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Glossary term: Hertzsprung–Russell (HR) Diagram

Description: The Hertzsprung–Russell (or HR) diagram is a graph of two observational properties of stars: On the one axis, the total power emitted by stars (luminosity), and on the other axis either their effective temperature or spectral type. Where the effective temperature is used, it is shown on a logarithmic scale, increasing from right to left. The HR diagram is named after two scientists: Ejnar Hertzsprung and Henry Norris Russell who first made different versions of this graph in order to understand the properties of stars. The data points corresponding to the so-called "main sequence stars" lie on a diagonal band from the upper left to lower right in this graph. Data points corresponding to giant stars lie above and to the right of the main-sequence band. White dwarfs lie below and to the left of the band.

The HR diagram can also be a useful framework for representing the evolution of a star over time. Once a star has formed it will be positioned on the main sequence of the HR diagram, and its temperature and luminosity will remain roughly constant for some time. Later, as it evolves, the star's temperature will drop and its luminosity will increase. This means the star's position on the HR diagram moves up and to the right, away from the main sequence towards the giant branch. A star's evolution, specifically its changes in temperature and luminosity, can be represented by a curve in the HR diagram. Thus a star's evolutionary state can be determined from its temperature and luminosity using the HR diagram.

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Related Diagrams


A line of stars goes from cool faint stars to hot bright stars. Some stars lie above or below this line

Hertzsprung-Russell diagram

Caption: This diagram shows the temperature and luminosity of different stars. The size of each point represents the star’s radius and its colour is the colour the human eye would see. The stars range in colour from a washed-out blue to a washed-out reddish-orange. No star has a pure colour like red, green or blue as stars’ spectra include light from lots of different colours. However the reddest stars are commonly referred to as red and the bluest stars as blue. The sample of stars used to make this diagram was chosen to show a wide range of stars of different types so the relative number of each type of star is not representative of how commonly each type is found. From the top left to bottom right there is a long line of stars burning hydrogen in their cores. This is called the main sequence. On this line, one sees the stars Mintaka, Achenar, Sirius A, the Sun and Proxima Centauri. The objects around Proxima Centauri at the lower right end of the main sequence are known as red dwarfs. To the lower right of the red dwarfs are Teide 1 and Kelu-1 A. These two objects are brown dwarfs, objects too low in mass to have cores hot enough to fuse hydrogen for a sustained period of time. As they do not burn hydrogen, brown dwarfs are not considered main sequence stars. The name brown dwarf is unrelated to their colour. Above the main sequence, we find subgiants, giants and supergiants. These are stars that have finished burning hydrogen in their core and have evolved into larger objects. A star’s brightness depends on its temperature and size so giant stars are brighter than stars with a smaller radius but the same temperature. In time these objects will move towards the end of their lives and undergo either a planetary nebula phase or become supernovae. Stars which end their lives with a planetary nebula phase become a type of stellar remnant called a white dwarf. Such objects are much smaller than stars of the same temperature and thus are fainter and are found significantly below the main sequence. Stars which end their lives as supernovae become either black holes or neutron stars. These are not shown on this plot.
Credit: IAU OAE/Niall Deacon

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