Constellation Crux – The Southern Cross

The constellation Crux lies under the hind legs of the much larger constellation Centaurus, and its four bright stars, Acrux, Mimosa, Gacrux, and delta Crucis, mark a kite-like shape about 6 degrees long.  Some observers are disappointed at first seeing Crux, because it’s so small.  But there are enough fine sights here to last for many nights of patient contemplation. To paraphrase Rainer Maria Rilke, if you are bored by the constellation Crux, you are not yet poet enough to call forth its riches.  Let’s take a look…

The brightest star, Acrux, at the base of the cross, gets its name from a combination of “Alpha” and “Crux”.  A close look with a small telescope reveals Acrux as a pleasing double star.  The brighter of the two stars, which itself has another stellar companion too close to resolve, has a temperature of 30,000K and shines some 25,000x brighter than our sun.  The fainter star outshines our sun by 16,000x. Like most hot, massive stars, each component of Acrux are burning furiously through their store of fuel.  They will likely end their lives in supernova explosions in several million years, and since they are just 320 light years from Earth, will grow bright enough for a few weeks to cast shadows at midnight.

A map of the main stars of the Southern Cross (click to enlarge).

Mimosa, or beta Crucis, is also a hot blue star.  It lies 280 light years away.  The star takes its name from the Mimosa flower found in Central and South America.  Mimosa, Acrux, and delta Crucis likely share a common origin 10 million years ago with many other stars in Scorpius and Centaurus.

The red-orange Gacrux (gamma Crucis) makes a striking color contrast with the other bright blue-white stars of the constellation.  The star has burned through most of its nuclear fuel and has swollen and cooled to just 3,500K.  Gacrux lies fairly to Earth– just 88 light years– so it’s intrinsically much fainter than the Acrux and Mimosa.  It will expire gently, casting off its outer layers as a planetary nebula and leave behind a dim white dwarf.

One of the brightest stars in Crux isn’t a star at all, but splendid cluster of young stars aptly named the “Jewel Box”, or the kappa Crucis cluster (see top of page).  This star cluster, one of the treasures of the southern sky, is a lovely sight in binoculars and unforgettable in a small telescope.  We’ve looked at this cluster before… if you have a clear view of Crux, don’t miss it (see image at top of the page, from the University of Arizona)…