Star colours explained
Last updated 4 November 2026
A star's colour tells you its surface temperature. The coolest stars glow red or orange, at around 3,000 to 4,000°C; middling stars like the Sun are yellow-white, at about 5,500°C; and the hottest stars are white or blue-white, at 10,000°C and above. From the UK, compare orange Betelgeuse with blue-white Rigel in Orion to see the difference for yourself.
Why stars are different colours
Stars shine because they are hot, the same way metal glows when it is heated. Any hot object gives out light across a range of colours, and the mix depends on temperature: as it heats up, the peak of its light moves from red through orange and yellow to white and then blue. An electric hob ring or a blacksmith's iron shows it, glowing dull red first, then orange, then brighter and whiter. Stars follow the same physics at far higher temperatures, and physicists call this kind of glow black-body radiation.
So, against everyday habit, where red means hot and blue means cold, in astronomy red stars are the coolest and blue stars the hottest.
The star types: O, B, A, F, G, K, M
Astronomers sort stars into spectral classes by the patterns of lines in their light, which mainly reflect temperature. From hottest to coolest they run O, B, A, F, G, K, M, traditionally remembered as "Oh Be A Fine Girl/Guy, Kiss Me".
| Class | Colour | Approximate surface temperature | Example seen from the UK |
|---|---|---|---|
| O | Blue | Above 30,000°C | Alnitak, in Orion's Belt (its brightest component) |
| B | Blue-white | About 10,000–30,000°C | Rigel, Spica, Regulus |
| A | White | About 7,500–10,000°C | Sirius, Vega, Deneb |
| F | Yellow-white | About 6,000–7,500°C | Procyon, Polaris |
| G | Yellow | About 5,000–6,000°C | The Sun |
| K | Orange | About 3,500–5,000°C | Arcturus, Aldebaran, Pollux |
| M | Red-orange | Below about 3,500°C | Betelgeuse, Antares |
The ranges are approximate and differ slightly between sources. The Sun's surface is about 5,500°C (around 5,800 kelvin).
The most colourful stars to see from the UK
- Betelgeuse and Rigel (Orion, winter): the classic pair, orange-red at the top left and blue-white at the bottom right. See our Orion guide.
- Aldebaran (Taurus, winter): an orange eye in the Bull's face, beside the blue-white Pleiades.
- Arcturus (Boötes, spring and summer): a rich orange; set it against white Vega.
- Antares (Scorpius, summer): deep orange-red and very low in the south from the UK. Its name is often translated as "rival of Mars".
- Albireo (Cygnus, summer and autumn): a small telescope splits it into a golden star and a blue one, one of the loveliest colour contrasts in the sky. See summer constellations.
- Capella (Auriga, overhead in winter): a warm yellow, close to the Sun's colour.
To see colour better, use binoculars, or defocus them slightly so each star becomes a small disc: the colour spreads over a larger area and is easier to see.
Why most stars look white
Our eyes are poor at colour in dim light. The colour-sensitive cells (cones) need fairly bright light; in the dark we rely on rods, which are more sensitive but colour-blind. So only the brightest stars show obvious colour to the naked eye, and even then as soft pastels rather than vivid hues. Photographs exaggerate star colours because the camera collects light for longer.
Why there are no green stars
A star gives out light across the whole rainbow, not one colour. A star whose light peaks in the green, like the Sun, is also giving out plenty of red and blue, and our eyes blend the mix into white or yellowish-white. No temperature makes a glowing body look green to the human eye, so there are no green stars, and purple stars are missing for similar reasons.
The colour of a named star. Every star we name carries its catalogue reference from the European Space Agency's Hipparcos mission or the Tycho-2 catalogue, so you can look up its measured properties in any astronomical database. At magnitude 6.5 or fainter it is too faint for the eye to see its colour, or the star at all, without binoculars or a telescope. See can you see a named star?
Do colour and size go together?
Not directly. Colour depends on surface temperature, not size. Betelgeuse and Antares are cool red supergiants, hundreds of times wider than the Sun, while the commonest stars in the galaxy are also cool and red but tiny: red dwarfs, far too faint to see without a telescope. Proxima Centauri, the nearest star to the Sun, is one. White stars likewise range from bright, young stars like Vega to white dwarfs, the small, dense cores left behind by dead stars.
What about flashing colours?
A bright star low on the horizon can seem to flash red, green and blue. That is not its true colour but the Earth's atmosphere bending different colours by different amounts; our guide to why stars twinkle explains it. A star's real colour is the steady tint it shows high in the sky. For more on the bright stars named here, see the brightest stars in the UK sky.
Common questions
What does a star's colour tell you?
Its surface temperature. Red and orange stars are the coolest, at roughly 3,000 to 4,000°C; yellow stars like the Sun are around 5,500°C; and white and blue-white stars are the hottest, from about 7,500°C to well over 30,000°C.
Are blue stars hotter than red stars?
Yes. In astronomy blue stars are the hottest and red stars the coolest. As a glowing object gets hotter, its light shifts from red through yellow and white to blue.
What colour is the Sun?
The Sun is a G-type star and is essentially white seen from space. It looks yellow from the ground, and orange or red at sunrise and sunset, because the atmosphere scatters away some of its blue light.
Are there any green stars?
No. A star whose light peaks in green also gives out lots of red and blue light, and the eye blends them into white. No star looks green, although a bright star twinkling low in the sky can flash green for a moment.
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