The so-called Jean's Mass defines the mass that is critically unstable to gravitational collapse for a given mass whose only internal support is thermal pressure.
4 pi Rho 3/2 -3/2
MJ = --------- ( pi Vc) (G Rho)
3
where Rho is the gas density ( gm/cc), and Vc ( cm/sec)
is the thermal sound speed in the gas
which is given by
2
1/2 m V = 3/2 K T
where m is the mass, in grams, of a typical gas atom, T is the gas temperature and
K is Boltzman's Constant.
What this all means is that the minimum mass, in grams, that can collapse depends on its density and temperature:
1 3/2
MJ proportional to ----- T
1/2
Rho
which just means that cold clouds ( 10 - 20 K) yield the smallest masses after
collapse, and the mass is also minimized by gas with the lowest density.
Typical interstellar clouds have densities of about 1000 atoms/cc
and lead to objects with about 0.1 - a few times the mass of the Sun, but
brown dwarfs can also be produced with masses as low as 0.01 times the mass of
the Sun.