The physical processes of emission lines in cosmic nebula I think I understand how absorption lines in cosmic bodies occur. But after reading about the emission lines in quasars I am wondering more and more about the physical processes causing the emission lines in cosmic nebula. I hope you can explain these processes to me, more precisely from where to where energy needs to transition and if there are temperature requirements on the involved objects. Below are some of the statements that I encountered and I don't understand:

Emission clouds/nebula need to be ionized - But they need to be only partially ionized, right? If they are fully ionized energy transitions of the electrons wouldn't be possible, so emission lines wouldn't be possible either and there would be only scattering.

Only hot gases create emission lines - Is this really true? Wouldn't it be possible, that a cosmic cloud much cooler than the object illuminating it would also cause emission lines (see drawing below)?

Regarding emission lines from stars I read that they occur due to recombination - is this true? But why only discrete initial energy of the electrons would allow them to recombine with a (partially) ionized atom?


So can you explain the physical processes of nebula emission lines in a bit more detail than the links do? Tnx
Just a short answer, and likely others will fill in more details.

If there is ionization of some atoms, then generally there is recombination as well - you will have both processes going on, roughly in balance with each other.  Typically when an electron recombines with an atom, it does so into some excited state.  Then as it drops from that excited state into lower states, it emits one or more photons on the way down.  This is how ionization gives you emission lines - it’s the inverse process that matters for the emission.

To have emission lines, the cloud only needs to be hotter than the background behind it.  So in your sketch, an observer at the bottom would see an emission spectrum (as you’ve drawn), but an observer to the right would see an absorption spectrum (with the same lines) - from that perspective, more light is lost relative to the (hot) background source than is re-emitted in that direction by the cloud.


This link may help with some additional explanation of these ideas.