A star is a massive ball of hot plasma held together by its own gravity, generating light and heat through nuclear fusion in its core. Fusion happens when hydrogen atoms are squeezed together under enormous pressure and temperature, forming helium and releasing tremendous energy in the process.
Stars vary enormously in size, temperature, and color. Small, cooler stars glow red or orange, while massive, extremely hot stars burn blue or white. Our Sun is a fairly average, medium-sized yellow star compared to the wide range found across the universe.
Every star exists in a delicate balance between two forces: gravity pulling inward and the pressure from fusion pushing outward. As long as a star has fuel to burn, that balance holds steady, which is why stars can shine for millions or even billions of years.