Colours of Stars

Constellation of Orion
Credit: Rogelio Bernal Andreo

Stars shine in lots of different colours. Their colour tells us the temperature of the star.

Some stars shine with a red-ish colour. This means they are quite cool for stars - only 3000 °C or less. Other stars shine with blue-ish glow. This means they are much hotter -over 30,000 °C! The Sun is shines with a yellow-ish colour and is about 6,000 °C. This makes the Sun a relatively cool star. 

The temperature of a star depends on how much mass it has. Very massive stars, which can be over ten times the mass of the Sun, are the hottest and usually glow blue. Smaller stars, with less than half the mass of the Sun, are the coolest and usually glow red.

Using the technique of spectroscopy, stars can be classified by their colour (or temperature) into a series of letters which denote their spectral type. The hottest stars are denoted by the letter O, with the sequence progressing through B, A, F, G, K to the coolest M stars . Each spectral type is split further by the numbers 0 - 9 so that a B0 star is bluer (and therefore hotter) than a B9 star, which in turn, is slightly bluer than an A0 star.

The B-V color index is a way of measuring the colour of a star. This quantity is calculated using the apparent magnitude of a star seen through 2 different telescope filters. The B filter lets through wavelengths of light centred on the blue band of light. The V filter lets through wavelengths of light close to green-yellow band of light.