The Pleiades, an open cluster of stars in the constellation of Taurus. The Pleiades (ˈpliːədiːz or /ˈplaɪədiːz/ also known as M 45, the '''Seven Sisters''', Seven Stars, SED, Matariki An open cluster is a group of up to a few thousand Stars that were formed from the same Giant molecular cloud, and are still loosely gravitationally In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture Taurus (it looks like a bull (ˈtɔrəs bull, symbol, Unicode ♉ is one of the Constellations of the Zodiac. NASA photo

A star is a massive, luminous ball of plasma. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Sun (Sol is the Star at the center of the Solar System. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Other stars are visible in the night sky, when they are not outshone by the Sun. For most of its life, a star shines because thermonuclear fusion in its core releases energy that traverses the star's interior and then radiates into outer space. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus The core of the Sun is considered to extend from the center to about 0 Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical

Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy. Luminosity has different meanings in several different fields of science The total mass of a star is the principal determinant in its evolution and eventual fate. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD

A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. [1] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Convection in the most general terms refers to the movement of molecules within Fluids (i The star's internal pressure prevents it from collapsing further under its own gravity. Gravitation is a natural Phenomenon by which objects with Mass attract one another Once the hydrogen fuel at the core is exhausted, those stars having at least 0. Fuel is any material that is burned or altered in order to obtain energy 4 times the mass of the Sun[2] expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements. [3]

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. [4]

## Observation history

Historically, stars have been important to civilizations throughout the world. A Civilization is a society in which large numbers of people share a variety of common elements They have been used in religious practices and for celestial navigation and orientation. A religion is a set of Tenets and practices often centered upon specific Supernatural and moral claims about Reality, the Cosmos Celestial navigation, also known as astronavigation, is a Position fixing technique that was devised to help sailors cross the featureless oceans without having to Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable. The celestial spheres or celestial orbs were the fundamental celestial entities of the cosmological celestial mechanics first invented by Eudoxus, and developed by Aristotle By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is [5] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices. A solar calendar is a Calendar whose dates indicate the position of the earth on its revolution around the Sun (or equivalently the apparent position of the sun moving [6] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to the nearest star, the Sun. The Gregorian calendar is the most widely used Calendar in the world today A solar calendar is a Calendar whose dates indicate the position of the earth on its revolution around the Sun (or equivalently the apparent position of the sun moving

The oldest accurately-dated star chart appeared in Ancient Egypt in 1,534 BCE. Ancient Egypt was an Ancient Civilization in eastern North Africa, concentrated along the lower reaches of the Nile River in what is now [7] Islamic astronomers gave Arabic names to many stars which are still used today, and they invented numerous astronomical instruments which could compute the positions of the stars. Arabic (ar الْعَرَبيّة (informally ar عَرَبيْ) in terms of the number of speakers is the largest living member of the Semitic language In the 11th century, Abū Rayhān al-Bīrūnī described the Milky Way galaxy as multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the A lunar eclipse occurs whenever the Moon passes through some portion of the Earth's shadow [8]

In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. Astronomy in China has a very long history Oracle bones from the Shang Dynasty ( 2nd millennium BC) record eclipses and novae [9] Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. Tycho Brahe, born Tyge Ottesen Brahe ( December 14 1546 &ndash October 24 1601) was a Danish nobleman In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have other planets, possibly even Earth-like, in orbit around them,[10] an idea that had been suggested earlier by such ancient Greek philosophers as Democritus and Epicurus. Giordano Bruno (1548 – February 17, 1600) was an Italian Philosopher best-known as an early proponent of Heliocentrism and An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 Democritus ( Greek:) was a pre-Socratic Greek Materialist Philosopher (born at Abdera in Thrace ca [11] By the following century the idea of the stars as distant suns was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the solar system, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley. Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements Richard Bentley ( January 27, 1662 &ndash July 14, 1742) was an English Theologian, classical scholar and [12]

The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Geminiano Montanari ( June 1 1633 - October 13 1687) was an Italian astronomer, lens -maker and proponent of the Algol (β Per / Beta Persei known colloquially as the Demon Star, is a bright Star in the Constellation Perseus. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions from the time of the ancient Greek astronomers Ptolemy and Hipparchus. Edmond Halley FRS (ˈɛdmənd ˈhɔːlɪ ( November 8, 1656 &ndash January 14, 1742) was an English Astronomer The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for Claudius Ptolemaeus ( Greek: Klaúdios Ptolemaîos; after 83 &ndash ca Hipparchus ( Greek; ca 190 BC &ndash ca 120 BC was a Greek Astronomer, Geographer, and Mathematician of the Hellenistic The first direct measurement of the distance to a star (61 Cygni at 11. Not be confused with 16 Cygni, a more distant system containing two G-type stars harboring the Gas giant planet 16 Cygni Bb. 4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by Friedrich Wilhelm Bessel (22 July 1784 &ndash 17 March 1846 was a German Mathematician, Astronomer, and systematizer of the Bessel functions Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between Parallax measurements demonstrated the vast separation of the stars in the heavens. [10]

William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British During the 1780s, he performed 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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The Galactic Center is the rotational center of the Milky Way Galaxy. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. Sir John Frederick William Herschel 1st Baronet KH, FRS ( March 7, 1792 &ndash May 11, 1871)was an [13] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems. A binary star is a Star system consisting of two Stars orbiting around their Center of mass.

The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy. Joseph von Fraunhofer ( March 6, 1787 &ndash June 7, 1826) was a German optician Father Pietro Angelo Secchi SJ (29 June 1818 &ndash 26 February 1878] was an Italian astronomer. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere. Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared In 1865 Secchi began classifying stars into spectral types. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics [14] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s. Annie Jump Cannon ( December 11, 1863 &ndash April 13, 1941) was an American Astronomer whose cataloging work was instrumental

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. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 day period. Edward Charles Pickering ( July 19 1846 – February 3 1919) was an American Astronomer and Physicist, brother A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. Friedrich Georg Wilhelm von Struve (Vasily Yakovlevich Struve ( April 15, 1793 &ndash November 23, 1864 ( Julian calendar Sherburne Wesley Burnham ( December 12 1838 &ndash March 11 1921) was an American Astronomer. The elements of an orbit are the parameters needed to specify that Orbit uniquely given a model of two point-masses obeying the Newtonian laws of motion and the The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827. [15]

The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. A photograph (often shortened to photo) is an Image created by Light falling on a light-sensitive surface usually Photographic film or an electronic Karl Schwarzschild discovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. Karl Schwarzschild ( October 9, 1873 - May 11, 1916) was a German Jewish Physicist and Astronomer. The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals. Introduction When a Metallic surface is exposed to Electromagnetic radiation above a certain threshold Frequency, the light is absorbed and Electrons In the broadest sense a photometer is any instrument used to measure Illuminance or Irradiance. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope. Albert Abraham Michelson ( December 19, 1852 &ndash May 9, 1931) was a Polish - American Physicist known Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of The Mount Wilson Observatory (MWO is an Astronomical observatory in Los Angeles County California. [16]

