the Hertzsprung-Russell diagram back to teaching back to the course start of previous section first page of this section previous page next page last page of this section start of next section help on navigating these pages symbols, constants and quantities

The values of mass, radius, luminosity and effective temperature do not vary independently over the ranges listed above. There are, in fact, two major relationships: luminosity with effective temperature and luminosity with mass. The first of these relations is known as the Hertzsprung-Russell diagram (HR diagram for short) or the colour-magnitude diagram. A schematic HR diagram for stars in the solar neighbourhood is shown in figure 2.

Figure 2:  Schematic Hertzsprung-Russell diagram.

Most of the stars lie in four groups and there are large regions of the HR diagram which contain no stars. The narrow band which contains around 90% of the stars and runs diagonally across the HR diagram from hot, bright stars to faint, cool ones is known as the main sequence. The other groups are known as the giants, supergiants and white dwarfs. The giants and supergiants have higher luminosities than main-sequence stars of the same effective temperature. Given that luminosity is the energy radiated per second by the whole star, and each square centimetre of a giant star radiates the same energy per second as each square centimetre of a main-sequence star of the same effective temperature, it follows that the giants and supergiants must have larger radii than main-sequence stars. Similarly, the white dwarfs have lower luminosities than main-sequence stars of the same effective temperature, which means that they must have smaller radii.

©Vik Dhillon, 27th September 2010