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.
Written by Jim Kaler. Return to STARS.