Important conceptual work on the physical basis 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. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons This allowed the chemical composition of the stellar atmosphere to be determined. [17]

With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies,[18] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy[19]). A supernova (plural supernovae or supernovas) is a stellar Explosion. The Local Group is the group of galaxies that includes our galaxy the Milky Way. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A star catalogue, or star catalog, is an Astronomical catalogue that lists Stars In Astronomy, many stars are referred to simply by catalogue But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth. The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18 [20] In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster—the most distant stars resolved have up to hundred million light years away[21] (see Cepheids). The Virgo Supercluster or Local Supercluster is the galactic Supercluster that contains the Local Group, the latter containing in its turn The Local Group is the group of galaxies that includes our galaxy the Milky Way. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The Virgo Supercluster or Local Supercluster is the galactic Supercluster that contains the Local Group, the latter containing in its turn The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away[22]—ten times the distance of the most distant star cluster previously observed.

## Star designations

The concept of the constellation was known to exist during the Babylonian period. Designations of stars (and other celestial bodies are done by the International Astronomical Union (IAU In ancient times only the Sun and Moon, a few hundred Stars and the most easily visible Planets had names A star catalogue, or star catalog, is an Astronomical catalogue that lists Stars In Astronomy, many stars are referred to simply by catalogue Babylon was a City-state of ancient Mesopotamia, the remains of which can be found in present-day Al Hillah, Babil Province, Iraq 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 and these became the basis of astrology. The ecliptic is the apparent path that the Sun traces out in the sky during the year Astrology (from Greek grc ἄστρον astron, "constellation star" and grc -λογία -logia) is a group of Systems Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations. Arabic (ar الْعَرَبيّة (informally ar عَرَبيْ) in terms of the number of speakers is the largest living member of the Semitic language Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome.

As well as certain constellations and the Sun itself, stars as a whole have their own myths. The word mythology (from the Greek grc μυθολογία mythología, meaning "a story-telling a legendary lore" [23] They were thought to be the souls of the dead or gods. An example is the star Algol, which was thought to represent the eye of the Gorgon Medusa. In Greek mythology, a gorgon ( Greek: γοργώ or γοργών transl In Greek mythology, Medusa ( Greek: Μέδουσα (Médousa "guardian protectress" was a monstrous Chthonic female character gazing upon

To the Ancient Greeks, some "stars," known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken. Greek religion encompasses the collection of beliefs and rituals practiced in Ancient Greece in the form of both popular public religion and cult practices. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University [23] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance Roman mythology, or more appropriately Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its 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 created a series of star maps and applied Greek letters as designations to the stars in each constellation. Johann Bayer (1572 &ndash March 7, 1625) was a German Astronomer and Lawyer. A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Later the English astronomer John Flamsteed came up with a system using numbers, which would later be known as the Flamsteed designation. John Flamsteed FRS ( 19 August, 1646 - 31 December, 1719) was an English Astronomer and the first Flamsteed designations for Stars are similar to Bayer designations except that they use numbers instead of Greek letters Numerous additional systems have since been created as star catalogues have appeared. A star catalogue, or star catalog, is an Astronomical catalogue that lists Stars In Astronomy, many stars are referred to simply by catalogue

The only body which has been recognized by the scientific community as having the authority to name stars or other celestial bodies is the International Astronomical Union (IAU). [24] A number of private companies (for instance, the "International Star Registry") purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them,[24] and many in the astronomy community view these organizations as frauds preying on people ignorant of star naming procedure. The International Star Registry (ISR founded in 1979, enables buyers to name a Star as a gift or memorial In the broadest sense a fraud is a Deception made for personal gain or to damage another individual [25]

## Units of measurement

Most stellar parameters are expressed in SI units by convention, but CGS units are also used (e. The centimetre-gram-second system ( CGS) is a system of physical units. g. , expressing luminosity in ergs per second). An erg is the unit of Energy and Mechanical work in the centimetre-gram-second (CGS system of units symbol "erg" Mass, luminosity, and radii are usually given in solar units, based on the characteristics of the Sun:

 solar mass: $M_\odot = 1.9891 \times 10^{30}$ kg[26] solar luminosity: $L_\odot = 3.827 \times 10^{26}$ watts[26] solar radius: $R_\odot = 6.960 \times 10^{8}$ m[27]

Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit (AU)—approximately the mean distance between the Earth and the Sun (150 million km or 93 million miles). Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. The solar luminosity, L_\odot is a unit of Luminosity ( power emitted in the form of Photons conventionally used by Astronomers to The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. In Astronomy, the solar radius is a unit of Length used to express the size of Stars It is equal to the current radius of the Sun. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the

## Formation and evolution

Main article: Stellar evolution

Stars are formed within molecular clouds; large regions of high density (though still less dense than the inside of an earthly vacuum chamber) in the interstellar medium. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar vacuum chamber is a rigid enclosure from which air and other gases are removed by a Vacuum pump. These clouds consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming nebula is the Orion Nebula. A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a Diffuse nebula situated south of Orion 's Belt [28] As massive stars are formed from these clouds, they powerfully illuminate and ionize the clouds from which they formed, creating an H II region. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An H II region (also known as Emission nebula) is a cloud of glowing Gas and plasma, sometimes several hundred Light-years across

### Protostar formation

Main article: Star formation

The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae (massive stellar explosions) or the collision of two galaxies (as in a starburst galaxy). Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. A supernova (plural supernovae or supernovas) is a stellar Explosion. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter A starburst galaxy is a Galaxy in the process of an exceptionally high rate of Star formation, compared to the usual star formation rate seen in most galaxies Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force. The Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation

Artist's conception of the birth of a star within a dense molecular cloud. NASA image

As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. A Bok globule is a dark cloud of dense dust and gas in which Star formation sometimes takes place These can contain up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite A protostar is a large object that forms by contraction out of the gas of a Giant molecular cloud in the Interstellar medium. [29] These pre-main sequence stars are often surrounded by a protoplanetary disk. A pre-main sequence star (PMS star or PMS object is a Star in the stage when it has not yet reached the Main sequence. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star The period of gravitational contraction lasts for about 10–15 million years.

Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. T Tauri stars ( TTS) are a class of variable Stars named after their prototype – T Tauri. Herbig Ae/Be stars are Pre-main sequence stars - young ( Spectral types A and B These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects. Herbig-Haro objects are small patches of nebulosity associated with newly-born Stars and are formed when Gas ejected by young Stars collides [30]

### Main sequence

Main article: Main sequence

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Such stars are said to be on the main sequence and are called dwarf stars. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. [31] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4. 6 billion years ago. [32]

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. For most stars, the amount of mass lost is negligible. The Sun loses 10−14 solar masses every year,[33] or about 0. 01% of its total mass over its entire lifespan. However very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution. [34] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence. [35]

An example of a Hertzsprung-Russell diagram for a set of stars that includes the Sun (center). The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD (See "Classification" below. )

The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K At the end of their lives, they simply become dimmer and dimmer. [2] However, since the lifespan of such stars is greater than the current age of the universe (13. 7 billion years), no such stars are expected to exist yet.

Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[36] and modify the strength of the stellar wind. [37] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " )

### Post-main sequence

As stars of at least 0. 4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 For example, in about 5 billion years, when the Sun is a red giant, it will expand out to a maximum radius of roughly 1 AU (150,000,000 km), 250 times its present size. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand As a giant, the Sun will lose roughly 30% of its current mass. [32][38]

In a red giant of up to 2. 25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core. [39] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. Helium fusion is a kind of Nuclear fusion, with the nuclei involved being Helium. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium. [40]

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.

#### Massive stars

Betelgeuse is a red supergiant star approaching the end of its life cycle

During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Betelgeuse (ˈbiːtəldʒuːz or /ˈbɛtəldʒuːz/ ( α Ori α Orionis Alpha Orionis is a Semiregular variable star located 640 Light-years away from Red supergiants ( RSG s are Supergiant Stars ( luminosity class I) of spectral type K or M Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium.

The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 The carbon burning process is a set of Nuclear fusion reactions that take place in massive Stars (at least 4 MSun at birth that have used up This process continues, with the successive stages being fueled by neon (see neon burning process), oxygen (see oxygen burning process), and silicon (see silicon burning process). Neon (ˈniːɒn is the Chemical element that has the symbol Ne and Atomic number 10 The neon burning process is a set of Nuclear fusion reactions that take place in massive Stars (at least 8 MSun) Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The oxygen burning process is a set of Nuclear fusion reactions that take place in massive Stars that have used up the lighter elements in their cores Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 In Astrophysics, Silicon burning is a two week sequence of Nuclear fusion reactions that occur in massive Stars with a minimum of about 8–11 Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth. [41]

The final stage is reached when the star begins producing iron. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy—the process would, on the contrary, consume energy. Binding energy is the Mechanical energy required to disassemble a whole into separate parts Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. Nuclear fission is the splitting of the nucleus of an atom into parts (lighter nuclei) often producing Free neutrons and other smaller nuclei which may [39] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

#### Collapse

An evolved, average-size star will now shed its outer layers as a planetary nebula. A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die If what remains after the outer atmosphere has been shed is less than 1. 4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. [42] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion White dwarfs will eventually fade into black dwarfs over a very long stretch of time. A black dwarf is a hypothetical Star, created when a White dwarf becomes sufficiently cool to no longer emit significant Heat or Light

The Crab Nebula, remnants of a supernova that was first observed around 1050 AD

In larger stars, fusion continues until the iron core has grown so large (more than 1. The Crab Nebula  (catalogue designations M 1 NGC 1952 Taurus A is a Supernova remnant and Pulsar wind nebula in the Constellation 4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. In Nuclear physics, beta decay is a type of Radioactive decay in which a Beta particle (an Electron or a Positron) is emitted Electron capture (sometimes called inverse beta decay) is a Decay mode for Isotopes that will occur when there are too many Protons in the The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called A supernova (plural supernovae or supernovas) is a stellar Explosion. Supernovae are 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 existed before. [43]

Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula[43]) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves X-ray bursters are a class of binary stars which have periodic outbursts luminous in X-rays. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e [44] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion Quark matter or QCD matter (see QCD) refers to any of a number of theorized phases of matter whose degrees of freedom include Quarks and Gluons Within a black hole the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. 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. [43]

## Distribution

A white dwarf star in orbit around Sirius (artist's impression). NASA image

In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit around each other. A multiple star consists of three or more Stars which appear from the Earth to be close to one another in the sky The most common multi-star system is a binary star, but systems of three or more stars are also found. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of co-orbiting binary stars. [45] Larger groups called star clusters also exist. Star clusters are groups of Stars which are gravitationally bound These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. A stellar association is a very loose Star cluster, looser than both Open clusters and Globular clusters Stellar associations will normally contain from A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite.

It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth. [46]

Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe. In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that [47] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered. [48] Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the observable universe. Names of numbers larger than a quadrillion are almost never used for reasons discussed further below [49] That is 230 billion times as many as the 300 billion in the Milky Way.

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39. Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 9 trillion (1012) kilometres, or 4. 2 light-years away. Light from Proxima Centauri takes 4. 2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle (5 miles per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there. NASA 's Space Shuttle, officially called the Space Transportation System ( STS) is the Spacecraft currently used by the United States [50] Distances like this are typical inside galactic discs, including in the vicinity of the solar system. A disc is a component of Disc galaxies, such as Spiral galaxies, or Lenticular galaxies. [51] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the

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. [52] Such collisions can produce what are known as blue stragglers. Blue stragglers ( BSS) are Stars in open or Globular clusters that are hotter and bluer than other cluster stars having the same Luminosity These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster . [53]

## Characteristics

The Sun is the nearest star to Earth

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

### Age

Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13. 7 billion years old—the observed age of the universe. The age of the Universe is the time elapsed between the theory of the Big Bang and the present day The oldest star yet discovered, HE 1523-0901, is an estimated 13. HE 1523-0901 is the designation given to a Red giant Star located in the Milky Way galaxy 2 billion years old. [54]

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 about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. [55][56]

### Chemical composition

When stars form they are composed of about 70% hydrogen and 28% helium, as measured by mass, with a small fraction of heavier elements. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen 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. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age. [57] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system. [58]

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. [59] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron. Mu Leonis (μ Leo / μ Leonis is a Star in the Constellation Leo. [60] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements. In astrophysics peculiar stars have distinctly unusual metal abundances at least in their surface layers Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24 Rare earth elements and rare earth metals are according to IUPAC, the collection of seventeen Chemical elements in the Periodic table, namely [61]

### Diameter

Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through the atmosphere. The Sun is also a star, but it 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. R Doradus (also called HD 29712) is the name of a Red giant Mira variable Star in the far-southern Constellation Dorado 057 arcseconds. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. [62]

The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required in order to produce images of these objects. The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of Another technique for measuring the angular size of stars is through occultation. In Shia Islam The Occultation is a term used to designate the hidden state of the Imam of the Time. 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. [63]

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun—about 0. Supergiants are among the most massive Stars In the Hertzsprung-Russell diagram they occupy the top region of the diagram Betelgeuse (ˈbiːtəldʒuːz or /ˈbɛtəldʒuːz/ ( α Ori α Orionis Alpha Orionis is a Semiregular variable star located 640 Light-years away from Orion (ɒˈraɪən a Constellation often referred to as The Hunter, is a prominent constellation one of the largest and perhaps the best-known and most conspicuous 9 billion kilometres. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand However, Betelgeuse has a much lower density than the Sun. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different [64]

