TheInfoList

A star is an
astronomical object In , an astronomical object or celestial object is a naturally occurring , association, or structure that exists in the . In , the terms ''object'' and ''body'' are often used interchangeably. However, an astronomical body or celestial body i ...
consisting of a luminous
spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric In mathematics, a quadric or quadric surface (quadric hypersurface in higher dimensions), is a generalization of conic sections (ellipse In math ...

of
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood plasma ...
held together by its own
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

. The nearest star to
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

is the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

. Many other stars are visible to the naked eye at
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of ambient Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospher ...

, but due to their immense distance from Earth they appear as
fixed Fixed may refer to: * Fixed (EP), ''Fixed'' (EP), EP by Nine Inch Nails * ''Fixed'', an upcoming 3D adult animated film directed by Genndy Tartakovsky * Fixed (typeface), a collection of monospace bitmap fonts that is distributed with the X Window ...
points of light in the sky. The most prominent stars are grouped into
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest ...

s and asterisms, and many of the brightest stars have proper names.
Astronomer An astronomer is a in the field of who focuses their studies on a specific question or field outside the scope of . They observe s such as s, s, , s and – in either (by analyzing the data) or . Examples of topics or fields astronomers stud ...

s have assembled
star catalogue A star catalogue (English in the Commonwealth of Nations, Commonwealth English) or star catalog (American English) is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a gre ...
s that identify the known stars and provide standardized
stellar designation In astronomy, stars have a variety of different stellar designations and names, including catalogue designations, current and historical proper name A proper noun is a noun A noun () is a word In linguistics, a word of a spoken language ...
s. The
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
contains an estimated to stars, but most are invisible to the naked eye from Earth, including all individual stars outside our
galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

, the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

. A star's life with the
gravitational collapse Gravitational collapse is the contraction of an astronomical object In , an astronomical object or celestial object is a naturally occurring , association, or structure that exists in the . In , the terms ''object'' and ''body'' are often ...
of a gaseous
nebula A nebula ( for 'cloud' or 'fog'; pl. nebulae, nebulæ or nebulas) is a distinct body of s (which can consist of , , , ; possibly as ). Originally, the term was used to describe any diffused , including beyond the . The , for instance, was once ...

of material composed primarily of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

, along with helium and trace amounts of heavier elements. The total mass of a star is the main factor that determines its
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

and eventual fate. For most of its active life, a star shines due to
thermonuclear fusion Thermonuclear fusion is the process of atoms combining or “fusing” together with huge amounts of heat. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the resulting energy is released in an uncontrolled manner, as it is ...
of hydrogen into
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining") ...

in its core, releasing energy that traverses the star's interior and then into
outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting ...
. At the end of a star's lifetime, its core becomes a
stellar remnant In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects hav ...
: a
white dwarf A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant ...

, a
neutron star A neutron star is the collapsed core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this par ...
, or, if it is sufficiently massive, a . Almost all naturally occurring elements heavier than
lithium Lithium (from el, λίθος, lithos, lit=stone) is a with the Li and  3. It is a soft, silvery-white . Under , it is the least dense metal and the least dense solid element. Like all alkali metals, lithium is highly and flammable, a ...

are created by
stellar nucleosynthesis Stellar nucleosynthesis is the nucleosynthesis, creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the Big Bang nucleosynthesis, original creation of hydrogen, heli ...
in stars or their remnants. Chemically enriched material is returned to the
interstellar medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
by stellar mass loss or
supernova A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

explosions and then recycled into new stars. Astronomers can determine stellar properties including mass, age,
metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astron ...
(chemical composition), variability,
distance Distance is a numerical measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, which can be used to compare with other objects or eve ...
, and motion through
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

by carrying out observations of a star's ,
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a Continuum (theory), continuum. The word was first used scientifically in optics to describe the ...
, and changes in its position on the sky over time. Stars can form orbital systems with other astronomical objects, as in the case of
planetary system A planetary system is a set of gravity, gravitationally bound non-Star, stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems m ...
s and
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster Star clusters are large groups of star ...

s with
two 2 (two) is a number, numeral (linguistics), numeral and numerical digit, digit. It is the natural number following 1 and preceding 3. It is the smallest and only even prime number. Because it forms the basis of a Dualistic cosmology, duality, it ...

or
more More or Mores may refer to: Computing * MORE (application), outline software for Mac OS * more (command), a shell command * MORE protocol, a routing protocol * Missouri Research and Education Network Music Albums * More! (album), ''More!'' (album ...
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 Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered ...
or a galaxy.

# Etymology

The word "star" ultimately derives from the
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
root "h₂stḗr" also meaning star, but further analyzable as h₂eh₁s- ("to burn", also the source of the word "ash") + -tēr (agentive suffix). Compare
Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an appa ...

stella,
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
aster,
German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

Stern. Some scholars believe the word is a borrowing from
AkkadianAkkadian or Accadian may refer to: * The Akkadian language Akkadian ( ''akkadû'', ''ak-ka-du-u2''; logogram: ''URIKI'')John Huehnergard & Christopher Woods, "Akkadian and Eblaite", ''The Cambridge Encyclopedia of the World's Ancient Languages' ...

"istar" (venus), however some doubt that suggestion. Star is cognate (shares the same root) with the following words:
asterisk The asterisk , from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a Typography, typographical symbol. It is so called because it resembles a conventional image of a star (heraldry), star. Computer scientists and mathem ...

,
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

, astral,
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest ...

