On Christmas night, my 5-year-old cousin was enjoying his new toy telescope and asked me why a star he was looking at was red.
He knows that our sun is yellow, and I explained that stars can be different colours - some stars can be red and some stars can be blue. That was a good enough answer for him.
However, I would like to explain how and why that can happen.
Our sun is a star, much like all the other stars you see in the night sky. The fact that it is so much closer to us than those other stars is why it seems so much bigger and brighter.
The sun is about 5 billion years old, and has about 5 billion more years until it reaches the end stages of its life. You may have heard people predict that in 5 billion years the sun is going to die. This is partially true.
At the sun's core, hydrogen is fused into helium, and that is what keeps it bright and hot. Eventually there won't be hydrogen left in the core to fuse, so the hydrogen around the core starts to fuse. This will make the outer layers of the sun "puff out".
This means the sun will look bigger, but as it does this it will cool down, becoming redder, and making it a red giant.
The colour of a star depends on where it is in its life cycle (its age), and how much mass or fuel it has. Stars change colour as part of their life cycle, like our sun becoming a red giant. However, stars can also be different colours to start with when they are born.
Stars with more fuel burn hotter and are bluer than stars with less fuel. It might feel counter intuitive that a blue star is hotter than a red star, but take a moment to think about heating up metal. When heated, metal goes from red hot to white hot and then probably starts melting.
Stars that are smaller and cooler than the sun, and are in the main part of their lives, are more orange or red, and are called red dwarfs.
Stars that are bigger and hotter than the sun look whiter than our sun, which appears yellow.
Then there are the stars that are even bigger and hotter. They are the blue giants. The blue giants start out with a lot of fuel but, because they are so big, they get through it faster and reach the ends of their lives sooner.
The red dwarfs on the other hand, are very conservative about their fuel use and can live for more than the 13.7 billion years that the universe has been around for.
If you want to spot some of these coloured stars for yourself, check out the constellation of Orion (which includes the saucepan) and you'll see a red giant called Betelgeuse and a blue supergiant called Rigel. Or with a pair of binoculars or a telescope, check out the Jewel Box Cluster near the Southern Cross, shown in the image, with its red and blue stars.
- Eloise Birchall is a PhD Student at the Research School of Astronomy and Astrophysics at the Australian National University.