### 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 the Velocity of an object in the direction of the line of sight (i The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for

Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units The proper motion of a star is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. By determining the parallax of a star, the proper motion can then be converted into units of velocity. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements. [65]

Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. The space velocity of an Astronomical object is its actual motion relative to the Sun or the Local standard of rest. Among nearby stars, it has been found that 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. [66] Comparison of the kinematics of nearby stars has also led to the identification of 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 These are most likely groups of stars that share a common point of origin in giant molecular clouds. [67]

### Magnetic field

Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman-Doppler imaging

The magnetic field of a star is generated within regions of the interior where convective circulation occurs. A stellar magnetic field is a Magnetic field generated by the motion of conductive plasma inside a Main sequence ( Hydrogen -burning T Tauri stars ( TTS) are a class of variable Stars named after their prototype – T Tauri. In Astrophysics, Zeeman - Doppler imaging is a tomographic technique dedicated to the Cartography of stellar Magnetic fields In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Convection in the most general terms refers to the movement of molecules within Fluids (i This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. The dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a Magnetic field. 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. Starspots are equivalent to sunspots but located on other stars Coronal loops are arching magnetic fields that reach out into the corona from active regions. Coronal loops form the basic structure of the lower Corona and Transition region of the Sun. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares [68]

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, however, functioning as a brake to gradually slow the rate of rotation as the star grows older. 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. [69] During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity. The Maunder Minimum is the name given to the period roughly from 1645 to 1715, when Sunspots became exceedingly rare as noted by solar observers of

### Mass

One of the most massive stars known is Eta Carinae,[70] with 100–150 times as much mass as the Sun; its lifespan is very short—only several million years at most. Eta Carinae (η Carinae or η Car is a Hypergiant Luminous blue variable Star in the Carina constellation. A recent study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe. The Arches Cluster is the densest known Star cluster in the Milky Way and is located about 100 Light years away from the center of our galaxy, in the constellation [71] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. The Eddington luminosity (also referred to as the Eddington limit) in a star is defined as the point where the gravitational force inwards equals the continuum radiation force

The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3. In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby Star or stars NGC 1999 is a dust filled Bright nebula. It is a Reflection nebula, and shines from the light of the Variable star V380 5 times the mass of the Sun. NASA image

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[72] due to the complete absence of elements heavier than lithium in their composition. Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 This generation of supermassive, population III stars is long extinct, however, and currently only theoretical. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. AB Doradus is a pre-main sequence trinary star system in the Constellation Dorado. [73] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter. [74][75] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8. 3% of the solar mass, or about 87 times the mass of Jupiter. [76][75] Smaller bodies are called brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily

The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than 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. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared [17]

### Rotation

Main article: Stellar rotation

The rotation rate of stars can be approximated through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. Stellar rotation is the angular motion of a Star about its axis Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ Starspots are equivalent to sunspots but located on other stars Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. Achernar (α Eri / α Eridani / Alpha Eridani sometimes spelled Achenar, is the brightest star in the Constellation Eridanus and the eighth-brightest This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart. [77] By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1. 994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness [78]

Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. In Astronomy, the term compact star (sometimes compact object) is used to refer collectively to White dwarfs Neutron stars other exotic 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. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind. [79] 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 Crab Nebula  (catalogue designations M 1 NGC 1952 Taurus A is a Supernova remnant and Pulsar wind nebula in the Constellation [80] 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 at the core and the radius of the star and is often estimated from the star's color index. In Astronomy, the color index is a simple numerical expression that determines the color of an object which in the case of a Star gives its Temperature [81] It is normally given as the 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. Star The effective temperature of a Star is the temperature of a Black body with the same luminosity per surface area (\mathcal{F}_{Bol} In Physics, a black body is an object that absorbs all light that falls on it Note that the effective temperature is only a representative value, however, as stars actually have a temperature gradient that decreases with increasing distance from the core. [82] The temperature in the core region of a star is several million kelvins. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic [83]

The stellar temperature will determine the rate of energization or ionization of different 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). In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard [17]

Massive main sequence stars can have surface temperatures of 50,000 K. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Smaller stars such as the Sun have surface temperatures of a few thousand degrees. Red giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area. [84]

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Particle radiation is the radiation of Energy by means of fast-moving Subatomic particles. The particle radiation emitted by a star is manifested as the stellar wind[85] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos emanating from the star’s core. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive Alpha particles (named after and denoted by the first letter in the Greek alphabet, α consist of two Protons and two Neutrons bound together into a Beta particles are high-energy high-speed Electrons or Positrons emitted by certain types of Radioactive nuclei such as Potassium -40 Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost

The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion reaction. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena This energy is converted to other forms of electromagnetic energy, including visible light, by the time it reaches the star’s outer layers. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy. The surface gravity, g, of an astronomical or other object is the Gravitational acceleration experienced at its surface A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation If the distance of the star is known, 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 measured directly for stars in binary systems. A binary system is an astronomical term referring to two objects in space (usually Stars but also Planets galaxies or Asteroids which The technique of gravitational microlensing will also yield the mass of a star. Gravitational microlensing is an astronomical phenomenon due to the Gravitational lens effect [87]) With these parameters, astronomers can also estimate the age of the star. [88]

### Luminosity

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Radiant energy is the Energy of Electromagnetic waves The quantity of radiant energy may be calculated by integrating Radiant flux (or power For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux—the amount of energy radiated per unit area—across their entire surface. In the various subfields of Physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks The rapidly-rotating star Vega, for example, has a higher energy flux at its poles than along its equator. Vega (α Lyr / α Lyrae / Alpha Lyrae ( or) is the brightest Star in the Constellation Lyra, the fifth brightest star in the night The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the [89]

Surface patches with a lower temperature and luminosity than average are known as starspots. A sunspot is a region on the Sun 's surface ( Photosphere) that is marked by intense magnetic activity which inhibits Convection, forming Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,[90] and they also exhibit strong stellar limb darkening. Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or " Limb " of the image That is, the brightness decreases towards the edge of the stellar disk. [91] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features. A flare star is a Variable star which can undergo unpredictable dramatic increases in brightness for a few minutes Luyten 726-8 is a binary Star system that is one of Earth's nearest neighbors the closest star in the constellation Cetus [92]

### Magnitude

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light. Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32. 6 light-years), and it is directly related to a star’s luminosity.

Number of stars brighter than magnitude
Apparent
magnitude
Number
of Stars[93]
04
115
248
3171
4513
51,602
64,800
714,000

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[94] (the 5th root of 100 or approximately 2. In Mathematics, an n th root of a Number a is a number b such that bn = a. 512). This means that a first magnitude (+1. 00) star is about 2. 5 times brighter than a second magnitude (+2. 00) star, and approximately 100 times brighter than a sixth magnitude (+6. 00) star. 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 brightest stars, on either scale, have negative magnitude numbers. The variation in brightness between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2. 512; that is to say:

Δm = mfmb
2. 512Δm = variation in brightness

Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not equivalent for an individual star;[94] 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, however, Sirius appears brighter than Canopus. 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.

