1. The Hertzsprung-Russell Diagram (a.k.a. H-R Diagram)

2. Star Classification 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. Brightness or Magnitude
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 can be described in two ways: apparent magnitude and absolute magnitude.

4. Apparent and Absolute Magnitude
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. HR Diagram
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.