The Colour Magnitude Diagram (CMD)

The Colour Magnitude Diagram (or CMD) is a plot of observational data (see Figure 1) which shows how a population of stars can be plotted in terms of their brightness (or luminosity) and colour (or surface temperature). The fact that we are able to interpret a star's colour as a measure of its temperature is based on the idea that stars can be considered as black-body sources, enabling us to use Wien's Law. It is this temperature which we can use to plot the star's spectral type on the x-axis.

The first work in this area was conducted, in 1911, by the Danish astronomer Ejnar Hertzsprung, who produced a graph of stars' magnitudes against their colours. Independently in 1913, the American Henry Russell, showed that there did appear to be some sort of relationship between a star's luminosity and its temperature, and that stars fell into distinct groups. Such a plot is now known as a Hertzsprung-Russell (or H-R) diagram. These theoretical diagrams have since been reproduced for stellar populations such as open and globular clusters and even for galaxies.

If all stars were alike, all those with the same luminosity would have equal temperature and we might expect hotter stars to always be brighter than cooler ones. The diagram below suggests that stars populate specific areas of the CMD. In fact, Figure 1 goes even further and overlays a set of lines denoting where stars of equal radii lie.

Figure 1: A colour magnitude diagram.
Credit: ESO (http://www.eso.org/public/images/)

There appear to be four distinct areas where the stars lie:

  • A diagonal band of stars running from bright, blue stars to faint, red stars, known as the main sequence
  • A horizontal strip of extremely bright stars with a range of colours from blue to red (denoting a range of temperatures from hot to cool), known as supergiants
  • A grouping of red stars lying above (so brighter than) and to the right of the main sequence, known as red giants
  • A group of extremely faint and usually (but not always) blue or blue-white stars, known as white dwarfs. These stars are very often found in the middle of beautiful structures, known as planetary nebulae.

A really useful interactive applet which shows how black-body curves relate to a star's temperature has been produced by the Nebraska Astronomy Applet Project (NAAP).

Astronomers are able to produce these diagrams from open clusters or in fact any population of stars using a technique known as photometry.

Understand more about Wien's Law and spectral types.

Read more about the main sequence.

Find out about red giants, white dwarfs and supergiants.

Start learning about the technique of photometry.