As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14. LBV 1806-20 is a Luminous blue variable or possible Binary star located 30000&ndash49000 Light-years from the Sun, towards the center of the 2. This star is at least 5,000,000 times more luminous than the Sun. [95] The least luminous stars that are currently known are located in the NGC 6397 cluster. NGC 6397 is a Globular cluster in the Ara Constellation. It is located about 7200 Light-years from Earth, making it one of the The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth. [96]

## Classification

Surface Temperature Ranges for
Different Stellar Classes
[97]
ClassTemperatureSample star
O33,000 K or moreZeta Ophiuchi
B10,500–30,000 KRigel
A7,500–10,000 KAltair
F6,000–7,200 KProcyon A
G5,500–6,000 KSun
K4,000–5,250 KEpsilon Indi
M2,600–3,850 KProxima Centauri

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. Zeta Ophiuchi (ζ Oph / ζ Ophiuchi is a Star located in the Constellation of Ophiuchus. Rigel (ˈraɪʤəl (β Ori / β Orionis / Beta Orionis is the brightest star in the constellation Orion and the sixth brightest star in the sky with Visual Altair (α Aql / α Aquilae / Alpha Aquilae / Atair is the brightest Star in the Constellation Aquila and the twelfth brightest star This article is about the star Procyon is also the mammalian genus to which raccoons belong The Sun (Sol is the Star at the center of the Solar System. Epsilon Indi (ε Ind / ε Indi is an Orange dwarf Star approximately 12 Light-years away in the Constellation of Indus. Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics The twentieth century of the Common Era began on The hydrogen line, 21 centimeter line or HI line refers to the Spectral line created by changes in the energy state of neutral Hydrogen and [98] It was not known at the time that the major influence on the line strength was temperature, and when the classifications were reordered by temperature, it more closely resembled the modern scheme. [99]

There are different single-letter classifications of stars 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 have 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: class O0 and O1 stars may not exist. [100]

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 the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). A hypergiant ( luminosity class 0) is a Star with a tremendous Mass and Luminosity, showing signs of a very high rate of mass loss A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness In the context of Stellar nucleosynthesis, a hydrogen burning process can refer to either the Proton-proton chain reactions dominant in Main sequence These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type. [100] Our Sun is a main sequence G2V yellow dwarf, being of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters, can follow 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. [100]

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 index. [101]

## Variable stars

Main article: Variable star
The asymmetrical appearance of Mira, an oscillating variable star. For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson Mira, ˈmaɪrə also known as Omicron Ceti (or ο Ceti / ο Cet is a Red giant Star estimated 200-400 Light years away in NASA HST image

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation Mira variables, named after the star Mira (ˈmairə are a class of pulsating Variable stars characterized by very red colors pulsation periods longer than 100 days [102]

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events. [102] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars. A flare star is a Variable star which can undergo unpredictable dramatic increases in brightness for a few minutes

Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White 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. [4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion. [103] Some novae are also recurrent, having periodic outbursts of moderate amplitude. [102]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots. [102] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2. 3 to 3. 5 over a period of 2. 87 days.

## Structure

Main article: Stellar structure

The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. Stars of different mass and age have varying internal structures. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. In Vector calculus, the gradient of a Scalar field is a Vector field which points in the direction of the greatest rate of increase of the scalar The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. In atmospheric sciences ( Meteorology, Climatology and related fields the pressure gradient (typically of air, more generally of any Fluid) The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic 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. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus [104][105]

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions 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 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core. [106]

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and 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.

This diagram shows a cross-section of a solar-type star. NASA image

The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. The radiation zone is the middle zone in the Sun 's interior Energy travels out of the core into the radiation zone In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. The convection zone of a Star is the range of radii in which energy is transported primarily by Convection. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity as in the outer envelope. Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation especially visible Light. [105]

The occurrence of convection in the outer envelope of a main sequence star depends on the 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. [107] Red dwarf stars with less than 0. 4 solar masses are convective throughout, which prevents the accumulation of a helium core. [2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified. [105]

The portion of a star that is visible to an observer is called the photosphere. 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 out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear. A sunspot is a region on the Sun 's surface ( Photosphere) that is marked by intense magnetic activity which inhibits Convection, forming

Above the level of the photosphere is the stellar atmosphere. The stellar atmosphere is the outer region of the volume of a Star, lying above the Stellar core, Radiation zone and Convection zone. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. The chromosphere (literally "colour sphere" is a thin layer of the Sun 's atmosphere just above the Photosphere, roughly 10000 kilometres deep In Solar physics, a spicule is a dynamic jet of about 500km diameter on the Sun. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres. A corona is a type of plasma " atmosphere " of the Sun or other celestial body extending millions of Kilometres into space most easily [108] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star. [107] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse. A solar eclipse occurs when the Moon passes between the Sun and the Earth so that the Sun is wholly or partially obscured

From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the [109]

## Nuclear fusion reaction pathways

Overview of the proton-proton chain
The carbon-nitrogen-oxygen cycle

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in Stars to build the nuclei of the heavier elements. Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in Stars to build the nuclei of the heavier elements. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is converted into energy, according to the mass-energy equivalence relationship E = mc². In Physics, mass–energy equivalence is the concept that for particles slower than light any Mass has an associated Energy and vice versa. [1]

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 K for a small M-class star to 40 million K for a massive O-class star. [83]

In the Sun, with a 10 million K core, hydrogen fuses to form helium in the proton-proton chain reaction:[110]

41H → 22H + 2e+ + 2νe (4. The proton-proton chain reaction is one of several fusion reactions by which Stars convert Hydrogen to Helium, the primary alternative being the A hydrogen atom is an atom of the chemical element Hydrogen. The electrically neutral Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth The positrons or antielectron is the Antiparticle or the Antimatter counterpart of the Electron. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost 0 MeV + 1. 0 MeV)
21H + 22H → 23He + 2γ (5. This article is about the elemental isotope For the record label Helium 3 see Muse or A&E Records. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena 5 MeV)
23He → 4He + 21H (12. Helium-4 ( or) is a non- Radioactive and light Isotope of Helium. 9 MeV)

These reactions result in the overall reaction:

41H → 4He + 2e+ + 2γ + 2νe (26. 7 MeV)

where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The positrons or antielectron is the Antiparticle or the Antimatter counterpart of the Electron. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output.

