The Stars at Night....

http://cde.nwc.edu/SCI2108/course_documents/the_sun/hr_diagram/two/hr_diagram2.htm 

Hetrzsprung and  Russell independently made an interesting observation in 1912 when they attempted to correlate the spectral class of a star with its absolute magnitude. They discovered that 90% of the stars in the Universe fall within a narrow band called "The Main Sequence." They noticed that as a star's surface temperature increased, so does the absolute magnitude. Bluish stars shine brightly, and reddish stars shine more dimly.

To put thing into perspective, consider that to light our sky to the current level with a low end M class star, we would have to be 1000 times closer than we presently are to the Sun, or a mere 150,000 km. At the high end of the O class, we would have to be 200 times farther away than we are now, or at a distance 5 times greater than that of Pluto. This relationship is shown in the chart below, and is called the HR Diagram. It is THE most important diagram in all of Astronomy for it holds the clues to the evolution of all stars, including our own.

 

http://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram