: Stars form within cold, dense clouds of gas and dust known as molecular clouds or stellar nurseries. Gravity causes these clouds to collapse into hot, spinning "protostars".
: Once the core temperature reaches a critical point, nuclear fusion begins, and the star enters its most stable phase, fusing hydrogen into helium. : Stars form within cold, dense clouds of
Every star undergoes a distinct life cycle that can last from a few million to trillions of years, depending on its initial mass. Every star undergoes a distinct life cycle that
: When a star exhausts its hydrogen, its outer layers expand. Small stars like the Sun become red giants and eventually shed their layers to become white dwarfs. Massive stars may end in spectacular supernova explosions, leaving behind neutron stars or black holes. Classification and Properties Massive stars may end in spectacular supernova explosions,