What are the physical laws that determine the internal structure
of stars? We first introduce the Mass-Luminosity Relation for
Main Sequence stars, as well as seeing how the mean density of stars
differs for stars on different parts of the H-R diagram. We then
introduce the Ideal Gas Law, which relates pressure, density, and temperature,
and show how the internal structure of a star is determined by
a continuous tug-of-war between internal pressure trying to blow
the star apart, and self-gravity trying to make it collapse. The
balance between the two is the state of Hydrostatic Equilibrium. How
the balance is maintained, and what happens when it is tipped in
favor of either will determine the appearance and subsequent evolution
of the star. Recorded 2006 January 18 in 1008 Evans Laboratory on the
Columbus campus of The Ohio State University.