Ian Czekala teaches a physical account of heating and cooling in H II regions. His nebular energy balance distinguishes the energy spent removing an electron from an atom from the excess photon energy that becomes electron kinetic energy.
In the lecture's account, photoionization supplies heating, while radiative recombination and collisional excitation followed by photon emission remove thermal energy. Balancing these processes determines the gas temperature. This explains why neither the ionization threshold nor the typical photon energy alone specifies the equilibrium temperature.[ref]
Heating and Cooling of HII regions by Ian Czekala. Opening physical overview and Heating by photoionization: excess energy and competing losses
Portrait of Ian Czekala
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