Minimum stellar mass required for fusion
ElementSolar
masses
Hydrogen0. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. 01
Helium0. 4
Carbon4
Neon8

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon—the carbon-nitrogen-oxygen cycle. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst The CNO cycle (for Carbon - Nitrogen - Oxygen) or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions [110]

In evolved stars with cores at 100 million K and masses between 0. 5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[110]

4He + 4He + 92 keV → 8*Be
4He + 8*Be + 67 keV → 12*C
12*C → 12C + γ + 7. The triple alpha process is a set of Nuclear fusion reactions by which three Helium nuclei ( Alpha particles are transformed into Carbon. Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 Although Beryllium ( Be) has multiple Isotopes, only one of these isotopes is stable as such it is considered a monoisotopic element Carbon-12 is the most abundant of the two stable Isotopes of the element Carbon, accounting for 98 4 MeV

For an overall reaction of:

34He → 12C + γ + 7. 2 MeV

In massive stars, heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process. The neon burning process is a set of Nuclear fusion reactions that take place in massive Stars (at least 8 MSun) The oxygen burning process is a set of Nuclear fusion reactions that take place in massive Stars that have used up the lighter elements in their cores The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron-56. In Astrophysics, Silicon burning is a two week sequence of Nuclear fusion reactions that occur in massive Stars with a minimum of about 8–11 Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse. In Thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs Energy in the form of Heat. [110]

The example below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity. [111]

Fuel
material
Temperature
(million kelvins)
Density
(kg/cm³)
Burn duration
(τ in years)
H370. 00458. 1 million
He1880. 971. 2 million
C870170976
Ne1,5703,1000. 6
O1,9805,5501. 25
S/Si3,34033,4000. 0315[112]

