Gravity collapses gas and dust in a nebula into a protostar; rising temperature can start fusion. A main-sequence star is stable when inward gravity is balanced by outward effects associated with its energy output. Sun-like stars become red giants, white dwarfs and eventually black dwarfs in the model; more massive stars become red supergiants, undergo supernovae and leave neutron stars or black holes. Example: Explain protostar formation. Gravity pulls the cloud together. Link cause and effect.