DELTA CRU (Delta Crucis). Overwhelmed by its neighbors in Crux, one of the most famous constellations
of the sky, and fourth brightest in the 4-star pattern, Delta
Crucis is the only one with no proper name (though given that Alpha
Crucis is called "Acrux" and Gamma Crucis
"Gacrux," no one would likely object to
"Delcrux.") Mid-third magnitude (2.78), this blue-white class B
(B2) star shines from a distance of 364 light years. Both the
class and the distance are similar to those of Acrux and Mimosa (Delta just a bit cooler and
fainter), and all three stars are clearly related by birth, though
are not close to being gravitationally bound together. Gacrux is
much closer and not part of the association. Taken by itself,
Delta is still a magnificent star. From its hot 22,550 Kelvin
surface it pours out the light (much of it in the ultraviolet) of
5600 Suns, from which we find a radius of
4.9 solar, a mass 8.5 solar, and an age of less than 30 million
years. From its spectrum, the star is classed as a subgiant, which
implies that it has recently given up hydrogen fusion in its core
and (with a dead helium core) is about to become a giant.
Consistently, like Mimosa, Delta Cru is a "Beta Cephei" variable that subtly changes
its brightness by a couple percent or so over a period of 3.7
hours. But the temperature and luminosity imply that the star is
only about half-way through its "dwarf" status, that is, its
hydrogen-fusing lifetime. The reason for the anomaly is not known.
Like many class B stars, Delta Cru is a fast spinner, rotating at
least 194 kilometers per second at its equator, giving a rotation
period less than 1.3 days. Also like many massive stars, it
produces a wind, which is estimated to blow about at a mass-loss
rate of about 1000 times that of the Sun. Delta Cru is just under
the limit at which stars explode, and it someday will produce a
massive white dwarf rather like Sirius-B.