Stars - Celestial Objects, Luminous Spheres, Stellar Evolution - A star is a massive, luminous sphere of plasma held together by gravity, with energy produced through nuclear fusion in its core. Stars vary in size, temperature, brightness, and lifespan, depending on their mass. They are classified using the spectral classification system (O, B, A, F, G, K, M), with O-type stars being the hottest and M-type stars the coolest. The Sun, a G-type main-sequence star, is the closest star to Earth and provides the heat and light necessary for life. Stars are born in nebulae, where gravity causes clouds of gas to collapse, forming protostars that eventually ignite nuclear fusion. Throughout their lifetimes, stars evolve based on their mass—low-mass stars become red giants before shedding their outer layers as planetary nebulae and becoming white dwarfs, while massive stars explode as supernovae, leaving behind neutron stars or black holes. Stars are the primary sources of chemical elements, synthesizing heavier elements through nuclear fusion and enriching the universe with carbon, oxygen, iron, and other elements essential for planets and life. Modern telescopes, including JWST and Hubble, study stars across the universe, helping astronomers understand stellar evolution, galaxy formation, and the origins of planetary systems.
