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| Via chandra.harvard.edu |
A star is born when a cloud of gas and dust collapses to the point where the material in the center of the clump is so dense and hot that the nuclear fusion of hydrogen nuclei into helium nuclei can occur. The outflow of energy released by these reactions provides the pressure necessary to halt the collapse.
Depending on their mass, stars reach the end of their evolution as a white dwarf, neutron star or black hole. The cycle begins anew as an expanding supershell from one or more supernovas trigger the formation of a new generation of stars. Brown dwarfs have a mass of only a few percent of that of the Sun and cannot sustain nuclear reactions, so they never evolve.
