![]() Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Nuclear fusion powers a star for most of its existence. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. ![]() All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. The table shows the lifetimes of stars as a function of their masses. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. Stellar evolution is the process by which a star changes over the course of time. Changes to stars over their lifespans Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
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