Sunday space | Why is the sun white and stars all different colours?

Rigel, the star on the top left, looks distinctly blue. If you look at Betelgeuse, which is on the bottom right, it will look pretty red.

These colours are very important to understanding the properties of a star. Imagine you are sitting around a campfire. In the outer parts of the fire, the flames will be more orange and red. As you get towards the centre of the fire, it will appear bluer. You'll also notice the cooler flames are the red ones, while the hotter flames are bluer.

This is the same with most stars. A redder star will be a bit colder, and a blue star will be hotter. Measuring the colours of stars also helps us understand the temperature of the star.

In fact, Rigel, is called a blue super-giant, and Betelgeuse a red super-giant. Blue super-giants like Rigel emit most of their light in the blue colour, and in the red for red super-giants like Betelgeuse.

There is another aspect to the colour - the age. When stars are born, they are using only hydrogen as their fuel. It is burning hotter as well, meaning bluer stars are also hotter.

As a star uses up its fuel, it starts to burn more helium, and is a bit colder. This makes most red stars also older stars.

The sun is classified as a G-type star, also known as a yellow dwarf star. But the sun's colour is not the yellow we imagine, despite all the drawings and picture: it is white.

The peak temperature of the sun puts it in the green colour range. However, in visible light, it emits red, blue and green colours fairly evenly. What happens when you combine even amounts of red, blue and green? You get white. Why does it look more yellow, though, in the sky? The same reason the sky is blue during the day, and more orange-pink-red at sunrise or sunset - our atmosphere.

Called Rayleigh scattering, the more atmosphere light from the sun has to travel through, the more it scatters. Shorter (bluer light) is scattered more than red light. When the sun is on the horizon, light has to go through lots of atmosphere, and the blue light scatters away, a lot more than red, giving sunrise or sunset that beautiful colour. On Mars, because the atmosphere is different, sunrise and sunset is blue.

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When the sun is higher in the sky, it doesn't have to travel through as much atmosphere, so the blue light doesn't scatter away as much. If you look at the colour of the sun, it changes. It appears more orange on the horizon, yellow as it gets higher, and then close to white when it is right above in the sky.

Even the moon appears more yellow on the horizon and whiter when it is higher. And as the moon emits no light of its own, it is only reflected sunlight. This scattering effect is essentially zero, though, for distant stars, so the colours we see in the sky are fairly representative of the star itself.

So next time you use a sun emoji, make sure to find the white-coloured one.