[kmaitreys]

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).

[mananaysiempre]

Sorry, yes, there’s a terminological disconnect here: M31, say, is the “Andromeda Galaxy” to us, but the “Andromeda Nebula” to Hubble’s contemporaries circa 1920. The recognition that at least some of the cloudy (nebulous, literally) stuff in the sky is galaxies (and that the universe fits more than one) was the very point of the fight I mentioned. The world before it was thought to be drastically smaller in a way that I find difficult to think about.

[kmaitreys]

That's right. Historically, nebula was anything that looked cloudy, so a lot of astrophysical objects that we now understand are distinct, were simply labelled as nebulous. M31, as you said, being a great example.

Modern astrophysics still carries the baggage of obsolete terminology to this day, from names of objects to names of units.