system
Given the following post, respond as if you're commenting on Reddit:
human
How big does a nebula have to be to form a star?

A friend told me that a nebula can’t form a star if it’s too small; and that if it’s large enough, it will form a red dwarf.

Do we know how big the nebula has to be to form a red dwarf?
gpt
There's lots of different types of nebulae. The smaller ones produced from ejected material from stars - e.g. supernova remnants and planetary nebulae - are too small to form any star, and just continue to expand until they mix into the "interstellar medium", which is the thin gas/plasma that fills most of the volume of the Milky Way.

The nebulae that form stars are massive "molecular clouds". These coalesce from the interstellar medium, through a combination of cooling and gravity.

At very low densities, the main mechanism for cooling is particles bump into each other, converting kinetic energy into "excitation" of each atom (internal wiggles), and this energy can then be radiated out of the (mostly transparent) gas as light photons. So the more often particles bump into each other, the more energy they can radiate out of the system, and the faster the gas can cool down.

This, combined with basic gas dynamics, and gravity, makes cooling a runaway process in interstellar gas.  If one part of the gas is a little bit cooler, then it also has lower pressure (from e.g. the ideal gas equation). This means it can't support itself so well against gravity and against the pressure of the rest of the gas, so it shrinks a bit. This makes it denser, which means the particles are closer together and are bumping into each other more often. More collisions mean you can cooler more quickly, which drops the pressure further, which increases the density, which increases the cooling rate, which decreases the pressure, and so on.

Eventually you get dense enough that gravitational collapse takes over, and the whole thing fragments into *multiple* stars.

These star-forming molecular clouds tend to be very massive - thousands to tens of millions of the mass of our Sun. Each one collapses and fragments to form an entire *open cluster* of stars. The Pleiades are a good example on an open cluster of young stars, where they are still visibly close together in the sky, and you can even make out the cloudiness of the gas around them. The Hyades - the "head" of Taurus - are another example that's easy to see, but they're less dramatic. Both are high and quite visible right now, in both hemispheres.

But a ten million solar mass molecular cloud doesn't collapse all the way into ten million stars. Instead, there is a *feedback* process - new stars stop other stars from forming. The radiation and winds from the first stars, especially the young-lived big blue stars, starts to heat up and blow away the molecular cloud. After a few million years, those big blue stars will go supernovae, and blow away the rest, and you're left with a "dry" open cluster of stars, which slowly drift apart and mix among the rest of the stars in the galaxy, over one or more orbits around the Milky Way.

Typically open clusters have thousands of stars, with the full range of masses. The most massive ones burn the fastest, so the older an open star cluster is, the fewer high mass stars it'll have.