Plenty to explore in southern skies

THIS week I would like to encourage you to explore the colour of some stars in the southern sky. Since colour is used by astronomers to infer stellar surface temperatures, it’s possible to learn something about the universe with the power of the human eye. With the moon reaching firstquarter phase on Saturday, the early evening is the perfect time to wrap up warm and head out for some stargazing.

Our chart shows a wideangle view of the whole southern sky at 9pm this week. The ‘‘pointers’’, Alpha and Beta Centauri, are high in the south; they are called the pointers because a line drawn from the brighter star (Alpha Centauri) through the dimmer star (Beta Centauri) points towards the Southern Cross.

Alpha Centauri, the brighter and more yellow of the pointer stars, is one of the closest stars in the sky; in fact, it’s just over 4.3 light years away. Its yelloworange colour tells us that its surface temperature is roughly 4700degC. By comparison, our star (the sun) has a surface temperature of 5500degC.

Although it is less bright (but more blue) than its neighbour, Beta Centauri (sometimes called Hadar) is actually bigger and much further away; astronomers tell us that its blue colour represents a surface temperature of almost 25,000degC and have measured its distance at just over 390 light years.

Three of the four stars which comprise the Southern Cross are blue, with surface temperatures higher than 20,000degC; the dimmest of the four main stars, Gamma (sometimes called Gacrux), is orangered, with a much lower surface temperature than the other three at just over 3200degC.

A line drawn along the long axis of the Southern Cross extending towards the horizon will pass between the Magellanic Clouds (large, LMC, and small, SMC), and eventually reach another bright bluewhite star, Achernar, which is 15deg above the southern horizon at 9pm.

If you are looking south, to the right of Achernar is Canopus, a whitecoloured star whose surface temperature is about 10,000degC.

Isn’t it amazing what can be seen with the unaided eye?