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Star

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Various stars in the Large Magellanic Cloud.
False-color imagery of the Sun, a G-type main-sequence star, the closest to Earth

A star is type of astronomical object consisting of a luminous sphere of superheated gas called plasma bounded together by gravity. The nearest star to our planet, the Earth, is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Astronomers have proposed various star catalogs that identify the known stars and provide standardized stellar designations, such as the Bayer designation, the Henry Draper designations, and the Hipparcos catalogue. However, most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through highly powerful telescopes.

For at least a definite portion of its life, a star would shine due it fusing hydrogen into helium in its core, releasing energy that traverses the star's interior and is expelled into space. 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 can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its stellar evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

A star's life begins as star formation with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[1] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[2] In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[3] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is more massive than 3 solar masses, a Black hole.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[4] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

Sizes[edit]

See List of largest stars.

Classification and life span[edit]

See Stellar Classification.

Variability[edit]

See Category:Variable stars.

References[edit]

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  4. Iben, Icko Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55–114. Bibcode 1991ApJS...76...55I. doi:10.1086/191565.


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