They all start as nebulae, vast clouds of dust and gas. Then -- stirred up by a streaking comet or distant supernova -- the resulting force moves through the cloud, causing particles to clump up. The clumps accumulate more mass and gravity, pulling in even more particles. In a million years or so, the clump develops into a dense, small body called a protostar. That protostar keeps drawing in more gas and growing even hotter.
Once the protostar gets hot enough, its hydrogen atoms start fusing, generating helium and an outflow of energy. At this point, the outward push of fusion still doesn't measure up to the inward pull of the star's gravity. If the protostar gathers enough mass, a bipolar flow happens. The star blasts the remaining gas and dust away. At this point, the young star stabilizes and becomes a main sequence star.
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