines with a luminosity 2400 times that
of the Sun, which leads to a radius of 3.1
times solar and a mass 8 times solar. Moreover, evolutionary
theory shows that the star is not a subgiant at all, but is near
the "zero-age main sequence," that is, it is very young true dwarf
that is just starting to fuse its internal hydrogen into helium.
Mu-1's projected equatorial rotation velocity of 48 kilometers per
second leads to a rotation period less than 3 1/4 days. The
fainter of the pair, Mu-2, is the more interesting, its measured
rotation speed of 230 kilometers per second consistent with its
classification as a "Be" (class B-emission) star that has a
surrounding disk that radiates emission lines of the hydrogen
spectrum (like
Gamma Cas or Zeta Tauri).
It is also formally mis-
classed as a "high mass X-ray binary." Old observations suggested
X-ray emission that later proved not to be present. Even if X-rays
had been present, the term has come to mean that a star has a
compact neutron-star companion like
X Persei, and is not just an X-
ray star in an otherwise ordinary binary system. With a
temperature of 16,500, Mu-2 radiates 425 solar luminosities into
space. Its radius of 2.5 times that of the Sun gives it a short
rotation period of under half a day. Carrying a mass of five times
that of the Sun, like Mu-1, Mu-2 is a young star just beginning its
career. Together, they are part of the vast "Lower-Centaurus Crux"
association of hot stars. The two are separated by at least 3900
Astronomical Units, and given their masses must orbit with a period
of at least 68,000 years (explaining why we cannot yet see orbital
motion). At the stated separation, from Mu-1, Mu-2 would shine (to
the eye, disallowing ultraviolet radiation) with the light of 6
full Moons, while from Mu-2, Mu-1 would appear three times as
bright. While there is a possibility that Mu-1 is just over the
edge of the stars that blow up as supernovae, the odds are that
both will die as massive white dwarfs -- and that over time, they
will separate from each other thanks to the gravitational pulls of
other stars.