Review

#05

Very patchy - there are dense clouds as well as large zones of low density, hot gas.

#07

An emission nebula is a warmish cloud of gas that manages to have a significant optical glow.

#12

These are cold and rather dense clouds of gas that also have a relatively large amount of dust in them. They are ``dark'' in the sense that (a) they tend to block optical light from behind them and (b) they do not glow much in the optical themselves.