IGCSE Physics / Stellar evolution
Classifying stars by colour revision notes
Edexcel International GCSE Physics (4PH1), chapter 27: Stellar evolution, specification point 8.7, 8.8. Free lesson.
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Stars are classified by colour because their colour depends on their surface temperature. The order from coolest to hottest is red, orange, yellow, white, blue.
How colour changes as temperature rises
A heated metal bar glows different colours as its temperature increases. Stars follow the same pattern, so their colour gives you a way to compare their surface temperatures. From coolest to hottest: red, orange, yellow, white, blue. Red is the first glow of a cooler object. Blue indicates the hottest stars.
What each star colour tells you
Astronomers classify stars by colour. A star’s colour depends on the temperature of its surface, which is the outside layer we see. The Sun is a yellow star, around the middle of the colour order. Red stars are the coolest. Betelgeuse is red. Orange stars are a little hotter than red stars. Arcturus is orange. Yellow stars are hotter than orange stars. The Sun is yellow. White stars are hotter than yellow stars. Sirius is white. Blue stars are the hottest. Rigel is blue.
Why red does not mean hottest for stars
In everyday life, red is often used to mean hot and blue to mean cold. For stars, that idea is the wrong way round: red is the glow of a cooler surface, while blue means a higher surface temperature. Red stars have the lowest surface temperature. Blue stars have the highest surface temperature. White stars are hotter than yellow stars, but cooler than blue stars. White is second from the hottest end.
Comparing stars by colour
Betelgeuse and Rigel are both bright stars in the constellation of Orion, which is a pattern of stars in the sky. Betelgeuse looks reddish and Rigel looks bluish white, so Rigel has the much hotter surface. Colour tells you surface temperature only. It does not tell you a star’s size or distance. A star’s core is far hotter than its surface, so specify surface temperature when comparing stars. Betelgeuse: red, cooler surface. Rigel: blue-white, much hotter surface.
Compare the surface temperatures
To compare two stars, place their colours in the order from coolest to hottest. Then state which star has the higher surface temperature and link your answer to the colour order.