Institute of Astronomy

 

Ask an Astronomer - Stars

Twinkling stars

Published on 28/02/2011 
Question: 

Why do stars twinkle and planets do not?

In fact, both stars and planets twinkle! The twinkling is due to the turbulent air in the Earth's atmosphere, blurring and distorting the image of the star. The twinkling therefore has more of an effect nearer to the horizon, where the light must travel through more of the densest parts of the Earth's atmosphere. You can see this for yourself! Compare the twinkling of a star near the horizon (such as Sirius), and one close to zenith (straight up). Objects such as the Sun, Moon and the planets are called extended sources, because the light is emitted from a disc. Objects such as distant stars are called point sources, because they appear to be a point as they are very far away. In fact, the light from extended sources can be thought of as many point sources spread over an area. The turbulent air in the atmosphere causes a point source to appear to move around on the sky ever so slightly. However if we spread many point sources over the face of the planet, all point sources move around, but we do not notice a change in the total light from the object.

The brightest star

Published on 28/02/2011 
Question: 

What is the brightest star in the night sky?

We believe our galaxy, the Milky Way, contains roughly 200 THOUSAND MILLION stars (that is, 200,000,000,000). However, if you were to count the number of stars in the night sky with a naked eye, you would only be able to count about 6,000. Besides our star, the Sun, the brightest star in the night sky is Sirius, in the constellation Canis Major (the “Big Dog”). Sirius is almost twice as bright as the next brightest star, Canopus, in the constellation Carina.

Stars

Published on 25/02/2011 
Question: 

What is a star?

A star is a huge sphere of hot, glowing gas. The star produces its own heat and light through a process called nuclear fusion. This process forces lighter chemical elements to become heavier chemical elements. When this happens a large amount of energy is released and this energy is what causes the star to heat up and 'shine'.

Stars come in a range of sizes and colours. Our Sun is a fairly typical yellow of M type star. Stars which are hotter than our Sun tend to look bluer while those which are cooler look red. So the next time you look at the night sky, see if you can see stars of different colours!