,
Esther Esther is described in all versions of the Book of Esther The Book of Esther (Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historica ...

.

# Observation history

Historically, stars have been important to
civilization  A civilization (or civilisation) is a complex society A complex society is a concept that is shared by a range of disciplines including anthropology, archaeology, history and sociology to describe a stage of social formation. The concep ...

s throughout the world. They have been part of religious practices, used for
celestial navigation Celestial navigation, also known as astronavigation, is the ancient and modern practice of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their p ...

and orientation, to mark the passage of seasons, and to define calendars. Early astronomers recognized a difference between "
fixed stars The fixed stars ( la, stellae fixae) compose the background of astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical object, physical entity, association, or structure that exists i ...
", whose position on the
celestial sphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...

does not change, and "wandering stars" (
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

s), which move noticeably relative to the fixed stars over days or weeks. Many ancient astronomers believed that the stars were permanently affixed to a
heavenly sphere The celestial spheres, or celestial orbs, were the fundamental entities of the cosmology, cosmological models developed by Plato, Eudoxus of Cnidus, Eudoxus, Aristotle, Ptolemy, Copernicus, and others. In these celestial models, the diurnal mo ...
and that they were immutable. By convention, astronomers grouped prominent stars into asterisms and
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest ...

s and used them to track the motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create
calendars A calendar is a system of organizing days. This is done by giving names to periods of time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, ...
, which could be used to regulate agricultural practices. The
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun. The oldest accurately dated
star chart A star chart or star map, also called a sky chart or sky map, is a celestial cartography, map of the night sky. Astronomers divide these into grids to use them more easily. They are used to identify and locate constellations and astronomical obj ...
was the result of ancient
Egyptian astronomy Egyptian astronomy began in prehistory, prehistoric times, in the Prehistoric Egypt, Predynastic Period. In the 5th millennium BCE, the stone circles at Nabta Playa may have made use of astronomical alignments. By the time the historical Ancient Eg ...
in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in th ...

in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC). The first
star catalogue A star catalogue (English in the Commonwealth of Nations, Commonwealth English) or star catalog (American English) is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a gre ...
in
Greek astronomy#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
was created by
Aristillus Aristyllus ( el, Ἀρίστυλλος; fl. c. 261 BC) was a Greek astronomy, astronomer, presumably of the school of Timocharis (c. 300 BC). He was among the earliest meridian-astronomy observers. Six of his stellar declinations are pres ...
in approximately 300 BC, with the help of
TimocharisTimocharis of Alexandria ( grc-gre, Τιμόχαρις or Τιμοχάρης, ''gen.'' Τιμοχάρους; c. 320–260 BC) was a Greek astronomer and philosopher. Likely born in Alexandria ) , name = Alexandri ...
. The star catalog of
Hipparchus Hipparchus of Nicaea (; el, Ἵππαρχος, ''Hipparkhos'';  BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of precession of the ...
(2nd century BC) included 1020 stars, and was used to assemble
Ptolemy Claudius Ptolemy (; grc-koi, Κλαύδιος Πτολεμαῖος, , ; la, Claudius Ptolemaeus; AD) was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes ...
's star catalogue. Hipparchus is known for the discovery of the first recorded ''
nova A nova (plural novae or novas) is a transient astronomical eventA transient astronomical event, often shortened by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field out ...

'' (new star). Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens,
Chinese astronomers Astronomy in China has a long history stretching from the Shang Dynasty, being refined over a period of more than 3,000 years. The Ancient China, Ancient Chinese people have identified stars from 1300BCE, as Chinese star names later categorized in ...
were aware that new stars could appear. In 185 AD, they were the first to observe and write about a
supernova A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

, now known as the
SN 185 SN 185 was a transient astronomical eventA transient astronomical event, often shortened by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. ...
. The brightest stellar event in recorded history was the
SN 1006 SN 1006 was a supernova that is likely the brightest observed stellar event in recorded history, reaching an estimated −7.5 Apparent magnitude, visual magnitude, and exceeding roughly sixteen times the brightness of Venus. Appearing between Ap ...

supernova, which was observed in 1006 and written about by the Egyptian astronomer
Ali ibn Ridwan Abu'l Hassan Ali ibn Ridwan Al-Misri () (c. 988 - c. 1061) was an Arab The Arabs (singular Arab ; singular ar, عَرَبِيٌّ, ISO 233: , Arabic pronunciation: , plural ar, عَرَبٌ, ISO 233: , Arabic pronunciation: ) are an ethnic g ...
and several Chinese astronomers. The
SN 1054 SN 1054 is a supernova (bright spot on the lower left), a type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supe ...
supernova, which gave birth to the
Crab Nebula The Crab Nebula (catalogue designations M1, NGC 1952, Taurus Taurus is Latin for 'bull' and may refer to: * Taurus (constellation), one of the constellations of the zodiac * Taurus (mythology), one of two Greek mythological characters named ...

