The Hertzsprung-Russell (H-R) diagram is a graph in which the colour of stars is plotted against their brightness.
The color of a star depends on its temperature.
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 dwarf stars lie to either side of the main sequence stars.
The H-R diagram shows how bright each color star should be. If the star actually looks dimmer, it must be further away. By comparing the brightness predicted by the H-R diagram against how bright a star really looks, astronomers can work out how far away it is (see distances).
The H-R diagram was devised independently by Ejnar Hertzsprung in 1911 and Henry Russell in 1913.