Speaking Wednesday hot, we say that the average kinetic energy increases. When the volumetric heating increases the kinetic energy of each particle. And each particle in a unit of time can increase their energy on the ultimate value in the absence of thermodynamic equilibrium, this means that the heating rate environment is directly proportional to the number of particles in unit volume, ie concentration ? (n, T) = nG (T). The function G (T) [erg / c] is called the efficiency of heating and is calculated through the elementary processes of interaction and radiation.
Ultraviolet radiation stars (photoionization)
Classic photoeffect: quantum energy spent on ionization of atoms with an arbitrary level i and the kinetic energy of the electron. Then the electrons colliding with various particles and the kinetic energy is transferred into energy of chaotic motion, gas heated.
However, it is not so simple. Interstellar gas is composed of hydrogen, which can be ionized only hard UV. And the main «interceptors» UV quanta are atoms of impurities: iron, silicon, sulfur, Kali, etc. They play an important role in establishing the heat balance of cold gas.