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.