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A star is an astronomical object that is made up of a bright spheroid of plasma that is kept together by gravity. The Sun is the closest star to Earth. Many more stars are visible to the human eye at night, but they appear as stationary points of light in the sky due to their great distance from Earth. Many of the brightest stars have proper names, and the most notable stars are organized into constellations and asterisms. Astronomers have put together star catalogs that list all of the known stars and give them standardized names. The observable universe comprises an estimated 1022 to 1024 stars, although most of them, including all individual stars beyond our galaxy, the Milky Way, are invisible to the naked eye from Earth.

The birth of a star begins with the gravitational collapse of a gaseous nebula containing mostly hydrogen, helium, and trace quantities of heavier components. A star’s overall mass is the most important element in determining its evolution and ultimate fate. A star glows throughout the majority of its active life because of the thermonuclear fusion of hydrogen into helium in its core, which releases energy that travels through the star’s interior and eventually radiates into space. A star’s core becomes a stellar remnant at the end of its life: a white dwarf, a neutron star, or, if big enough, a black hole.

Stellar nucleosynthesis in stars or their remains produces almost all naturally occurring metals heavier than lithium. Stellar-mass loss or supernova explosions return chemically enriched material to the interstellar medium, which is then recycled into new stars. By observing a star’s apparent brightness, spectrum, and changes in its location on the sky over time, astronomers may estimate stellar characteristics like mass, age, metallicity (chemical composition), variability, distance, and motion across space.

As in the case of planetary systems and star systems with two or more stars, stars can form orbital systems with other celestial objects. The gravitational interaction between two such stars with a reasonably tight orbit can have a major influence on their evolution. A star cluster or galaxy, for example, might be part of a much bigger gravitationally linked structure.

Early astronomers distinguished between “fixed stars” (planets) that remain stationary on the celestial sphere and “wandering stars” (planets) that shift considerably relative to the fixed stars over days or weeks. Many ancient astronomers thought that the stars were immutably fixed to a celestial sphere. Astronomers utilized asterisms and constellations to follow the movements of the planets and the presumed location of the Sun, as is customary. Calendars were created using the motion of the Sun against the background stars (and the horizon) to control agricultural activities. The Gregorian calendar is a solar calendar based on the angle of the Earth’s rotating axis relative to its local star, the Sun, which is now used virtually everywhere in the globe.

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