Life and death of stars
Stars are great spheres of intensely hot gas that are undergoing nuclear reactions (similar to those in hydrogen bombs). Stars form in vast clouds of gas and dust known as nebulae, and have a long lifespan – a few million to tens of billions of years – before they exhaust their supply of fuel. The largest stars are about 120 times the mass of the sun, and are known as supergiants. The smallest, known as red dwarfs, are about one-tenth the sun’s mass. The manner in which stars die depends on their size. The products of many star deaths are also known as nebulae, for they are also great clouds of gas and dust, but in this case ejected from the periphery of an exploding star.
The Southern Cross
Visible year round from New Zealand, the Southern Cross constellation (Crux) and its associates the Pointers are among the brightest stars in the southern sky. Four bright stars form the ends of an imaginary cross with a long axis and short crossbar. The long axis always points in the direction of the South Celestial Pole, and for this reason it serves as a night-time navigational aid.
The Coal Sack and Jewel Box
Along the eastern edge of the Southern Cross is a dark region called the Coal Sack nebula. It is a star nursery, where young stars are forming from dense clouds of glowing gas and dust compressed under intense gravitational force. Just above the Coal Sack and alongside the second brightest star of the Southern Cross is the Jewel Box, a colourful cluster of about 50 stars that can be seen with a telescope.
Easy as ABC
The ABC is a useful way of remembering the defining stars of the southern hemisphere:
A is for Alpha Centauri, B is Beta Centauri, and C is the cross.
The Centaurus constellation lies to the east of the Southern Cross. Its two brightest stars, Alpha and Beta Centauri, are commonly known as the Pointers, because an imaginary line between the two stars points towards the cross.
Alpha Centauri appears as the third brightest star in the night sky, after Sirius and Canopus. It shines with a yellow light, and is not a single star, but a triple star system. Two stars orbit around each other every 80 years and both are visible with a good telescope. The third star lies far beyond them, and is so small and faint that it was only discovered in 1915. This is Proxima Centauri, the closest star to our sun at only 4.22 light years away. It appears to orbit the other two stars every 500,000 years.
Beta Centauri, the 11th brightest star, shines with a blue-white light. It is the nearest of the pointers to the Southern Cross, and is 526 light years from earth. It is a double star system, consisting of two giant stars about 15 times bigger than the sun.