Heating and Cooling
The ISM is usually far from thermodynamic equilibrium. Collisions establish a Maxwell-Boltzmann distribution of velocities, and the 'temperature' normally used to describe interstellar gas is the 'kinetic temperature', which describes the temperature at which the particles would have the observed Maxwell-Boltzmann velocity distribution in thermodynamic equilibrium. However, the interstellar radiation field is typically much weaker than a medium in thermodynamic equilibrium; it is most often roughly that of an A star (surface temperature of ~10,000 K) highly diluted. Therefore, bound levels within an atom or molecule in the ISM are rarely populated according to the Boltzmann formula (Spitzer 1978, ยง 2.4).
Depending on the temperature, density, and ionization state of a portion of the ISM, different heating and cooling mechanisms determine the temperature of the gas.
Read more about this topic: Interstellar Medium
Famous quotes containing the words heating and, heating and/or cooling:
“If the factory people outside the colleges live under the discipline of narrow means, the people inside live under almost every other kind of discipline except that of narrow means—from the fruity austerities of learning, through the iron rations of English gentlemanhood, down to the modest disadvantages of occupying cold stone buildings without central heating and having to cross two or three quadrangles to take a bath.”
—Margaret Halsey (b. 1910)
“If the factory people outside the colleges live under the discipline of narrow means, the people inside live under almost every other kind of discipline except that of narrow means—from the fruity austerities of learning, through the iron rations of English gentlemanhood, down to the modest disadvantages of occupying cold stone buildings without central heating and having to cross two or three quadrangles to take a bath.”
—Margaret Halsey (b. 1910)
“As a bathtub lined with white porcelain,
When the hot water gives out or goes tepid,
So is the slow cooling of our chivalrous passion,
O my much praised but-not-altogether-satisfactory lady.”
—Ezra Pound (1885–1972)