Classifying Stars by Colour and Temperature — Stellar Evolution | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Life cycle of stars and the Hertzsprung-Russell diagram appear as descriptive multi-mark questions.

Look at the night sky carefully and you can pick out different colours: Betelgeuse in Orion looks distinctly orange-red; Rigel in the same constellation is a brilliant blue-white; the Sun is yellow; many faint stars look a quiet red
A star's colour is set by the temperature of its surface. Hot objects glow blue-white; cooler objects glow red. This is exactly the same physics as a piece of metal heated in a furnace: at moderate heat it glows red, at higher heat it turns orange, then yellow, then white-hot, then blue-white

Hotter star → bluer colour

Cooler star → redder colour

Star colour	Surface temperature (K)	Examples
Blue	≈ 30 000 K	Spica, Mintaka
Blue-white	≈ 10 000 K	Rigel, Sirius, Vega
White	≈ 7500 K	Deneb
Yellow	≈ 6000 K	The Sun, Alpha Centauri A
Orange	≈ 4500 K	Aldebaran, Arcturus
Red	≈ 3000 K	Betelgeuse, Proxima Centauri

Be careful: in everyday life we associate red with hot (red-hot iron, red flames) and blue with cold (cold water taps, ice). For stars it is the other way around: blue is the hottest, red is the coolest. The flame analogy is closer to the truth than the tap one, since a Bunsen burner's blue cone is hotter than its yellow tip

A star's surface temperature depends on its mass and the stage of its life it is in
Massive young stars on the main sequence are blue-white and very hot
Lighter main-sequence stars (like the Sun) are yellow
Aging stars that have swelled into red giants or red supergiants are cool and red, even though they may be very large and very bright
As a star expands, its surface cools; as a star contracts, its surface heats up. So when the Sun eventually becomes a red giant, its outer layers swell out and cool to red; when it then collapses to a white dwarf, those layers contract again and heat to white-hot