When we look at the sky on a dark night what we see is a snapshot of the stellar population. We can represent this snapshot by plotting some of the characteristics of stars on the H-R diagram. The H-R diagram is a plot of a star's temperature as a function of its luminosity (i.e. how bright they are). Different stars have different colours, and one can determine its temperature by studying its colour. We can then use the distrubution of stars in the H-R diagram together with stellar models to trace out the evolution of a star.

If you look at the H-R diagram, above to the left, you will see that most stars are gathered on a diagonal band. That line is called the Main Sequence and stars remain on the Main Sequence for most of its life. You'll also see that there are some stars at the top right that are brighter than those of the main group with the same temperature, they are the Red Giants. The stars at the bottom left are known as White Dwarfs.

To find out more about the evolution of stars click on the part you're interested in using the image above. A short explanation shows up below the H-R diagram and for some groups you will see where in the diagram they are located.