Hertzsprung-Russell Diagram

The Hertzsprung-Russell (H-R) diagram is a chart displaying the number of stars that fit into particular categories. The H-R diagram display stars' temperature and brightness. In the chart below the horizontal axis (the X axis, left to right) shows the color and temperature of the star (the two are related). The vertical axis (the Y axis, up and down) shows the luminosity (brightness) of the star. To read the chart, you should know the following:

1. Temperature decreases from left to right (the highest temperature stars are located on the LEFT side of the H-R diagram and the lowest temperatures stars are on the RIGHT). Therefore, Supergiants are some of the lowest temperature stars in our universe.

2. Luminosity increases from the bottom to the top (the brightest stars are located at the TOP of the H-R diagram and the dimmest stars are on the BOTTOM). Therefore, Supergiants are also some of the brightest stars in the universe.



Here's a simpler version of the Herztsprung-Russell Diagram.


Notice where most of the stars are located - the main sequence. The typical rule for stars is: the higher the temperature, the brighter the star. Most stars fit into the Main Sequence because they follow this rule. However, looking at the H-R diagram, you can see that the Supergiants, Giants, and White Dwarfs do not fit this rule and are therefore not located in the main sequence. Supergiants and Giants are very bright stars, but have low temperatures. The White Dwarfs are very dim stars, but have high temperatures.

There are several reasons why most stars are found to be in their main sequence:
1. The mass needed to become a supergiant is very rare. Most stars are far smaller in mass
2. The main sequence is the longest phase in the life span of a star. It can last for billions of years.
3. The Universe itself is between 13-14 billion years old. That is not enough time for a large number of stars to become dwarfs.





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