Beta Crucis - Becrux, Mimosa

Beta Crucis is a blue-white star of magnitude 1.28, also known as Becrux or Mimosa. It is the second brightest star in the constellation Crux (after α Cru or Acrux), and the 19th brightest star in the night sky. Becrux is the easternmost of the four stars forming the Southern Cross.

Since Beta Crucis lies at roughly declination -60°, it is only visible south of the Tropic of Cancer, and therefore did not receive an ancient traditional name. "Becrux" is simply a contraction of the "Be" in Beta plus the name of the constellation, Crux.

"Mimosa" is a recent name which comes from Latin and means "actor." It may refer derive from the name of the beautiful southern hemisphere flower, based on the star's color. Beta Crucis is simply known as the Third Star of the Cross in Chinese.

Properties

Mimosa is a magnificent blue-white, very hot class B0.5 giant star 350 light years away, with a temperature up to 27,600 K. Such heat causes the star to radiate most of its light in the invisible ultraviolet. If all that radiation is taken into account, β Cru's luminosity climbs to 34,000 suns. From these figures, its calculated radius is 8.1 times the Sun's, in good agreement with the value of 8.4 solar radii found from its small angular size.

Mimosa is also a multiply-periodic β Cepheid variable that fluctuates between magnitudes 1.23 and 1.31, with periods of 5.68, 3.87, and 2.91 hours. Mimosa has an iron content only about half solar, a common occurrence among local young stars. No more than 10 or so million years old, it seems more to be nearing the end of its hydrogen-fusing stage. Most likely, the star will become a red supergiant, which will develop an iron core, and blow up as a supernova - increasing the Galaxy's metal content with time.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]