Astaris a luminous sphere ofplasmaheld together by its owngravity. The nearest star toEarthis theSun. Other stars are visible from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped intoconstellationsandasterisms, and the brightest stars gained proper names. Extensivecatalogues of starshave been assembled by astronomers, which provide standardizedstar designations.For at least a portion of its life, a star shines due tothermonuclear fusionofhydrogenintoheliumin its core, releasing energy that traverses the star's interior and thenradiatesintoouter space. Once the hydrogen in the core of a star is nearly exhausted, almost all naturally occurring elements heavier than helium are created bystellar nucleosynthesisduring the star's lifetime and, for some stars, bysupernova nucleosynthesiswhen it explodes. Near the end of its life, a star can also containdegenerate matter.Astronomerscan determine themass, age,metallicity(chemical composition), and many other properties of a star by observing its motion through space,luminosity, andspectrumrespectively. The total mass of a star is the principal determinant of itsevolutionand 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, known as aHertzsprungRussell diagram(HR diagram), allows the age and evolutionary state of a star to be determined.A star's lifebeginswith thegravitational collapseof a gaseousnebulaof 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.[1]The remainder of the star's interior carries energy away from the core through a combination ofradiativeandconvectiveprocesses. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogenfuelat the core is exhausted, a star with at least 0.4 times the mass of the Sun[2]expands to become ared giant, in some cases fusing heavierelementsat the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of its matter into the interstellar environment, where it will contribute to the formation of a new generation of stars with a higher proportion of heavy elements.[3]Meanwhile, the core becomes astellar remnant: awhite dwarf, aneutron star, or (if it is sufficiently massive) ablack hole.Binaryand multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stableorbits. 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 astar clusteror agalaxy.Contents[hide] 1Observation history 2Designations 3Units of measurement 4Formation and evolution 4.1Star formation 4.2Main sequence 4.3Postmain sequence 4.3.1Massive stars 4.3.2Collapse 5Distribution 6Characteristics 6.1Age 6.2Chemical composition 6.3Diameter 6.4Kinematics 6.5Magnetic field 6.6Mass 6.7Rotation 6.8Temperature 7Radiation 7.1Luminosity 7.2Magnitude 8Classification 9Variable stars 10Structure 11Nuclear fusion reaction pathways 12See also 13References 14Further reading 15External linksObservation history

People have seen patterns in the stars since ancient times.[5]This 1690 depiction of the constellation ofLeo, the lion, is byJohannes Hevelius.[6]

The constellation ofLeoas it can be seen by the naked eye. Lines have been added.Historically, stars have been important tocivilizationsthroughout the world. They have been part of religious practices and used forcelestial navigationand orientation. Many ancient astronomers believed that stars were permanently affixed to aheavenly sphere, and that they were immutable. By convention, astronomers grouped stars intoconstellationsand used them to track the motions of theplanetsand the inferred position of the Sun.[5]The motion of the Sun against the background stars (and the horizon) was used to createcalendars, which could be used to regulate agricultural practices.[7]TheGregorian calendar, currently used nearly everywhere in the world, is asolar calendarbased on the angle of the Earth's rotational axis relative to its local star, the Sun.The oldest accurately datedstar chartappeared in ancientEgyptian astronomyin 1534 BC.[8]Theearliest known star catalogueswere compiled by the ancientBabylonian astronomersofMesopotamiain the late 2nd millennium BC, during theKassite Period(ca.15311155 BC).[9]The firststar catalogueinGreek astronomywas created byAristillusin approximately 300 BC, with the help ofTimocharis.[10]The star catalog ofHipparchus(2nd century BC) included 1020 stars, and was used to assemblePtolemy's star catalogue.[11]Hipparchus is known for the discovery of the first recordednova(new star).[12]Many of the constellations and star names in use today derive from Greek astronomy.In spite of the apparent immutability of the heavens,Chinese astronomerswere aware that new stars could appear.[13]In 185 AD, they were the first to observe and write about asupernova, now known as theSN 185.[14]The brightest stellar event in recorded history was theSN 1006supernova, which was observed in 1006 and written about by the Egyptian astronomerAli ibn Ridwanand several Chinese astronomers.[15]TheSN 1054supernova, which gave birth to theCrab Nebula, was also observed by Chinese and Islamic astronomers.[16][17][18]