vik dhillon: phy213 - the observed properties of stars - the Hertzsprung-Russell diagram
the Hertzsprung-Russell diagram
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.
