A level Physics · Edexcel 9PH0 · Topic 10B
The Hertzsprung-Russell diagram
Key points
- The Hertzsprung–Russell (H–R) diagram plots luminosity (vertical, log scale) against surface temperature (horizontal, decreasing to the right).
- Main sequence stars (like the Sun) fuse hydrogen to helium in their cores; most stars spend most of their lives here.
- Red giants and supergiants are cool but very luminous, so they must be huge (top right). White dwarfs are hot but dim, so they are small (bottom left).
- A low-mass star like the Sun leaves the main sequence to become a red giant, then sheds its outer layers and becomes a white dwarf.
- A high-mass star becomes a red supergiant, then explodes as a supernova, leaving a neutron star or a black hole.
- A star's mass decides its path: heavier stars are hotter and brighter on the main sequence and have shorter lives.
- Use to explain the relative sizes of stars in different regions.
Topic notesPremium
Plotting stars by luminosity and temperature reveals clear patterns rather than a random scatter. The Hertzsprung–Russell (H–R) diagram groups stars by type and shows the path each star takes through its life.
Unlock the full notes with Premium
Premium includes full topic notes, the paper generator, self-quizzes and progress tracking.
Practise this topicPremium
Premium members can generate an exam-style paper with a mark scheme, or take a quick quiz, on The Hertzsprung-Russell diagram.