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

: Once the core temperature reaches a critical point, nuclear fusion begins, and the star enters its most stable phase, fusing hydrogen into helium. Stars are massive, luminous spheres of plasma held

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