Astronomers have seen stars forming within a nebular cloud. as the nebular cloud condenses and its own gravitational attraction collapses it, heat and energy build up creating

nuclear fusion

Explanation:

Nuclear fusion is a stage that happens during star formation. This begins in the gravitational collapse of a giant nebula cloud. These clouds can span several dozen cubic light years of space and contain large amounts of matter. When gravity collapses the cloud, heat and energy accumulate, the cloud splits into smaller pieces, each centered around a concentration of matter. As these concentrations increase in mass, gravitation corresponds, and with it the whole process accelerates, with the collapse itself creating thermal energy.

Eventually, these parts condense under heat and pressure into gaseous spheres called proto-stars. If a proto-star does not concentrate enough mass, it never reaches the pressure and heat necessary for nuclear fusion, and becomes a brown dwarf.

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