, was also observed by Chinese and Islamic astronomers.
Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous
astronomical instruments Astronomical instruments include: *Alidade *Armillary sphere *Astrarium *Astrolabe *Astronomical clock *the Antikythera mechanism, an astronomical clock *Blink comparator *Bolometer *the Canterbury Astrolabe Quadrant *Celatone *Celestial sphere *Cha ...

that could compute the positions of the stars. They built the first large
observatory An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been cons ...

research institutes, mainly for the purpose of producing ''
Zij A zij ( fa, زيج, zīj) is an astronomy in medieval Islam, Islamic astronomical book that tabulates ephemeris, parameters used for astronomy, astronomical calculations of the apparent place, positions of the Sun, Moon, stars, and planets. Etym ...
'' star catalogues. Among these, the ''
Book of Fixed Stars The ''Book of Fixed Stars'' ( ar, كتاب صور الكواكب ', literally ''The Book of the Shapes of Stars'') is an astronomical text written by Abd al-Rahman al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 9 ...
'' (964) was written by the
Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, Persian people, the majority ethnic group in Iran, not to be conflated with the Iranian peoples ** Persian language, an Iranian ...

astronomer
Abd al-Rahman al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 903 in Rey, Iran – May 25, 986 in Shiraz Shiraz (; fa, شیراز, Šîrâz ) is the List of Iranian cities by population, fifth-most-populous city of Iran and the capit ...
, who observed a number of stars,
star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered ...
s (including the
Omicron Velorum Omicron Velorum (ο Vel, ο Velorum) is a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...
and Brocchi's Clusters) and
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

(including the
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

). According to A. Zahoor, in the 11th century, the Persian
polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific pro ...

scholar
Abu Rayhan Biruni Abu Rayhan al-Biruni (973 – after 1050) was an Iranian peoples, Iranian in scholar and polymath during the Islamic Golden Age. He has been variously called as the "founder of Indology", "Father of Comparative religion, Comparative Religion ...
described the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

galaxy as a multitude of fragments having the properties of stars, and gave the
latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

s of various stars during a
lunar eclipse A lunar eclipse occurs when the moves into the . This can occur only when the , Earth, and Moon are exactly or very closely aligned (in ) with Earth between the other two, and only on the night of a . The type and length of a lunar eclipse dep ...

in 1019. According to Josep Puig, the astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of
refraction In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force ...

from sublunary material, citing his observation of the
conjunction Conjunction may refer to: * Conjunction (astronomy), in which two astronomical bodies appear close together in the sky * Conjunction (astrology), astrological aspect in horoscopic astrology * Conjunction (grammar), a part of speech * Logical conjun ...
of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence. Early European astronomers such as
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; 14 December 154624 October 1601) was a Danish astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. T ...

identified new stars in the
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

(later termed ''novae''), suggesting that the heavens were not immutable. In 1584,
Giordano Bruno Giordano Bruno (; ; la, Iordanus Brunus Nolanus; born Filippo Bruno, January or February 1548 – 17 February 1600) was an Italian Dominican friar A friar is a brother and a member of one of the mendicant orders founded in the twelfth ...

suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them, an idea that had been suggested earlier by the ancient
Greek philosophers Ancient Greek philosophy arose in the 6th century BC, at a time when the inhabitants of ancient Greece were struggling to repel devastating invasions from the east. Greek philosophy continued throughout the Hellenistic period The Hellenistic pe ...
,
Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient w ...

and
Epicurus Epicurus, ''Epíkouros'', "ally, comrade" (341–270 BC) was an and who founded , a highly influential school of . He was born on the Greek island of to parents. Influenced by , , , and possibly the , he turned against the of his day and e ...

, and by medieval Islamic cosmologists such as
Fakhr al-Din al-Razi Fakhr al-Dīn al-Rāzī or Fakhruddin Razi ( fa, فخر الدين رازي) (26 January 1150 - 29 March 1210) often known by the sobriquet Sultan of the theologians, was a Persian polymath, Islamic scholar and a pioneer of inductive logic. He w ...
. By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System,
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

suggested that the stars were equally distributed in every direction, an idea prompted by the theologian
Richard Bentley Richard Bentley FRS (; 27 January 1662 – 14 July 1742) was an English classical scholar, critic, and theologian. Considered the "founder of historical philology Philology is the study of language A language is a structured system ...

. The Italian astronomer
Geminiano Montanari Geminiano Montanari. Geminiano Montanari (1 June 1633 – 13 October 1687) was an Italian astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. The ...

recorded observing variations in luminosity of the star
Algol Algol , designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation A constellation is an area on the celestial s ...
in 1667.
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects su ...

published the first measurements of the
proper motion Proper motion is the astrometry, astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the m ...

of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
astronomers Ptolemy and Hipparchus.
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
. His son
John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint, and did botanical wor ...
repeated this study in the southern hemisphere and found a corresponding increase in the same direction. In addition to his other accomplishments, William Herschel is noted for his discovery that some stars do not merely lie along the same line of sight, but are physical companions that form binary star systems. The science of stellar spectroscopy was pioneered by
Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a Bavarian physicist and optical lens manufacturer. He made optical glass and achromatic telescope objective lenses, invented the spectroscope, and developed diffraction gra ...

and
Angelo Secchi Angelo Secchi (; 28 June 1818 – 26 February 1878) was an Italian Catholic priest, astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They ...

