- Devised by Ejnar Hertzsprung and Henry Russell, the Hertzsprung-Russell (H-R) diagram is a graph to plot the temperature of stars against their brightness. The temperature of a star is indicated by its colour,
- Cool stars are red or reddish-yellow
- Hot stars burn white or blue.
- Medium-sized stars form a diagonal band called the main sequence across the graph.
- The whiter and hotter a main sequence star is, the brighter it shines. White stars and blue-white stars are usually bigger and younger.
- The redder and cooler a star is, the dimmer it glows. Cool red stars tend to be smaller and older.
- Giant stars and white dwarfs lies to either side of the main sequence stars.
- The H-R diagram shows how bright each star should be. If the star actually looks dimmer, it must be further away.