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H-R Diagram

01H-R Diagram

Axes & regions

This section covers the axes, scales and principal stellar regions of the Hertzsprung-Russell diagram.

Reading an H-R diagram

A Hertzsprung-Russell (H-R) diagram shows how stars are distributed according to their surface temperature and intrinsic brightness.

  • The vertical axis is absolute magnitude, with bright, more negative magnitudes at the top and faint, more positive magnitudes at the bottom.
  • The required absolute-magnitude scale runs approximately from −10-10 at the top to +15+15 at the bottom.
  • The horizontal axis can show surface temperature, decreasing from about 50 000 K50\,000\,\mathrm{K} on the left to 2500 K2500\,\mathrm{K} on the right.
  • Alternatively, it can be labelled with spectral classes in the order O B A F G K M.
  • Temperature therefore runs in the opposite direction from the usual increasing numerical x-axis.
absolute magnitudespectral class¡10¡50+5+10+15OBAFGKMsurface temperature / K50000250001100075006000500035002500main sequencegiantssupergiantswhite dwarfsSun

Stars do not appear randomly across the diagram. They cluster into recognisable regions, especially the main sequence, giants, supergiants and white dwarfs.

The H-R diagram represents stable stellar stages. Brief transitional events such as supernovae do not form persistent regions, while black holes are absent because they are not directly visible through emitted light.

Exam Tip: On a blank H-R diagram, place −10-10 towards the top and +15+15 at the bottom. On the horizontal axis, either show temperature falling from roughly 50 00050\,000 to 2500 K2500\,\mathrm{K} or place the classes O B A F G K M from left to right.

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