Stellar Astronomy: Part 4 – Of Dwarfs and Supergiants

It is rather amazing what we can know about stars simply by analyzing their light and with rigorous application of logic and mathematics.  We can measure the distance to any nearby star using parallax.  And by multiplying a star’s apparent brightness by the square of its distance, we can determine its actual luminosity.  Furthermore, we can know both the composition and the surface temperature of a star by analyzing the absorption lines of its starlight using a spectroscope.  This led to the field of stellar classification and the discovery of the main sequence.  But why does this sequence exist?  Why are blue main sequence stars so much brighter than red main sequence stars?