Stars condense from clouds of gas, primarily hydrogen, and especially in the concentrations of such gases we call nebulae. As a galaxy rotates, it may produce "density waves" that pass through the gas in its spiral arms. This could be a special trigger to setting off major outbreaks of star births. But, whatever movements make parts of a cloud of hydrogen gas dense enough, the fact remains that these regions do begin to contract under the force of their own increased gravity. As one of these prestellar nebulae contracts, it begins to rotate, for the energy of its previous random motions are conserved. It also begins to flatten and may eventually become a proplyd, a protoplanetary disk that will give rise to planets. And the energy of the inward-falling material produces heat—at the center of the prestellar nebula, a protostar begins to glow.
But it is not yet very hot. If there is not enough mass to keep the heating increasing, the object may become a brown dwarf, or, if even less massive, only a Jupiter-like planet (if such a planet is traveling through space alone or with other planets but with no star, it is called a planemo.) If there is enough mass, the protostar keeps warming, possibly beginning to shine out from open parts of what is now a cocoon nebula. And with similar neighboring cocoons it may be observed as a Harbig-Haro object. Leftover dark gas may appear against bright regions as black clumps known as Bok globules. As their glow brightens but is still only caused by the energy of gravitational contraction, the infant stars may vary in brightness and become T Tauri stars.
The protostar keeps contracting and heating up, moving leftward on the H-R diagram, reaching the main sequence at a height (luminosity) determined by its mass (the more massive, the higher on the H-R diagram). The object has achieved a great enough central pressure and temperature— about 10 or 15 million K—for nuclear fusion to begin. Hydrogen finally starts to be converted to helium. The outward-pushing pressure from the fusion process meets the inward-pulling force of gravity and finds equilibrium. A star is born.
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