. By comparing the spectra of stars such as
Sirius Sirius () is the list of brightest stars, brightest star in the night sky. Its name is derived from the Ancient Greek language, Greek word (, 'glowing' or 'scorching'). The star is designated α Canis Majoris, Latinisation of name ...

to the Sun, they found differences in the strength and number of their
absorption lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency range, c ...
—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types. The modern version of the stellar classification scheme was developed by Annie J. Cannon during the early 1900s. The first direct measurement of the distance to a star (
61 Cygni 61 Cygni is a binary star system in the constellation Cygnus (constellation), Cygnus, consisting of a pair of Stellar classification, K-type dwarf stars that orbit each other in a period of about 659 years. Of apparent magnitude 5 ...
at 11.4
light-years A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric un ...
) was made in 1838 by
Friedrich Bessel Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germa ...
using the
parallax Parallax is a displacement or difference in the apparent positionThe apparent place of an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the ...

technique. Parallax measurements demonstrated the vast separation of the stars in the heavens. Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. discovered the first
spectroscopic binary A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called '' multiple star systems''. These systems, especially when more distant, often appear to the unaided eye as ...
in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as
Friedrich Georg Wilhelm von Struve Friedrich Georg Wilhelm von Struve (russian: link=no, Василий Яковлевич Струве, trans. ''Vasily Yakovlevich Struve''; 15 April 1793 – ) was a Baltic German astronomer and geodesist from the famous Struve family. He is best k ...
and S. W. Burnham, allowing the masses of stars to be determined from computation of
orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the sa ...

. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827. The twentieth century saw increasingly rapid advances in the scientific study of stars. The
photograph A photograph (also known as a photo) is an image An SAR radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz de Tenerife is visible as the purple and white ...
became a valuable astronomical tool.
Karl Schwarzschild Karl Schwarzschild (; 9 October 1873 – 11 May 1916) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, se ...

discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the
photographic magnitude Photography is the art, application, and practice of creating durable image An SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz de Teneri ...
. The development of the
photoelectric The photoelectric effect is the emission of electron The electron is a subatomic particle, symbol or , whose electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electrom ...

photometer A photometer is an instrument that measures the strength of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is t ...

allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an
interferometer . The two light rays with a common source combine at the half-silvered mirror to reach the detector. They may either interfere constructively (strengthening in intensity) if their light waves arrive in phase, or interfere destructively (weakening i ...

on the Hooker telescope at
Mount Wilson Observatory The Mount Wilson Observatory (MWO) is an Observatory#Astronomical observatories, astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson (California), Mount Wilson, a peak in the San Gabriel ...
. Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution.
Cecilia Payne-Gaposchkin Cecilia Helena Payne-Gaposchkin (''née'' Payne; – ) was a British-born American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They ob ...
first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis. The spectra of stars were further understood through advances in
quantum physics Quantum mechanics is a fundamental theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, ...
. This allowed the chemical composition of the stellar atmosphere to be determined. With the exception of rare events such as supernovae and
supernova imposter Supernova impostors are stellar explosions that appear at first to be a supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe ...
s, individual stars have primarily been observed in the
Local Group Local group may refer to: * The Local Group Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way The Local Group is the galaxy group that includes the Milky Way The Milky Way is th ...

, and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy) and its satellites. Individual stars such as Cepheid variables have been observed in the M87 and galaxies of the
Virgo Cluster The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline ...

, as well as luminous stars in some other relatively nearby galaxies. With the aid of
gravitational lensing A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. This effect is know ...

, a single star (named
Icarus In Greek mythology, Icarus (; grc, Ἴκαρος, Íkaros, ) was the son of the master craftsman Daedalus, the creator of the Labyrinth. Icarus and Daedalus attempt to escape from Crete by means of wings that Daedalus constructed from feather ...
) has been observed at 9 billion light-years away.

# Designations

The concept of a constellation was known to exist during the
Babylon ''Bābili(m)'' * sux, 𒆍𒀭𒊏𒆠 * arc, 𐡁𐡁𐡋 ''Babil'' * grc-gre, Βαβυλών ''Babylṓn'' * he, בָּבֶל ''Bavel'' * peo, 𐎲𐎠𐎲𐎡𐎽𐎢 ''Bābiru'' * elx, 𒀸𒁀𒉿𒇷 ''Babili'' *Kassite The Kassites ...

ian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

and these became the basis of
astrology Astrology is a pseudoscience that claims to divination, divine information about human affairs and terrestrial events by studying the movements and relative positions of Celestial objects in astrology, celestial objects. Astrology has be ...
. Many of the more prominent individual stars were given names, particularly with
Arabic Arabic (, ' or , ' or ) is a Semitic language The Semitic languages are a branch of the Afroasiatic language family originating in the Middle East The Middle East is a list of transcontinental countries, transcontinental region ...
or
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...
designations. As well as certain constellations and the Sun itself, individual stars have their own
myths Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psychology), attitude that something is the ca ...

. To the
Ancient Greeks Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era was ...
, some "stars", known as
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

s (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

,
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

,
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

,
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

were taken. (
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

were
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
and
Roman gods The Roman deities most familiar today are those the Romans identified with Greek counterparts (see ''interpretatio graeca ''Interpretatio graeca'' (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the I ...
, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.) Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer
Johann Bayer The constellation Orion in Bayer's ''Uranometria'' Johann Bayer (1572 – 7 March 1625) was a German lawyer A lawyer or attorney is a person who practices law, as an advocate, attorney at lawAttorney at law or attorney-at-law, usual ...
created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

was invented and added to
John Flamsteed John Flamsteed FRS (19 August 1646 – 31 December 1719) was an English astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astr ...

