2.  Stars can be classified in three ways:  Size – How massive the star is  Temperature – A stars color reveals its temperature. ▪ Red stars have a cooler temperature, and blue stars have a warmer temperature.  Brightness/Magnitude – The amount of light a star gives off depends on its size and temperature.

3.  How bright a star looks from Earth depends upon both how far the star is from Earth and how bright the star actually is.  A stars brightness can be described in two ways: apparent magnitude and absolute magnitude.

4.  Apparent magnitude is the brightness of a star as seen from Earth.  Absolute magnitude is brightness of a star as if it were 32.6 light years from Earth.  The brightness of the stars is compared to the brightness of our Sun. We call this luminosity.

5.  An HR diagram shows the two most important characteristics of stars, which are temperature and absolute magnitude (brightness) and/or luminosity.  Scientists plot the surface temperatures of stars and their brightness on a graph.