The Hertzsprung-Russell diagram shows the relationship between a star's temperature and its luminosity. It is also often called the H-R diagram or colour-magnitude diagram. It is a very useful graph because it can be used to chart the life cycle of a star. We can use it to study groups of stars in clusters or galaxies.
If all stars were alike, all those with the same luminosity would have the same temperature. We might also expect hotter stars to always be brighter than cooler ones. By looking at the chart, we can see this is not the case. There are groups of bright, hot stars and groups of bright, cool stars. There are also groups of dim, hot stars and groups of dim, cool stars.
Most stars, including the Sun, plot in a band which runs from the top-left to the bottom-right of the chart. Stars in this area of the chart are in the main-sequence stage of their lives. We can use the chart to see that the temperature of main-sequence stars increases with brightness. This is because the star's mass controls both its temperature and brightness at this stage.
Red giant and red supergiant stars fall in the top-right of the chart. This tells us they are brighter than main sequence stars but also redder and cooler. This is because they expand and cool as they reach the final stages of their lives. However, because of their large size, they remain very bright.
In the bottom-left of the chart, we find hot stars that are dimmer than main-sequence stars. This is because they have small radii but contain a lot of mass. These are white dwarf stars.
Stars tend to spend about 90% of their life in the main-sequence stage. After this, they evolve into giant stars for the remaining 10 % of their lives. Finally, they will either explode as a supernova or become white dwarf stars.
The chart has been used by astronomers since the early 20th century. In 1911, Danish astronomer Ejnar Hertzsprung made a chart of stars' magnitudes and colours. Two years later, Henry Norris Russell showed that stars could be put into groups based on their luminosity and temperature. The chart is named after both scientists.
In the images on this page, stars' surface temperatures are plotted on the x-axis (horizontal axis). Stars' luminosity is plotted along the y-axis (vertical axis). Be aware that the x-axis of the H-R diagram does not always use the temperature. It can also use spectral class (OBAFGKM), or colour. However, all show the same relationship with a star's luminosity. We can use class, colour, or temperature because stars are considered black-body sources.