's star catalogue in his book ''"Historia coelestis Britannica"'' (the 1712 edition), whereby this numbering system came to be called ''
Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception Visual perception is the ...
'' or ''Flamsteed numbering''. The internationally recognized authority for naming celestial bodies is the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
(IAU). The International Astronomical Union maintains the
Working Group on Star NamesThe International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) exists to promote and safeguard the science of astronomy through international cooperation, assign official ...
(WGSN) which catalogs and standardizes proper names for stars. A number of private companies sell names of stars which are not recognized by the IAU, professional astronomers, or the amateur astronomy community. The
British Library The British Library is the national library A national library is a library established by a government as a country's preeminent repository of information. Unlike public library, public libraries, these rarely allow citizens to borrow book ...

calls this an
commercial enterprise Business is the activity of making one's living or making money by producing or buying and selling Product (business), products (such as goods and services). Simply put, it is "any activity or enterprise entered into for profit." Having a busi ...
, and the
New York City Department of Consumer and Worker Protection The New York City Department of Consumer and Worker Protection (DCWP), formerly the Department of Consumer Affairs (DCA), is an agency of the Government of New York City The Government of New York City, headquartered at New York City Hall in Low ...
issued a violation against one such star-naming company for engaging in a deceptive trade practice.

# Units of measurement

Although stellar parameters can be expressed in
SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system The metric system is a system of measurement A sys ...
or
Gaussian units Gaussian units constitute a metric system The metric system is a system of measurement A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been import ...
, it is often most convenient to express
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
,
luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...

, and in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of ''nominal'' solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters: : The
solar mass The solar mass () is a standard unit of mass in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial obje ...
M was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the G. Since the product of the Newtonian gravitational constant and solar mass together (GM) has been determined to much greater precision, the IAU defined the ''nominal'' solar mass parameter to be: : The nominal solar mass parameter can be combined with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters. Large lengths, such as the radius of a giant star or the
semi-major axis In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...
of a binary star system, are often expressed in terms of the
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metri ...

—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the
astronomical constant An astronomical constant is any of several physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant (m ...
to be an exact length in meters: 149,597,870,700 m.

# Formation and evolution

Stars condense from regions of
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...
of higher matter density, yet those regions are less dense than within a
vacuum chamber A vacuum is space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
. These regions—known as ''
molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
s''—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the
Orion Nebula The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the Orion (constellation), constellation of Orion. It is one of the brightest nebulae, and is visible to t ...

. Most stars form in groups of dozens to hundreds of thousands of stars. in these groups may powerfully illuminate those clouds,
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
izing the hydrogen, and creating
H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...
s. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation. All stars spend the majority of their existence as ''
main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar Color index, color versus Absolute magnitude, brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams afte ...
stars'', fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass: * ''Very low mass stars'', with masses below 0.5 , are fully convective and distribute
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining") ...

evenly throughout the whole star while on the main sequence. Therefore, they never undergo shell burning and never become
red giant A red giant is a luminous giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Ast ...

s. After exhausting their hydrogen they become helium white dwarfs and slowly cool. As the lifetime of stars is longer than the
age of the universe In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chro ...
, no such star has yet reached the white dwarf stage. * * *

## Star formation

The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). When a region reaches a sufficient density of matter to satisfy the criteria for
Jeans instability In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a ...
, it begins to collapse under its own gravitational force. As the cloud collapses, individual conglomerations of dense dust and gas form "
Bok globule In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...
s". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ph ...

, a
protostar A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star (i.e. that of the Sun or lower), it lasts about 5 ...
forms at the core. These
pre-main-sequence star A pre-main-sequence star (also known as a PMS star and PMS object) is a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown ...
s are often surrounded by a
protoplanetary disk A protoplanetary disk is a rotating circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments i ...

and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 million years for a star like the sun, up to 100 million years for a red dwarf. Early stars of less than 2 are called
T Tauri star T Tauri stars (TTS) are a class of variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates. This variation may be caused by a change in emitted light or by something partly blocking t ...
s, while those with greater mass are
Herbig Ae/Be star A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young (<10Myr) star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lin ...
s. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the
angular momentum In , angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of . It is an important quantity in physics because it is a —the total angular momentum of a closed system remains constant. In three , the ...

of the collapsing star and result in small patches of nebulosity known as
Herbig–Haro object Herbig–Haro (HH) objects are bright patches of nebula, nebulosity associated with newborn stars. They are formed when narrow jets of partially plasma (physics), ionised gas ejected by stars collide with nearby clouds of gas and dust at several ...
s. These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed. Early in their development, T Tauri stars follow the
Hayashi track The Hayashi track is a luminosity–temperature relationship obeyed by infant stars of less than in the pre-main-sequence phase (PMS phase) of stellar evolution. It is named after Japanese astrophysicist Chushiro Hayashi. On the Hertzsprung–R ...
—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the
Henyey track The Henyey track is a path taken by pre-main-sequence stars with masses >0.5 Solar mass in the Hertzsprung–Russell diagram after the end of the Hayashi track. The astronomer Louis G. Henyey and his colleagues in the 1950s showed that the p ...
. Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed. A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.

## Main sequence

Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions in the core region. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago. Every star generates a
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 every year, or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 each year, significantly affecting their evolution. Stars that begin with more than 50 can lose over half their total mass while on the main sequence. The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 , called
red dwarf A red dwarf is the smallest and coolest kind of star on the main sequence. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs c ...
s, are able to fuse nearly all of their mass while stars of about 1 can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature. Since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 are expected to have moved off the main sequence. Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical
concentration In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in t ...

of these elements in a star, its
metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astron ...
. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields, which affects the strength of its stellar wind. Older,
population II During 1944, Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the s ...
stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their
atmospheres The standard atmosphere (symbol: atm) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a ...
.

## Post–main sequence

As stars of at least 0.4 exhaust the supply of hydrogen at their core, they start to fuse hydrogen in a shell surrounding the helium core. The outer layers of the star expand and cool greatly as they transition into a
red giant A red giant is a luminous giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Ast ...