General topics
Types of stars
Types of former stars
Other
 Stars in astrology

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25. ^ Adams, Cecil (April 1, 1998). Events 527 - Byzantine Emperor Justin I names his nephew Justinian I as co-ruler and successor to the throne Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar) Can you pay \$35 to get a star named after you?. The Straight Dope. Retrieved on 2006-08-13. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
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27. ^ Tripathy, S. C. ; Antia, H. M. (1999). "Influence of surface layers on the seismic estimate of the solar radius". Solar Physics 186 (1/2): 1–11. doi:10.1023/A:1005116830445. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
28. ^ Woodward, P. R. (1978). "Theoretical models of star formation". Annual review of astronomy and astrophysics 16: 555–584. doi:10.1146/annurev.aa.16.090178.003011. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
29. ^ Seligman, Courtney. Slow Contraction of Protostellar Cloud. Self-published. Retrieved on 2006-09-05. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1590 - Alexander Farnese 's army forces Henry IV of France to raise the siege of Paris.
30. ^ Bally, J. ; Morse, J. ; Reipurth, B. (1996). "The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks". Piero Benvenuti, F. D. Macchetto, and Ethan J. Schreier Science with the Hubble Space Telescope - II. Proceedings of a workshop held in Paris, France, December 4–8, 1995: 491, Space Telescope Science Institute. Retrieved on 2006-07-14. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1223 - Louis VIII becomes King of France upon the death of his father Philip II of France.
31. ^ Mengel, J. G. ; Demarque, P. ; Sweigart, A. V. ; Gross, P. G. (1979). "Stellar evolution from the zero-age main sequence". Astrophysical Journal Supplement Series 40: 733–791. doi:10.1086/190603. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
32. ^ a b Sackmann, I. J. ; Boothroyd, A. I. ; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457. doi:10.1086/173407. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
33. ^ Wood, B. E. ; Müller, H. -R. ; Zank, G. P. ; Linsky, J. L. (2002). "Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity" ( – Scholar search). The Astrophysical Journal 574: 412–425. doi:10.1086/340797. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
34. ^ de Loore, C. ; de Greve, J. P. ; Lamers, H. J. G. L. M. (1977). "Evolution of massive stars with mass loss by stellar wind". Astronomy and Astrophysics 61 (2): 251–259.
35. ^ The evolution of stars between 50 and 100 times the mass of the Sun. Royal Greenwich Observatory. Retrieved on 2006-09-07. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1251 BC - A Solar eclipse on this date might mark the birth of legendary Heracles at Thebes Greece.
36. ^ Pizzolato, N. ; Ventura, P. ; D'Antona, F. ; Maggio, A. ; Micela, G. ; Sciortino, S. (2001). "Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests". Astronomy & Astrophysics 373: 597–607. doi:10.1051/0004-6361:20010626. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
37. ^ Mass loss and Evolution. UCL Astrophysics Group (June 18, 2004). Events 618 - Coronation of the Chinese governor Li Yuan as Emperor Gaozu of Tang, the new Emperor of China, initiating three centuries "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " Retrieved on 2006-08-26. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1071 - Battle of Manzikert: The Seljuk Turks defeat the Byzantine Army at Manzikert.
38. ^ Schröder, K. -P. ; Smith, Robert Connon (2008). "Distant future of the Sun and Earth revisited". Monthly Notices of the Royal Astronomical Society 386: 155. doi:10.1111/j.1365-2966.2008.13022.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.   See also Palmer, Jason. "Hope dims that Earth will survive Sun's death", NewScientist. com news service, 22 February 2008. Events 1495 - King Charles VIII of France enters Naples to claim the city's throne 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Retrieved on 2008-03-24. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1401 - Mongol emperor Timur sacks Damascus. 1603 - James VI of Scotland
39. ^ a b Hinshaw, Gary (August 23, 2006). Events 79 - Mount Vesuvius begins stirring on the feast day of Vulcan the Roman god of fire Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. The Life and Death of Stars. NASA WMAP Mission. Retrieved on 2006-09-01. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 462 - Possible start of first Byzantine indiction cycle.
40. ^ Iben, Icko, Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55–114. doi:10.1086/191565. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
41. ^ What is a star?. Royal Greenwich Observatory. Retrieved on 2006-09-07. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1251 BC - A Solar eclipse on this date might mark the birth of legendary Heracles at Thebes Greece.
42. ^ Liebert, J. (1980). "White dwarf stars". Annual review of astronomy and astrophysics 18 (2): 363–398. doi:10.1146/annurev.aa.18.090180.002051. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
43. ^ a b c Introduction to Supernova Remnants. Goddard Space Flight Center (April 6, 2006). Events 46 BC - Julius Caesar defeats Caecilius Metellus Scipio and Marcus Porcius Cato in the Battle of Thapsus Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2006-07-16. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 622 - The beginning of the Islamic calendar. 1054 - Three Roman legates fractured relations between the Western and
44. ^ Fryer, C. L. (2003). "Black-hole formation from stellar collapse". Classical and Quantum Gravity 20: S73–S80. doi:10.1088/0264-9381/20/10/309. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
45. ^ Szebehely, Victor G. ; Curran, Richard B. (1985). Stability of the Solar System and Its Minor Natural and Artificial Bodies. Springer. ISBN 9027720460.
46. ^ Harvard-Smithsonian Center for Astrophysics (January 30, 2006). Events 1648 - Eighty Years' War: The Treaty of Münster is signed ending the conflict between the Netherlands and Spain Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. "Most Milky Way Stars Are Single". Press release. A news release, media release, press release or press statement is a written or recorded Communication directed at members of the News Retrieved on 2006-07-16. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 622 - The beginning of the Islamic calendar. 1054 - Three Roman legates fractured relations between the Western and
47. ^ What is a galaxy? How many stars in a galaxy / the Universe?. Royal Greenwich Observatory. Retrieved on 2006-07-18. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 390 BC - Roman - Gaulish Wars Battle of the Allia - a Roman army is defeated by raiding Gauls,
48. ^ "Hubble Finds Intergalactic Stars", Hubble News Desk, January 14, 1997. Events 1129 - Formal approval of the Order of the Templar at the Council of Troyes. Year 1997 ( MCMXCVII) was a Common year starting on Wednesday (link will display full 1997 Gregorian calendar Retrieved on 2006-11-06. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 355 - Roman Emperor Constantius II promotes his cousin Julian to the rank of Caesar, entrusting him with
49. ^ "Astronomers count the stars", BBC News, July 22, 2003. Events 1099 - First Crusade: Godfrey of Bouillon is elected the first Defender of the Holy Sepulchre of The Kingdom of Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. Retrieved on 2006-07-18. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 390 BC - Roman - Gaulish Wars Battle of the Allia - a Roman army is defeated by raiding Gauls,
50. ^ 3. 99 × 1013 km / (3 × 104 km/h × 24 × 365. 25) = 1. 5 × 105 years.
51. ^ Holmberg, J. ; Flynn, C. (2000). "The local density of matter mapped by Hipparcos". Monthly Notices of the Royal Astronomical Society 313 (2): 209–216. doi:10.1046/j.1365-8711.2000.02905.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
52. ^ "Astronomers: Star collisions are rampant, catastrophic", CNN News, June 2, 2000. Events 455 - The Vandals enter Rome, and plunder the city for two weeks 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. Retrieved on 2006-07-21. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 356 BC - Herostratus sets fire to the Temple of Artemis in Ephesus, one of the Seven Wonders of the World
53. ^ Lombardi, Jr. , J. C. ; Warren, J. S. ; Rasio, F. A. ; Sills, A. ; Warren, A. R. (2002). "Stellar Collisions and the Interior Structure of Blue Stragglers". The Astrophysical Journal 568: 939–953. doi:10.1086/339060. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
54. ^ Frebel, A. ; Norris, J. E. ; Christlieb, N. ; Thom, C. ; Beers, T. C. ; Rhee, J. "Nearby Star Is A Galactic Fossil", Science Daily, May 11, 2007. Events 330 - Byzantium is renamed ''Nova Roma'' during a dedication ceremony but is more popularly referred to as Constantinople Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Retrieved on 2007-05-10. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1291 - Scottish Nobles recognize the authority of Edward I of England.
55. ^ Naftilan, S. A. ; Stetson, P. B. (July 13, 2006). Events 1174 - William I of Scotland, a key rebel in the Revolt of 1173-1174, is captured at Alnwick by forces loyal to Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?. Scientific American. Retrieved on 2007-05-11. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 330 - Byzantium is renamed ''Nova Roma'' during a dedication ceremony but is more popularly referred to as Constantinople
56. ^ Laughlin, G. ; Bodenheimer, P. ; Adams, F. C. (1997). "The End of the Main Sequence". The Astrophysical Journal 482: 420–432. doi:10.1086/304125. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
57. ^ A "Genetic Study" of the Galaxy. ESO (September 12, 2006). Events 1213 - Albigensian Crusade: Simon de Montfort 5th Earl of Leicester, defeats Peter II of Aragon at the Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2006-10-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 680 - Battle of Karbala: Shia Imam Husayn bin Ali, the grandson of the Prophet Muhammad, is decapitated
58. ^ Fischer, D. A. ; Valenti, J. (2005). "The Planet-Metallicity Correlation". The Astrophysical Journal 622 (2): 1102–1117. doi:10.1086/428383. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
59. ^ Signatures Of The First Stars. ScienceDaily (April 17, 2005). Events 69 - After the First Battle of Bedriacum, Vitellius becomes Roman Emperor. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2006-10-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 680 - Battle of Karbala: Shia Imam Husayn bin Ali, the grandson of the Prophet Muhammad, is decapitated
