One of the most useful tools for classifying stars is the Hertzsprung-Russell (H-R) diagram. It plots stars according to their surface temperature and colour or ‘spectral type’ (on the horizontal axis) and their luminosity (on the vertical axis). When a large number of randomly selected stars are plotted, a pattern soon emerges: most stars are arranged along a diagonal ribbon known as the ‘main sequence’, that runs between the faint, cool and red and the bright, hot and blue. Luminous cool stars and faint hot ones (‘red giants’ and ‘white dwarfs’) occupy regions to either side of the main sequence and are comparatively rare.
1. Main sequence
This is the region where stars spend the majority of their lives – a star’s position on the main sequence is largely determined by its mass.
2. Red giants
Most stars pass through this phase near the end of their lives, brightening and developing an atmosphere with a cool surface.
3. White dwarfs
These hot stars are faint because of their tiny size – they are the burnt-out, slowly cooling cores of stars like our own Sun.
These high-mass stars are brilliantly luminous and display a variety of colours as they move back and forth across the H-R diagram.