A star is any massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources. Of the tens of billions of trillions of stars in the observable universe, only a very small percentage are visible to the naked eye.

A star’s gas provides its fuel, and its mass determines how rapidly it runs through its supply, with lower-mass stars burning longer, dimmer, and cooler than very massive stars.

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What Is a Star? A star is a luminous sphere of plasma held together by gravity. It generates energy through nuclear fusion in its core — a process in which lighter atomic nuclei (typically hydrogen) combine to form heavier nuclei (like helium), releasing immense amounts of energy in the process.

The star’s size, temperature, and luminosity slowly evolve when it is in its main sequence, a stage that can last millions or billions of years. Our Sun is currently in the middle of its main sequence phase.

How does a star work? How do they form, live, and eventually die? Learn more about these distant objects and their major importance in the universe.