Star

A star is an massive astronomical object consisting of a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun , a G2 main sequence star, at approximate 150 million kilometers, and the second is Proxima Centauri, a red dwarf star, located 4.24 light-years away from Sun. Many other stars are visible to the naked eye at night, but due to their immense distance from Earth they appear as fixed points of light in the sky, furthermore Sirius A is the brightest star in the night sky with a apparent magnitude of -1.46. The most prominent stars are grouped into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars, but most are invisible to the naked eye from Earth, including all individual stars outside our galaxy, the Milky Way.
Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
== General ==
Unlike a planet, from which most light is reflected, a star shines because of its intense heat. Stellar astronomy is the study of stars. Every star known to humans has either a name or a systematic name, while most of the stars have Bayer designations.
Individual stars differ from each other due to their total mass, their composition, and their age. The total mass determines the course of evolution of a star, as well as its eventual fate. Initially, stars are composed primarily of hydrogen, with some helium and heavier trace elements that determine the metallicity of a star. Over the course of a star's evolution, a portion of the hydrogen is converted into heavier elements through the process of nuclear fusion. Part of the matter is then recycled back into the interstellar environment, where it is used to form a new generation of more metal-rich stars. 
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. For example, a nova occurs when a white dwarf accretes matter from a companion star.
== Classification ==
See also: Stellar classification
The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line. It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.
Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist. The Hertzsprung-Russell diagram can be used to interpret the temperature and luminosity of the spectral class or type of a star.
== Variable stars ==
Main article: Variable star
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira. Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events (and even nuclear fusion). This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova. The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion. Some novae are recurrent, having periodic outbursts of moderate amplitude.
Stars can vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots. A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.
== See Also ==
* The Sun
* Stellar classification
* Variable star
* Binary star
* Star types
=== Lists of stars ===
* List of largest stars
* List of nearest stars
* List of brightest stars
* List of star extremes
== References ==
Category:Stars
Category:Astronomy