Stars Colours Lifecycle H-R Diagram Links
The Hersprung-Russell diagram
The Hersprung-Russell diagram, named after the two astronomers who developed it independantly of each other, shows the relationship between colour, temperature and Absolute Magnitude. The brightest stars are also the largest.
Note that temperature decreases from left to right along the x axis. Note also that oC = K + 273.
Stars on the Main Sequence are converting Hydrogen to Helium. You can find out more about this topic and the stars in general by visiting the Enchanted Learning site's pages.