EVOLUTION OF STARS
The H-R diagram compares the brightness of a star with its temperature. The diagonal line running from the upper left to the lower right is called the Main Sequence. Stars lying on the Main Sequence are blue when they are bright and red when they are dim. Stars in the upper right (called Red Giants) are very bright, but still appear red. Stars near the bottom (known as White Dwarfs) are white, but not very bright. This diagram was developed independently by Ejnar Hertzsprung, a Danish astronomer, and Henry Norris Russell, an American astronomer.