The previous chapters having been about purely gravitational and orbital phenomena alone, this chapter introduces microphysical processes and relevant quantities. Adaptive conversions between units have already appeared in previous chapters, and now Planckian units are introduced for convenience in writing formulas, and the conversion to and from standard SI is explained. Planckian units continue to be used throughout the rest of the book. While the topic of quantum ideal gases (a photon gas, a degenerate electron gas, and of course a classical gas) is standard material in physics classes, they are briefly presented here, in an especially concise way using Planckian units. The physics at these tiny scales will be key in determining the macroscopic behaviour of stars and stellar objects in subsequent chapters.