. In some cases, they will fuse heavier elements at the core or in shells around the core. As the stars expand, they throw part of their mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly , 250 times its present size, and lose 30% of its current mass. See also As the hydrogen-burning shell produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 , the mass of the helium core becomes degenerate prior to
helium fusion The triple-alpha process is a set of nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). ...
. Finally, when the temperature increases sufficiently, core helium fusion begins explosively in what is called a
helium flash A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is p ...

, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the
horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...
of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the
red clump The red clump is a clustering of red giant A red giant is a luminous giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, su ...
, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch. After a star has fused the helium of its core, it begins fusing helium along a shell surrounding the hot carbon core. The star then follows an evolutionary path called the
asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in the ...
(AGB) that parallels the other described red-giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During the AGB phase, stars undergo
thermal pulse The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in the ...
s due to instabilities in the core of the star. In these thermal pulses, the luminosity of the star varies and matter is ejected from the star's atmosphere, ultimately forming a planetary nebula. As much as 50 – 70% of a star's mass can be ejected in this mass loss process. Because energy transport in an AGB star is primarily by
convection Convection is single or multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

, this ejected material is enriched with the fusion products dredged up from the core. Therefore, the planetary nebula is enriched with elements like carbon and oxygen. Ultimately, the planetary nebula disperses, enriching the general interstellar medium. Therefore, future generations of stars are made of the "star stuff" from past stars.

### Massive stars

During their helium-burning phase, a star of more than 9 solar masses expands to form first a
blue Blue is one of the three primary colours of pigments in painting and traditional colour theory In the visual arts The visual arts are art forms such as painting Painting is the practice of applying paint Paint is any pig ...
and then a
red supergiant Red is the color at the long wavelength end of the visible spectrum of light, next to Orange (colour), orange and opposite Violet (color), violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the ...
. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss, or from stripping of the outer layers. When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

(see
Carbon-burning process The carbon-burning process or carbon fusion is a set of nuclear fusion 400 px, The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above. Nuclear fusion is the process by which tw ...
). This process continues, with the successive stages being fueled by
neon Neon is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that co ...

(see
neon-burning process The neon-burning process (nuclear decay) is a set of nuclear fusion 400 px, The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above. Nuclear fusion is the process by which two o ...
),
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

(see
oxygen-burning processThe oxygen-burning process is a set of nuclear fusion 400 px, The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above. Nuclear fusion is the process by which two or more atoms are ...
), and
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

(see
silicon-burning process In , silicon burning is a very brief sequence of reactions that occur in massive s with a minimum of about 8–11 solar masses. burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long l ...
). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth. The final stage occurs when a massive star begins producing
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

. Since iron nuclei are more than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.

### Collapse

As a star's core shrinks, the intensity of radiation from that surface increases, creating such
radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is Ab ...

on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 , it shrinks to a relatively tiny object about the size of Earth, known as a
white dwarf A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant ...

. White dwarfs lack the mass for further gravitational compression to take place. The
electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant pressure in addition to, or in lieu of thermal pressure. The description applies to matter composed of electrons, proton A ...
inside a white dwarf is no longer a plasma. Eventually, white dwarfs fade into
black dwarf A black dwarf is a theoretical stellar remnant In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bo ...
s over a very long period of time. In massive stars, fusion continues until the iron core has grown so large (more than 1.4 ) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons,
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

s, and gamma rays in a burst of
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom An atom is the smallest unit of ordinary matter In classical physics ...

and
inverse beta decay Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to p ...
. The
shockwave of an attached shock on a sharp-nosed supersonic F/A-18F Super Hornet in transonic flight Flight or flying is the process by which an object (physics), object motion (physics), moves through a space without contacting any planetary surfac ...
formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before. A supernova explosion blows away the star's outer layers, leaving a
remnant Remnant or remnants may refer to: Religion * Remnant (Bible), a recurring theme in the Bible * Remnant (Seventh-day Adventist belief), the remnant theme in the Seventh-day Adventist Church * The Remnant (newspaper), ''The Remnant'' (newspaper), a ...
such as the Crab Nebula. The core is compressed into a
neutron star A neutron star is the collapsed core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this par ...
, which sometimes manifests itself as a
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the s ...

or
X-ray bursterX-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic power (light), the radiant power emitted by a light ...
. In the case of the largest stars, the remnant is a black hole greater than 4 . In a neutron star the matter is in a state known as
neutron-degenerate matter Degenerate matter is a highly dense state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a dail ...
, with a more exotic form of degenerate matter,
QCD matter Quark matter or QCD matter ( quantum chromodynamic) refers to any of a number of hypothetical phases of matter whose degrees of freedom In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that ma ...
, possibly present in the core. The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

### Binary stars

The evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its
Roche lobe The Roche lobe is the region around a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the ...
, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is overflowed, a variety of phenomena can result, including contact binaries, binaries, cataclysmic variables,
blue stragglers A blue straggler is a main-sequence star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the ...
, and
type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller wh ...
e. Mass transfer leads to cases such as the Algol paradox, where the most-evolved star in a system is the least massive. The evolution of binary and higher-order
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster Star clusters are large groups of star ...

s is intensely researched since so many stars have been found to be members of binary systems. Around half of Sun-like stars, and an even higher proportion of more massive stars, form in multiple systems and this may greatly influence such phenomena as novae and supernovae, the formation of certain types of star, and the enrichment of space with nucleosynthesis products. The influence of binary star evolution on the formation of evolved massive stars such as
luminous blue variable Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in both their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the ...
s, Wolf-Rayet stars, and the progenitors of certain classes of
core collapse supernova (bright spot on the lower left), a type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernova ...
is still disputed. Single massive stars may be unable to expel their outer layers fast enough to form the types and numbers of evolved stars that are observed, or to produce progenitors that would explode as the supernovae that are observed. Mass transfer through gravitational stripping in binary systems is seen by some astronomers as the solution to that problem.

