Why Are Stars Different Colours?

This post discusses aspects of various questions I've been sent, including 'how do the layers of the flame compare with the colours of a star?' (posed by Blah), and 'What are the different coclors of the stars and what does each color mean?' (posed by Kat), and it provides some background for answers to other questions about stars that are in the pipeline!


Everything that has a temperature emits radiation*. Sometimes this radiation is emitted at a wavelength that we can see- we call this visible light- and the object then appears to us to be glowing a certain colour. Imagine a blacksmith forging a horseshoe: as it is heated up, the metal starts to glow red. If we continue heating it, it glows  a bright yellow. Even when the horseshoe is 'cold'** it's emitting energy. We don't see it glow because it's emitting energy at wavelengths that our eyes can't detect - turn a sensitive enough infrared camera on it, and then you'll see it glow.

Now think back to your secondary school science lessons, particularly the ones in which you got to use the bunsen burners. The flame of the bunsen burner changed colour depending on how far you opened the air valve- a closed valve gave you a yellow/orange flame, and an open valve produced a hotter blue flame.

With the horseshoe, the colour it glowed was largely to do with how hot it was. With the bunsen burner, however, there is another factor than temperature that determines its colour: what is actually burning has an effect too. When it comes to stars, there are three things that determine their colour, as we see them:

• Temperature
• Composition (i.e. what they're made of)
• Doppler shift

By far the most important of these for stars is their temperature. Composition has a smaller effect but is useful in finding out what stars are made from, and doppler shift only makes itself apparent in stars that are moving either towards or away from us at considerable speeds (such as those in distant galaxies that are receding from us and so appear redder than they should be).

For the rest of this post I'll concentrate on the strongest factor in determining star colour, and that's its temperature.

Essentially, the colour of a star gives us a good idea of its temperature. Values given are rough surface temperatures in degrees Kelvin:

• Red stars are the coolest, measuring a paltry 3,000 K.
• Orange stars are the next up the scale, getting to a bit below 5,000 K before you get...
• ... yellow stars, like our Sun*** can get up to a toasty 6,000 K where they start to become a bit more...
• ...white. To get a properly white star, though, you need temperatures exceeding 7,500 K or so.
• Between 10,000 and 28,000 K a blue-ish tint is introduced.
• After this, up to the hottest stars (about 50,000**** K) are indicated by a distinctly blue colouration.

Star colours and their spectral types, going from cooler on the left to hotter on the right. Note that the hotter stars are also bigger stars! The letters are labels given to their 'spectral types'. You know what to do if you want more info on that...

* Any object that contains energy gives off some of this energy in the form of radiation. Measuring this energy is what gives us its temperature.
** It's only really relatively cold- it feels cold because it has a lower temperature than our hands do, but it still has energy.
*** Although you could also say that our Sun is green...
**** Crikey.