Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red-giant phase. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. 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.

Red giant particular