Stellar evolution is the process by which a star changes over the course of time. 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 age of the universe. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. 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. Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star, but as the preponderance of hydrogen gets used up, more and heavier elements are created by fusion processes in layers around the core. The outflow of energy from the core causes it to gradually expand and cool over time, thereby transforming it into a red giant. Stars with at least half to three-quarters of the mass of our Sun will expand further into a supergiant phase before running out of fuel and collapsing into an incredibly dense white dwarf. Stars with greater than three solar masses will continue to collapse until they become dense enough to begin thermonuclear fusion in their cores; these are known as supernovas. Finally, stars that are even more massive will be able to sustain nuclear fusion in their cores even after they collapse, forming neutron stars or black holes depending on their mass and rotation rate.