MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Hertzsprung-Russell diagram. A graph showing the brightness (vertical axis) and temperature (and hence colour; horizontal axis) of stars. The position of a star on the Hertzsprung-Russell diagram tells astronomers how much mass it has and how far it has evolved. Most young stars (including the Sun) fall into the main sequence running from the top left to bottom right. The more mass a star has the further it is towards the top left of this band. The clusters in the top right are giants and supergiants, old stars that have swollen in size and become brighter. The bottom cluster contains the white dwarfs, small faint stars that have shrunk and nearly burnt out.
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