# Distribution

Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical large galaxy like the Milky Way contains hundreds of billions of stars. There are more than 2 trillion (1012) galaxies, though most are less than 10% the mass of the Milky Way. Overall, there are likely to be between and stars (more stars than all the on planet
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

). Most stars are within galaxies, but between 10 and 50% of the starlight in large
galaxy cluster A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous sphe ...
s may come from stars outside of any galaxy. A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars exist. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars. Larger groups are called star clusters. These range from loose
stellar associations A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally unbou ...
with only a few stars to
open cluster An open cluster is a group of up to a few thousand star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest sta ...
s with dozens to thousands of stars, up to enormous
globular cluster A globular cluster is a sphere, spherical collection of stars. wiktionary:globular, Globular clusters are very tightly bound by gravity, with a high concentration of stars towards their centers. Their name is derived from Latin —a small sphere. ...
s with hundreds of thousands of stars. Such systems orbit their host galaxy. The stars in an open or globular cluster all formed from the same
giant molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", Je ...
, so all members normally have similar ages and compositions. Many stars are observed and most or all may have originally formed in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, 80% of which are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, more than two thirds of stars in the Milky Way are likely single red dwarfs. In a 2017 study of the
Perseus molecular cloud The Perseus molecular cloud (Per MCld) is a nearby (~1000 ly) giant molecular cloud in the constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically re ...
, astronomers found that most of the newly formed stars are in binary systems. In the model that best explained the data, all stars initially formed as binaries, though some binaries later split up and leave single stars behind. The nearest star to the Earth, apart from the Sun, is
Proxima Centauri Proxima Centauri is a small, low-mass star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star ...

, away. Travelling at the orbital speed of the
Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbit A low Earth orbit (LEO) is an Earth-centered orbit near the planet, often specified as having a period Period may refer to: Common uses * Era, a length or span of time * ...

, , it would take about 150,000 years to arrive. This is typical of stellar separations in galactic discs. Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos. Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common. Such collisions can produce what are known as
blue straggler A blue straggler is a main-sequence star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the ...
s. These abnormal stars have a higher surface temperature and thus are bluer than stars at the
main sequence turnoff The turnoff point for a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many oth ...
in the cluster to which they belong; in standard stellar evolution, blue stragglers would already have evolved off the main sequence and thus would not be seen in the cluster.

# Characteristics

Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.

## Age

Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed
age of the universe In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chro ...
. The oldest star yet discovered,
HD 140283 HD 140283 (or the Methuselah star) is a metal-poor subgiant star about 200 light years away from the Earth in the constellation Libra (constellation), Libra, near the boundary with Ophiuchus in the Milky Way Galaxy. Its apparent ...
, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old. (Due to the uncertainty in the value, this age for the star does not conflict with the age of the universe, determined by the Planck (spacecraft), Planck satellite as 13.799 ± 0.021). The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.

## Chemical composition

When stars form in the present Milky Way galaxy, they are composed of about 71% hydrogen and 27% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system. The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun. By contrast, the super-metal-rich star Mu Leonis, μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron. Chemically peculiar stars show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements. Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.

## Diameter

Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that Twinkling, twinkle because of the effect of the Earth's atmosphere. The Sun is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds. The disks of most stars are much too small in Angular diameter, angular size to be observed with current ground-based optical telescopes, and so Interferometry, interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed. Stars range in size from neutron stars, which vary anywhere from 20 to in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of the Sun with a much lower density.

## Kinematics

The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion. Radial velocity is measured by the doppler shift of the star's spectral lines and is given in units of km/second, s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements. When both rates of movement are known, the space velocity (astronomy), space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy. A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.

## Magnetic field

The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo theory, dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity. Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time. During the Maunder Minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.

## Mass

One of the most massive stars known is Eta Carinae, which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests as a rough upper limit for stars in the current era of the universe. This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses, but it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 limit on massive star formation. The first stars to form after the Big Bang may have been larger, up to 300 , due to the complete absence of elements heavier than
lithium Lithium (from el, λίθος, lithos, lit=stone) is a with the Li and  3. It is a soft, silvery-white . Under , it is the least dense metal and the least dense solid element. Like all alkali metals, lithium is highly and flammable, a ...

in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at . With a mass only 80 times that of Jupiter (), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core. For stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 . When the metallicity is very low, the minimum star size seems to be about 8.3% of the solar mass, or about 87 . Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants. The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.

## Rotation

The rotation rate of stars can be determined through spectroscopy, spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equatorial bulge, equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart. By contrast, the Sun rotates once every 25–35 days depending on latitude, with an equatorial velocity of 1.93 km/s. A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence. Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind. In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second. The rotation rate of the pulsar will gradually slow due to the emission of radiation.

## Temperature

The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index. The temperature is normally given in terms of an effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. The effective temperature is only representative of the surface, as the temperature increases toward the core. The temperature in the core region of a star is several million kelvins. The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below). Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they have a high luminosity due to their large exterior surface area.

