How Stars are Classified

Stars shine in many different colors. The color relates to a star's temperature and often its size. Think about the coil of an electric stove as the temperature rises. When you first turn it on, its black. As it gets hotter, it starts to glow a dull red. As it gets even hotter, it become brighter red and then orange. If it gets extremely hot, it may even turn blue white. Like a coil on a stove, a star's color depends on the temperature of its surface. Relatively cool stars are red, warmer stars are orange and yellow and the hottest stars are blue or white. 

The Hertzsprung-Russell (H-R) diagram is a chart that shows the brightness and color of stars. The color of a star is shown across the bottom of the H-R diagram. The colors are expressed as a letter, called a spectral class. They don't match the color name because they are left over from an older system that is no longer used.  For example, M is a red star. The luminosity (brightness) is show along the side. The brightest stars are closer to the top of the diagram. 

Sources
http://www.astro.uni-bonn.de/~javahrd/v071/index.html
http://lcogt.net/files/flash/hr-diagram/main.html

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