In our everyday experience things heat and cool quickly
by a combination of CONDUCTION, CONVECTION or RADIATION
In interstellar gas clouds the densities are incredibly
low and thus “conventional” heating and cooling mechanisms that involve large
numbers of atoms don’t work efficiently
For example, if there is only 1 atom of hydrogen gas
per cm3, the chance of that atom colliding with, or transferring thermal
energy to another atom is very small ---- however Interstellar space is big
and the time scales are very long!
