A question for my fellow DUers interested in the stars...

[Terminally_Chill]

4. not a complete answer.

as gravity crushes a cloud of interstellar gas into a smaller volume, it will create its own gravitational well which will pull in more gas, it will also heat up, if the temperature rise is too large, the pressure generated will halt the collapse of the gas mass until more material falls into the previously created well, which eventually overcomes the heat pressure, and becomes dense enough to initiate fusion.

if WATER is present in the interstellar gas, then the heat created by compression will be radiated away as IR and microwaves, (pure H and He cannot emit IR), the collapse will not stop and the density needed for fusion will occur before any additional mass can fall in.

astrophysicists theorize that only massive stars were formed in the immediate post big bang environment ( which was only H and He)