4PH1

Stellar Evolution

Astrophysics · 1 question type

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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.

Stars come in different colours

  • 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

Colour ↔ surface temperature

Hotter star → bluer colour

Cooler star → redder colour

Star colourSurface temperature (K)Examples
Blue≈ 30 000 KSpica, Mintaka
Blue-white≈ 10 000 KRigel, Sirius, Vega
White≈ 7500 KDeneb
Yellow≈ 6000 KThe Sun, Alpha Centauri A
Orange≈ 4500 KAldebaran, Arcturus
Red≈ 3000 KBetelgeuse, Proxima Centauri
A row of stars shaded by colour with their surface temperatures below, running from a deep blue star at 30 000 K on the left (labelled HOTTEST) through blue-white, white, pale yellow and yellow to orange and a red star at 3000 K on the right (labelled COOLEST), showing that hotter stars are bluer and cooler stars are redder
Source: Classification of Stars by Save My Exams
  • 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

What sets a star's temperature?

  • 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