A star is a massive, luminous sphere of hot plasma held together by its own gravity. Stars primarily consist of hydrogen and helium gas, with nuclear fusion occurring in their cores, where hydrogen atoms fuse into helium, releasing enormous amounts of energy in the form of light and heat. This energy production balances the inward pull of gravity, keeping the star stable for most of its life

. The nearest star to Earth is the Sun, which supports life on our planet by providing light and heat. Stars form in large clouds of gas and dust called stellar nurseries, where gravity pulls material together until nuclear fusion ignites

. Stars vary widely in size, brightness, temperature, and lifespan. Most stars, including the Sun, are classified as main sequence stars, where hydrogen fusion is ongoing in their cores. Over time, stars exhaust their fuel and evolve into different end states such as white dwarfs, neutron stars, or black holes depending on their mass

. Stars often exist in groups such as binary systems, star clusters, or galaxies, and they play a crucial role in creating heavier chemical elements through nucleosynthesis, which are then recycled into new stars and planets

. In summary, a star is a self-luminous celestial body powered by nuclear fusion, fundamental to the structure and evolution of the universe