Now we get to some real meat of astrophysics. We discuss the relationship between mass, luminosity, and lifespan of main sequence stars. It is an amazing discovery that mass is known to be tightly correlated to luminosity for Main Sequence stars. Also, the luminosity is tightly related to spectral type. So, just get the spectrum, and you know the mass! This is part of my complete intro Astronomy class that I taught at Willam Paterson University and CUNY Hunter. Hertzsprung-Russell diagram Luminosity/Radius/Temperature Relation Stellal Evolution Stellar Lifetime Main-Sequence Effective Temperatures from a Revised Mass-Luminosity Relation Based on Accurate Properties CNO Cycle Proton-Proton Chain 0:00 Introduction 0:01 Summary of Stellar Properties on the H-R Diagram 1:10 Stellar Radius as a Function of Mass 3:02 Stellar Luminosity as a Function of Mass 3:36 Stellar Masses and the Main Sequence 4:58 Stellar Masses from Close Binary Stars 6:06 Key Properties 9:46 Massive stars are rare! 22:05 The Nuclear Timescale 23:31 Main Sequence Lifetime 25:26 Consequences... 28:07 REVIEW QUESTIONS.