Deriving Planck's law of blackbody radiation. How to use it to tell the temperature of a star. Discussions about stars -- absorption lines and redshifts, and how to get the temperature correct anyway. Student demo of astronomy course software -- very cool. Counting photons is like counting phonons. (Phonons are quantized vibrational modes in solids.) Visual aid: model of a squishy crystal to demonstrate phonons. Debye law of heat capacity due to phonons in solids. Lecture Audio