The Mystery of Star Formation: Why Do Gas Clouds Collapse?

[wolram]

stars are formed from gas clouds, these gas clouds posses
potential energy and kinematic energy, they may also exhibit
a density contrast, theory tells us that collapse of the gas
cloud starts when PE overcomes KE, but i can find no explanation
for the trigger to this collapse ,put another way, why don't the
individual molecules just jiggle away for ever?

 

[Labguy]

stars are formed from gas clouds, these gas clouds posses
potential energy and kinematic energy, they may also exhibit
a density contrast, theory tells us that collapse of the gas
cloud starts when PE overcomes KE, but i can find no explanation
for the trigger to this collapse ,put another way, why don't the
individual molecules just jiggle away for ever?

It usually takes a shock wave or some kind of "pertubation event" to cause at least a "density contrast" (higher) enough for the big G to start a matter accumulation that is stronger than the gases' energy to jiggle away. That would be for the first stars formed, since second + generation stars form from gas clouds plus a considerable amount of heavier elements, sometimes just in the form of dust and sometimes in multi-billion ton clumps of any heavy elements that might be lingering around from the death of the first generation star(s). Short version.

 

[enigma]

The phenomena Labguy described is going on all over the universe, but one example which pops into my head is the Orion nebula. Presumably, there was a large supernova in that vicinity several million (billion?) years ago, and now it is a hotbed for star formation.

 

[Nereid]

stars are formed from gas clouds, these gas clouds posses
potential energy and kinematic energy, they may also exhibit
a density contrast, theory tells us that collapse of the gas
cloud starts when PE overcomes KE, but i can find no explanation
for the trigger to this collapse ,put another way, why don't the
individual molecules just jiggle away for ever?

Excellent question!

In addition to Labguy's and enigma's answers, wrt the very early years after matter and radiation decoupled (i.e. the time after the CMB was set free). There were density contrasts (as you say), of total matter (ordinary mass, in this case H and He, PLUS dark matter), left over from the time of inflation. Gravity ensured that, on balance, matter was attracted to regions of higher-than-surroundings density. The dark matter speeded up, passed the region peak density, and shot right on out again (dark matter doesn't collide with itself, or ordinary matter). However the H and He atoms did collide, and in so doing got 'excited' (electrons raised to higher energy levels, atoms got ionised), then 'relaxed' by emitting light (well, some of it was radio waves, IR, UV, ...). The light carried away energy, so the H and He gas 'cooled', and exited the region of peak density slower than the atoms went in. All 'on average', of course, and over-simplified (but not, I hope, too oversimplified).

 

[wolram]

most enlightening, for once i have an unambiguous picture of an event
that i could not fathom, thank you kind lady and gentlemen.