Stars -

Stars are massive, luminous spheres of plasma held together by their own gravity, primarily composed of hydrogen and helium. They are the fundamental building blocks of galaxies and serve as the universe's primary energy producers through nuclear fusion in their cores. While the Sun is the nearest and most well-studied star to Earth, astronomers estimate there are up to one septillion stars in the observable universe. Life Cycle of a Star

: 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". Stars are massive, luminous spheres of plasma held

: 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 Life Cycle of a Star : Stars form

Every star undergoes a distinct life cycle that can last from a few million to trillions of years, depending on its initial mass. Small stars like the Sun become red giants

Astronomers categorize stars based on their spectral characteristics , which correspond to their surface temperatures. Star Basics - NASA Science

: Once the core temperature reaches a critical point, nuclear fusion begins, and the star enters its most stable phase, fusing hydrogen into helium.