The primary way molecular clouds cool is by emitting the energy of the heat as light, which can escape the cloud.
A hot cloud of gas means there's lots of atoms and electrons zipping around, bumping into each other. If one of those bumps gives enough energy to an atom's electron, that electron gets excited to a higher energy level. When that excited electron jumps back down to a lower one, it emits the energy difference as light. Hydrogen and Helium are very simple atoms, meaning they only have 1-2 electrons to excite, few energy levels to excite to, and the minimum energy you need to make that first jump is higher. Conversely, "metals" have many more electrons and much more complex energy level structures with smaller, easier to accomplish jumps. Molecules, such as H2O, CO2, etc even more so. Therefore metals and molecules are much more likely to be able to take the energy of thermal collisions and convert it into outgoing light.
Each vertical line corresponds to one possible electron transition in the corresponding atom. You can imagine collisions as randomly firing along these spectra, and only when they hit one of the lines does light get emitted. So you can see, for H and He you're going have a way harder time getting hits. (And, in real life your shots aren't uniformly spread either, but skewed towards the red side at the temperatures molecular clouds are typically at. For hydrogen, you'd only have a single target to try to hit, vs the forest of lines that nitrogen has for example)
AsherKendrell t1_j65rcag wrote
Reply to Why are "metals" more effective at cooling molecular clouds than hydrogen and helium? by Thomas_Bonk
The primary way molecular clouds cool is by emitting the energy of the heat as light, which can escape the cloud.
A hot cloud of gas means there's lots of atoms and electrons zipping around, bumping into each other. If one of those bumps gives enough energy to an atom's electron, that electron gets excited to a higher energy level. When that excited electron jumps back down to a lower one, it emits the energy difference as light. Hydrogen and Helium are very simple atoms, meaning they only have 1-2 electrons to excite, few energy levels to excite to, and the minimum energy you need to make that first jump is higher. Conversely, "metals" have many more electrons and much more complex energy level structures with smaller, easier to accomplish jumps. Molecules, such as H2O, CO2, etc even more so. Therefore metals and molecules are much more likely to be able to take the energy of thermal collisions and convert it into outgoing light.
Take a look at these spectra for a visual: https://images.nagwa.com/figures/628134219594/1.svg
Each vertical line corresponds to one possible electron transition in the corresponding atom. You can imagine collisions as randomly firing along these spectra, and only when they hit one of the lines does light get emitted. So you can see, for H and He you're going have a way harder time getting hits. (And, in real life your shots aren't uniformly spread either, but skewed towards the red side at the temperatures molecular clouds are typically at. For hydrogen, you'd only have a single target to try to hit, vs the forest of lines that nitrogen has for example)