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A protostar will begin gravitational collapse only if the total gravitational potential energy exceeds twice the thermal energy. In other words, a gas has to be sufficiently cool and sufficiently dense to collapse. Also, as the protostar collapses about half of the gravitational PE is converted into heat, and about half is radiated into space. So suppose we have a protostar cloud with a thermal energy of T. If it is to collapse its gravitational PE should be >2T, say 3T. After some time, the protostar loses 2T in gravitational energy so that its gravitational potential energy is T. But half of the lost energy is now in heat, so the total thermal energy is now 2T. Will this protostar stop collapsing? How much must it collape before nuclear fusion begins?