Crux - the Southern Cross
The whole constellation Crux (the Cross) defines an area of sky, but the Southern Cross itself is an asterism of five stars in a cross (or kite) shape. [Image: Naskies]
The top of the cross is Gacrux (Gamma Crucis). Unlike the other three main stars, which are hot blue stars, Gacrux is a cool red giant. It is big. If it were in the Sun's place, it would extend halfway to the Earth.
Clockwise from Gacrux, the first star is Delta Crucis. It's the only one of the four stars with no name, only a Bayer designation of a Greek letter and the constellation name. Delta Crucis is about 10,000 times brighter than the Sun, and a rapidly rotating third magnitude star.
Alpha Crucis (Acrux), at the bottom of the cross, and Beta Crucis (Mimosa) are blue-white first magnitude stars. They're also multiple star systems, though astronomers have yet to agree on how many stars are in each system.
The most luminous part of Acrux is a binary star which can be resolved in amateur telescopes. The main star of the pair is also binary, but they can't be separated visually. Mimosa is a binary whose primary star is a hot blue-white giant, but whose secondary star hasn't been resolved in a telescope. It was detected by its effect on the stellar light spectrum. There's also a distant companion of an unknown type, discovered when the Chandra X-ray observatory detected its X-ray emission.
Tucked just inside the kite is Epsilon Crucis, a fourth magnitude orange giant about 230 light years away from us.
History and precession
Crux is a feature of the southern skies. Although visible for part of the year in the tropics, you can see it year round south of 34°S. Most people in the northern hemisphere will never see the Southern Cross, though several millennia ago it was visible even from northern Europe. The ancient Greeks had considered it part of the constellation Centaurus, but by 400 CE it was below the horizon in Athens.
A little wobble in the Earth's axis over long time periods changes which stars we can see at different latitudes. It's called precession. (“Ecliptic and Equinoxes” explains this – there's a link below this article.) In another 12,000 years, Crux will be visible in all of the USA and much of Europe.
The forgotten stars were reintroduced to Europeans by sixteenth century explorers and traders who went into the southern hemisphere. Celestial cartographers later separated Crux from Centaurus.
The northern hemisphere has a bright star within a degree of the north celestial pole, i.e., Polaris. So if you can find Polaris, you can orient yourself. Although there's also a star that's close to the south pole, Sigma Octans, it's no good as a guide star. It's so dim that it's barely visible even in a clear dark sky.
However the Southern Cross can be used to find south because the long axis of the cross points towards the south celestial pole. There is a little complication here in that some people mistakenly choose the asterism of the “False Cross” or the “Diamond Cross”. [Image: Starry Night Skies Photography] A good check on that is the Southern Pointers in Centaurus. Alpha and Beta Centauri are two prominent southern hemisphere stars, and they point to Gacrux.
Deep sky objects
The two notable deep sky objects in this little constellation are the Coalsack and the Jewel Box Cluster.
The Coalsack is a dark nebula, some of which edges into Musca and Centaurus. It's about 600 light years away, dark because its dust blocks light from the stars beyond it.
The Jewel Box (NGC 4755) is a young cluster of over 100 stars. To the unaided eye it looks like a fuzzy star, but seen through a good telescope, its blue and red giants have the effect, as John Herschel wrote, “of a casket of variously coloured precious stones.”
With its stars part of Centaurus, Crux wasn't associated with classical mythology, but it features extensively in the lore of the southern hemisphere peoples. Here are a few examples.
The Coalsack forms the head of the “Emu in the sky” in several Australian Aboriginal cultures. [Image credit: Museum of Victoria, Scienceworks]
Imagery of the Southern Cross is diverse. To Maoris it's “the Anchor”, and the Pointers are its rope as it anchors the canoe of the warrior Tamarereti. In some parts of Western Australia the Southern Cross is the deity Mirrabooka, a giant sky possum, sitting in a tree. In the Torres Strait, many islanders saw it as the trident of the creation deity Tagai's fishing spear. Gacrux is its handle.
It's been a stingray to some peoples of the East Indies and Brazil. The Kalapalo people in Brazil saw the stars of Crux as angry bees that emerged from the Coalsack, which was the hive. And in Botswana for the Tswana people the constellation was two giraffes, Acrux and Mimosa forming a male, and Gacrux and Delta Crucis forming the female.
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