protostar Primitive star formed from the breakup of interstellar clouds. After a fragment becomes detached, it continues to shrink under the influence of its own gravitation, drawing in more gas and dust and increasing in temperature and pressure. Eventually, the outward pressure associated with the rising temperature balances the inward pressure due to gravitation and collapse ceases, perhaps 10 000 years after separation from the cloud. At this stage, the fragment is called a protostar. When internal temperature exceeds 107K hydrogen ‘burning’ begins, marking the transition to a star.
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