The Hertzsprung-Russel diagram

The Hertzsprung-Russell diagram or H-R diagram, is a graphical representation of temperature and luminosity across the whole  evolutionary timescale of stars.

As described in supernovae & stellar lifecycle, stars are born when they initiate the fusion of hydrogen into helium.  As they continue to burn more and more elements in their cores, their temperatures rises, their envelope size increases, their colour becomes more red, and their luminosity increases.  This happens to all stars regardless of their mass, although the endpoints of low mass stars is significantly different to that of high mass stars.  All this can be rather confusing to understand, so the H-R diagram can graphically illustrate all the changes in all types of star in one.

H-R diagram

The diagram shows 4 main characteristics, colour (spectral class), temperature, mass, and luminosity.

The diagonal band seen running from top left to bottom right is called the main sequence.  This is where stars of all types will spend the vast majority of their lives.  Stars start out slightly above and to the right of their main sequence positions and drop into position when nuclear fusion begins.  They remain where they are until their reservoir of hydrogen is depleted.  It is here that stars begin their voyage away from the main sequence.

Stars with low masses (<5-8Mסּ) begin to swell in size as they begin to burn helium.  Although the core temperature increases, the surface temperature decreases as it increases in size, pushed outward by the internal radiative pressure from the core.  This massive increase in surface area coupled with a drop in surface temperature, leads to a rightward and upward movement in position on the H-R diagram as its colour becomes more red (due to its drop in temperature) and its luminosity increases (due to its increase in surface area).  Here it will remain until its helium supply has been diminished, staying put within the giant branch along with the likes of Aldeberan in Taurus, and Arcturus in Bootes.  Once the outer layers have been blown away (read stellar lifecycle) the remnant core (a white dwarf) with a surface temperature as high as 100,000K will move over to the left hand side of the H-R diagram, and slowly move to the bottom right hand side as it cool to become a black dwarf.

Large high mass stars also follow the same initial track off the main sequence when they run out of hydrogen.  Although the path they follow thereafter is not enormously different from a low mass star, its internal conditions and chemical composition are (read supernovae).