Hertzsprung-Russel Diagram

Hertzsprung-Russel diagrams draw relationships between the colour, temperature and luminosity of stars.

• X-Axis: Colour / Temp, linked to Spectral Class
• Y-Axis: Luminosity / Absolute Magnitude

Main sequence stars form a line from the top left to bottom right of the chart. The giants form bands near the top right, while white dwarfs are located in a line near the bottom left.

• Main sequence stars undergo fusion of H –> He (proton-proton)
• Heavier stars, i.e. giants, generally undergo CNO cycle fusion

Characteristics and Evolutionary Stage

Stars can be categorised into three main groups: main sequence, giants and white dwarves.

• Main sequence stars include 90% of stars. They are cool and dim in the bottom right while hot and bright in the top right.
• Giants tend to be massive in size and very bright despite their low temperatures. They are located in the upper-right.
• White dwarves are small and dim but very hot. They form a line near the bottom-left.

Stars generally begin in the main sequence band. But once hydrogen in the core runs out, fusion stops which causes the core to contract. Temperatures rapidly rise, igniting an outer layer of hydrogen and causing the star to expand to a giant.

Rising temperatures allow the CNO cycle to occur until helium in the core depletes also. The core contracts again, causing temperatures to rise again also. An outer layer of helium ignites.

If the star is less than 8 solar masses, it will not reach a sufficient temperature to burn carbon, becoming unstable. It is now a white dwarf.

If the star is greater than 8 solar masses, fusion will continue until all fuel runs out, at which point it will either collapse into a blackhole or become a neutron star which undergoes a supernova.