Crux & Centauris star map
Copyright: Melbourne Planetarium.
Highlights of the tour:
- a Jewel Box of stars
- variable stars
- the closest star
- the finest globular cluster
Crux is the small constellation more commonly known as the Southern Cross for the shape made by the four brightest stars. The two bright stars near the Cross, known as the Pointers, are not part of this constellation but are part of the neighbouring constellation, Centaurus. Centaurus was named after a strange creature, half man and half horse, which was part of Greek and Roman mythology. Alpha Centauri was worshipped by the Egyptians and temples were erected at Corinth and Delphi with special alignment to the rising of this star.
You should notice a colour difference in the four bright stars. Three are white but Gamma Crucis is orange. Gamma Crucis is cooler than the others. Close to Beta Crucis is Kappa Crucis, a beautiful galactic cluster of stars commonly known as the Jewel Box. It is one of the youngest clusters, perhaps only a few million years old.
Kappa Crucis lies on the edge of a dark area of sky called the Coal Sack. You will notice that there seem to be very few stars in this area. In fact, the Coal sack is a large dust cloud about 500 light years away. The dust cloud is blocking out the light from stars which lie beyond it. The few stars you see are in front of the cloud.
Between Epsilon and Alpha Crucis lies a faint star called R Crucis. This is a Cepheid variable. This type of star varies in brightness because its outer layers seem to be pulsating. They are important stars to astronomers because they are used to determine distances to nearby galaxies.
In your binoculars you can just fit the four bright stars of the Southern Cross. The brightest is Alpha Crucis, the star at the foot of the Cross. Alpha Crucis is actually a double star though you cannot see the two stars separately in binoculars. The stars are about 75 billion kilometres apart and revolve slowly around each other. The other bright stars are, clockwise from Alpha, Beta, Gamma and Delta Crucis. The fifth in the Cross is Epsilon Crucis. These stars do not all lie at the same distance. Delta Crucis is the farthest away at a distance of 570 light years while Gamma is only 220 light years away.
These two bright stars are often called the Pointers because they seem to point towards the Southern Cross. They are actually part of the constellation of Centaurus. The brighter of the two is Alpha Centauri. It has a yellowish colour and though it appears a single star it is actually a triple star system. Two of the stars revolve around each other while the third is thought to revolve around the other two. Alpha Centauri is the closest star system we ever see in the sky. It is only 4.3 light years away (just over 40 million kilometres). Beta Centauri, the other pointer, is a very hot star about 10,000 times brighter than the Sun. It is 490 light years away. Between the pointers lies R Centauri. This is a Mira variable. Unlike a Cepheid it varies in brightness over a long period. It takes 547 days to change from its brightest point to its faintest. At its faintest, you cannot see it, even with binoculars.
The bright star in your field of view is Epsilon Centauri. This star is about 570 light years away. North of Epsilon you should find a faint fuzzy blob. This is Omega Centauri, the best example of a globular cluster you will see. Globular clusters are named after their shape. There are over a million stars in this cluster, the core of which is about 100 light years in diameter. Globular clusters are among the oldest groups of stars in the galaxy and the stars in the cluster are probably about 13 billion years old. Omega Centauri is moving around the galaxy once every 100 million years so as you look at it, it is moving away from our region of space at a speed of over 200 kilometres per second.
The brightest star on which you have centred your binoculars is Lambda Centauri. It is about 370 light years away and is over 600 times brighter than the Sun. Nearby you will see NGC 3766, another galactic cluster like the Jewel Box. Again, these stars are relatively young.