This month, as an introduction to newcomers, let’s look at a famous group of stars, the Southern Cross, as well as a neighbouring star and see what we can find out about them by examining their colours. Like a gas burner on high, stars that are hotter burn with a white or bluish light.
As the temperature decreases, the colour changes to yellow, then to orange through to red. These colours are often subtle, but can tell us a lot about the properties of each star.
At about 8pm in mid-June, the Southern Cross is high in the south as shown on the star map above. To the left of the Cross are two bright stars called “The Pointers” because they “point” to the Cross. The Pointer star furthest to the left is Alpha Centauri, also known as Rigil Kentaurus, and is the nearest star system to our Sun.
Through a telescope, Alpha Centauri is seen to be a double star. The primary stars in the Alpha Centauri system are yellowish-white, much like our Sun, indicating they have similar temperatures.
Looking at the stars of the Southern Cross, you may notice the star “Acrux” at the bottom of the Cross is bluish white, showing it is a very hot, young star while the star at the top of the Cross, “Gacrux” has a reddish-orange colour, indicating it has a much lower temperature.
These reddish stars, known as Red Giants, are old stars nearing the end of their lives and are truly enormous objects that have expanded to many times the size of our Sun.
The photo of the Southern Cross and The Pointers (top right) was produced with a digital SLR camera on a tripod with a time exposure of five minutes. The lens was de-focussed at regular intervals during the exposure to “spread out” the light from each star to help show their different colours.
So with little more than an ordinary camera in a suburban backyard, we can start to learn about the physical properties of stars located many light years away!
Tamworth Regional Astronomy Club is seeking donations of disused or unwanted telescopes or binoculars. These will be refurbished by the club for use by the public on viewing nights. Contact Stephen 0422 554 556 if you can help.
Until next month, clear skies!