well, it does not seem that GRAVITY itself ignites collapsing stars. If you just calculate velocity of falling onto each other hydrogen atoms under their own weight, you find it to be high enough to start VERY SLOW p-p fusion when collapsing cloud has size of several Suns. Small clouds (or smal parts of bigger clouds) can not have enough free-fall speed to start even such slow fusion as Sun has, so they are just warm (few thousands K) brown dwarfs. Even Jupiter has some (almost negligible) nuclear fusion inside. Bigger clouds just due to their mass reach higher velocities of infall (temperatures) which correspond to much faster rate of nuclear reactions (according to formula exp(-E/kT)), and the release of energy per atom is higher thus atoms are moving faster, thus their pressure is greater and a star is "fattier" and shines brightly. If a cloud is very big, the temperature of falling gas and thus rate of nuclear reacrions during collapse is so high, that it does not let more gas to fall onto forming star, or may oscillate or even blow.

So, role of gravity is limited just by mutual attraction of hydrogen atoms to each other (by Newton's inverse square law), and as they fall onto each other they gain velocity as any other free-falling body does. Collisions distribute this velocity in all directions, i.e. due to collisions you have thermal motion. If protostar is thick enough, it becomes opaque for radiation, and then radiated by hot atoms infrared energy is absorbed by atoms nearby (or not so nerby) and thus is not lost, preventing cooling. So as protostar shrinks it gets hotter and hotter.

• Re: Re: Re: Re: Re: Re: Re: Re: Has science erred ! - skintless - February 1, 2001 - 04:45 UTC
  • Ignition of nuclear reactions in a protostar. - alexander - February 1, 2001 - 23:18 UTC