The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind, which streams from the outer layers as electrically charged protons and alpha particle, alpha and beta particles. A steady stream of almost massless neutrinos emanate directly from the star's core. The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers. The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere. Besides visible light, stars emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio frequency, radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics. Using the stellar spectrum, astronomers can determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in Binary star, binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.) With these parameters, astronomers can estimate the age of the star.

## Luminosity

The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power (physics), power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator. Patches of the star's surface with a lower temperature and luminosity than average are known as sunspot, starspots. Small, ''dwarf'' stars such as our Sun generally have essentially featureless disks with only small starspots. ''Giant'' stars have much larger, more obvious starspots, and they exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk. Red dwarf flare stars such as UV Ceti may possess prominent starspot features.

## Magnitude

The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the Extinction (astronomy), extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years). Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times (the nth root, 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude star, second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6. On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (Δ''L'') between two stars is calculated by subtracting the magnitude number of the brighter star (''m''b) from the magnitude number of the fainter star (''m''f), then using the difference as an exponent for the base number 2.512; that is to say: :$\Delta = m_\mathrm - m_\mathrm$ :$2.512^ = \Delta$ Relative to both luminosity and distance from Earth, a star's absolute magnitude (''M'') and apparent magnitude (''m'') are not equivalent; for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41. The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, the latter star appears the brighter of the two. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years. The List of most luminous stars, most luminous known stars have absolute magnitudes of roughly −12, corresponding to 6 million times the luminosity of the Sun. Theoretically, the least luminous stars are at the lower limit of mass at which stars are capable of supporting nuclear fusion of hydrogen in the core; stars just above this limit have been located in the NGC 6397 cluster. The faintest red dwarfs in the cluster are absolute magnitude 15, while a 17th absolute magnitude white dwarf has been discovered.

# 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. In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from ''0'' (hypergiants) through ''III'' (giant star, giants) to ''V'' (main sequence dwarfs); some authors add ''VII'' (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type. The Sun is a main sequence ''G2V'' yellow dwarf of intermediate temperature and ordinary size. There is additional nomenclature in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "''e''" can indicate the presence of emission lines; "''m''" represents unusually strong levels of metals, and "''var''" can mean variations in the spectral type. White dwarf stars have their own class that begins with the letter ''D''. This is further sub-divided into the classes ''DA'', ''DB'', ''DC'', ''DO'', ''DZ'', and ''DQ'', depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.

# Variable stars

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 variable, Cepheid and Cepheid-like stars, and long-period variables such as Mira variable, Mira. Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events. 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
nova A nova (plural novae or novas) is a transient astronomical eventA transient astronomical event, often shortened by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field out ...

e 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.

# Structure

The interior of a stable star is in a state of
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ph ...

: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star. As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 , fusion occurs in a slowly expanding shell around the degenerate matter, degenerate helium core. In addition to hydrostatic equilibrium, the interior of a stable star will maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below. The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (optics), opacity (making radiatative heat transfer inefficient) as in the outer envelope. The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers. Red dwarf stars with less than 0.4 are convective throughout, which prevents the accumulation of a helium core. For most stars the convective zones will vary over time as the star ages and the constitution of the interior is modified. The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear. Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicule (solar physics), spicules appear and solar flare, stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only . Beyond this is the stellar corona, corona, a volume of super-heated plasma that can extend outward to several million kilometres. The existence of a corona appears to be dependent on a convective zone in the outer layers of the star. Despite its high temperature, the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse. From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.

# Nuclear fusion reaction pathways

When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship ''E'' = ''mc''2. A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star. In the Sun, with a 16-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction: :4Hydrogen-1, 1H → 2deuterium, 2H + 2positron, e+ + 2neutrino, νe(2 x 0.4 Melectronvolt, eV) :2positron, e+ + 2positron, e → 2photon, γ (2 x 1.0 MeV) :21H + 22H → 2Helium-3, 3He + 2photon, γ (2 x 5.5 MeV) :23He → Helium-4, 4He + 21H (12.9 MeV) There are a couple other paths, in which He and He combine to form Be, which eventually (with the addition of another proton) yields two He, a gain of one. All these reactions result in the overall reaction: :41H → 4He + 2γ + 2νe (26.7 MeV) where γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts. Each individual reaction produces only a tiny amount of energy, but because enormous numbers of these reactions occur constantly, they produce all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV. In more massive stars, helium is produced in a cycle of reactions catalyst, catalyzed by carbon called the CNO cycle, carbon-nitrogen-oxygen cycle. In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 , helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium: :4He + 4He + 92 keV → isotopes of beryllium, 8*Be :4He + 8*Be + 67 keV → 12*C :12*C → carbon-12, 12C + γ + 7.4 MeV For an overall reaction of: :34He → 12C + γ + 7.2 MeV In massive stars, heavier elements can be burned in a contracting core through the
neon-burning process The neon-burning process (nuclear decay) is a set of nuclear fusion 400 px, The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above. Nuclear fusion is the process by which two o ...
and
oxygen-burning processThe oxygen-burning process is a set of nuclear fusion 400 px, The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above. Nuclear fusion is the process by which two or more atoms are ...
. The final stage in the stellar nucleosynthesis process is the
silicon-burning process In , silicon burning is a very brief sequence of reactions that occur in massive s with a minimum of about 8–11 solar masses. burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long l ...
that results in the production of the stable isotope iron-56. Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.