I feel uncomfortable labelling nebulae as collection of stars. The more appropriate term is stellar nursery if you want to allude to their role in star formation.
They themselves are just clouds of gas and dust where protostars have begun to form.
Stellar clusters are what you would call a collection of stars.
Also on the note of cosmology and astrophysics being strikingly young fields, I think that's fair statement if we consider their modern definitions. Although their core ideas have already been discussed in a lot of ancient civilizations. It was a lot more philosophical and less rooted in science though (except for the observational astronomy, which remains perhaps one of the oldest scientific discipline).