A star is a massive, luminous sphere of plasma which is held together by its own gravity. The closest star to Earth is the Sun, which is the source of most of the planets energy. Some other stars are visible to Earth during the night, appearing as a multitude of fixed luminous points due to their immense distance. Historically, the most prominent stars were grouped into constellations and asterisms, and the brightest stars gained proper names.
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates in outer space. Once the hydrogen in the core of a star is nearly exhausted, almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime and, for some stars, by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers are able to determine the mass, age, and metallicity, and many other properties of a star by observing its motion through space, luminosity, and spectrum, respectively. The total mass of a star is the principal determinant of its evolution and eventual fate. Other characteristics of stars, including diameter and temperature, change over the course of its life, while the star's environment affects its rotation and movement.
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.