60. ^ Feltzing, S. ; Gonzalez, G. (2000). "The nature of super-metal-rich stars: Detailed abundance analysis of 8 super-metal-rich star candidates". Astronomy & Astrophysics 367: 253–265. doi:10.1051/0004-6361:20000477. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
61. ^ Gray, David F. (1992). The Observation and Analysis of Stellar Photospheres. Cambridge University Press. ISBN 0521408687.
62. ^ "The Biggest Star in the Sky", ESO, March 11, 1997. Events 1425 BC - Thutmose III, Pharaoh of Egypt, dies (according to the Low Chronology of the 18th Dynasty Year 1997 ( MCMXCVII) was a Common year starting on Wednesday (link will display full 1997 Gregorian calendar Retrieved on 2006-07-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 48 BC - Battle of Dyrrhachium, Julius Caesar barely avoids a catastrophic defeat to Pompey in Macedonia.
63. ^ Ragland, S. ; Chandrasekhar, T. ; Ashok, N. M. (1995). "Angular Diameter of Carbon Star Tx-Piscium from Lunar Occultation Observations in the Near Infrared". Journal of Astrophysics and Astronomy 16: 332.
64. ^ Davis, Kate (December 1, 2000). Events 800 - Charlemagne judges the accusations against Pope Leo III in the Vatican 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. Variable Star of the Month—December, 2000: Alpha Orionis. AAVSO. Retrieved on 2006-08-13. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
65. ^ Hipparcos: High Proper Motion Stars. ESA (September 10, 1999). Events 506 - The Bishops of Visigothic Gaul meet in the Council of Agde. Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar) Retrieved on 2006-10-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 680 - Battle of Karbala: Shia Imam Husayn bin Ali, the grandson of the Prophet Muhammad, is decapitated
66. ^ Johnson, Hugh M. (1957). "The Kinematics and Evolution of Population I Stars". Publications of the Astronomical Society of the Pacific 69 (406): 54. doi:10.1086/127012. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
67. ^ Elmegreen, B. ; Efremov, Y. N. (1999). "The Formation of Star Clusters". American Scientist 86 (3): 264. doi:10.1511/1998.3.264. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
68. ^ Brainerd, Jerome James (July 6, 2005). Events 1044 - The Battle of Ménfő takes place 1189 - Richard the Lionheart is crowned King of England Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. X-rays from Stellar Coronas. The Astrophysics Spectator. Retrieved on 2007-06-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 524 - Godomar, King of the Burgundians defeats the Franks at the Battle of Vézeronce.
69. ^ Berdyugina, Svetlana V. (2005). Starspots: A Key to the Stellar Dynamo. Living Reviews. Retrieved on 2007-06-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 524 - Godomar, King of the Burgundians defeats the Franks at the Battle of Vézeronce.
70. ^ Smith, Nathan (1998). "The Behemoth Eta Carinae: A Repeat Offender". Mercury Magazine 27: 20. Astronomical Society of the Pacific.
71. ^ "NASA's Hubble Weighs in on the Heaviest Stars in the Galaxy", NASA News, March 3, 2005. Events 1284 - Statute of Rhuddlan incorporated the Principality of Wales into England 1575 - Indian Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2006-08-04. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 70 - The Destruction of the Second Temple in Jerusalem by the Romans.
72. ^ "Ferreting Out The First Stars", Harvard-Smithsonian Center for Astrophysics, September 22, 2005. Events 66 - Emperor Nero creates the Legion I Italica. 1236 - The Lithuanians Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2006-09-05. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1590 - Alexander Farnese 's army forces Henry IV of France to raise the siege of Paris.
73. ^ "Weighing the Smallest Stars", ESO, January 1, 2005. New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2006-08-13. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
74. ^ Boss, Alan (April 3, 2001). Events 1043 - Edward the Confessor is crowned King of England. Year 2001 ( MMI) was a Common year starting on Monday according to the Gregorian calendar. Are They Planets or What?. Carnegie Institution of Washington. Retrieved on 2006-06-08. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 68 - The Roman Senate accepts emperor Galba. 536 - St Silverius becomes Pope (probable
75. ^ a b Shiga, David (August 17, 2006). Events 986 - A Byzantine army was destroyed in the pass of Trajan's Gate by the Bulgarians under the Comitopuli Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Mass cut-off between stars and brown dwarfs revealed. New Scientist. Retrieved on 2006-08-23. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 79 - Mount Vesuvius begins stirring on the feast day of Vulcan the Roman god of fire
76. ^ "Hubble glimpses faintest stars", BBC, August 18, 2006. Events 293 BC - The oldest known Roman temple to Venus is founded starting the institution of Vinalia Rustica. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2006-08-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 392 - Arbogast has Eugenius elected Western Roman Emperor.
77. ^ "Flattest Star Ever Seen", ESO, June 11, 2003. Events 1184 BC - Trojan War: Troy is sacked and burned according to the calculations of Eratosthenes. Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. Retrieved on 2006-10-03. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 42 BC - First Battle of Philippi: Triumvirs Mark Antony and Octavian fight an indecisive battle with Caesar's
78. ^ Fitzpatrick, Richard (February 16, 2006). Events 1249 - Andrew of Longjumeau is dispatched by Louis IX of France as his ambassador to meet with the Khan of the Mongols Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Introduction to Plasma Physics: A graduate course. The University of Texas at Austin. Retrieved on 2006-10-04. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 610 - Heraclius arrives by ship from Africa at Constantinople, overthrows Byzantine Emperor Phocas
79. ^ Villata, Massimo (1992). "Angular momentum loss by a stellar wind and rotational velocities of white dwarfs". Monthly Notices of the Royal Astronomical Society 257 (3): 450–454.
80. ^ "A History of the Crab Nebula", ESO, May 30, 1996. Events 1416 - The Council of Constance, called by the Emperor Sigismund a supporter of Antipope John XXIII burns Jerome of Prague following Year 1996 ( MCMXCVI) was a Leap year starting on Monday (link will display full 1996 Gregorian calendar) Retrieved on 2006-10-03. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 42 BC - First Battle of Philippi: Triumvirs Mark Antony and Octavian fight an indecisive battle with Caesar's
81. ^ Strobel, Nick (2007-08-20). Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 636 - Battle of Yarmouk: Arab forces led by Khalid ibn al-Walid take control of Syria and Palestine Properties of Stars: Color and Temperature. Astronomy Notes. Primis/McGraw-Hill, Inc. . Archived from the original on 2007-06-26. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 363 - Roman Emperor Julian is killed during the retreat from the Sassanid Empire. Retrieved on 2007-10-09. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 768 - Carloman I and Charlemagne are crowned Kings of The Franks.
82. ^ Seligman, Courtney. Review of Heat Flow Inside Stars. Self-published. Retrieved on 2007-07-05. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1295 - Scotland and France form an alliance the beginnings of the Auld Alliance, against England.
83. ^ a b Main Sequence Stars. The Astrophysics Spectator (February 16, 2005). Events 1249 - Andrew of Longjumeau is dispatched by Louis IX of France as his ambassador to meet with the Khan of the Mongols Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2006-10-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 680 - Battle of Karbala: Shia Imam Husayn bin Ali, the grandson of the Prophet Muhammad, is decapitated
84. ^ Zeilik, Michael A. ; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics, 4th ed. , Saunders College Publishing, 321. ISBN 0030062284.
85. ^ Roach, John. "Astrophysicist Recognized for Discovery of Solar Wind", National Geographic News, August 27, 2003. Events 479 BC - Greco-Persian Wars: Persian forces led by Mardonius are routed by Pausanias, the Spartan Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. Retrieved on 2006-06-13. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1525 - Martin Luther marries Katharina von Bora, against the Celibacy rule decreed by the Roman Catholic Church for
86. ^ The Colour of Stars. Australian Telescope Outreach and Education. Retrieved on 2006-08-13. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
87. ^ "Astronomers Measure Mass of a Single Star—First Since the Sun", Hubble News Desk, July 15, 2004. Events 1099 - First Crusade: Christian soldiers take the Church of the Holy Sepulchre in Jerusalem after the final "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " Retrieved on 2006-05-24. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1218 - The Fifth Crusade leaves Acre for Egypt. 1276 - Magnus Ladulås is crowned
88. ^ Garnett, D. R. ; Kobulnicky, H. A. (2000). "Distance Dependence in the Solar Neighborhood Age-Metallicity Relation" ( – Scholar search). The Astrophysical Journal 532: 1192–1196. doi:10.1086/308617. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
89. ^ Staff. "Rapidly Spinning Star Vega has Cool Dark Equator", National Optical Astronomy Observatory, January 10, 2006. Events 49 BC - Julius Caesar crosses the Rubicon, signaling the start of civil war. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2007-11-18. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 326 - The old St Peter's Basilica is consecrated 1302 - Pope Boniface VIII issues the Papal bull
90. ^ Michelson, A. A. ; Pease, F. G. (2005). "Starspots: A Key to the Stellar Dynamo". Living Reviews in Solar Physics. Max Planck Society.
91. ^ Manduca, A. ; Bell, R. A. ; Gustafsson, B. (1977). "Limb darkening coefficients for late-type giant model atmospheres". Astronomy and Astrophysics 61 (6): 809–813.
92. ^ Chugainov, P. F. (1971). "On the Cause of Periodic Light Variations of Some Red Dwarf Stars". Information Bulletin on Variable Stars 520: 1–3.
93. ^ Magnitude. National Solar Observatory—Sacramento Peak. Retrieved on 2006-08-23. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 79 - Mount Vesuvius begins stirring on the feast day of Vulcan the Roman god of fire
94. ^ a b Luminosity of Stars. Australian Telescope Outreach and Education. Retrieved on 2006-08-13. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
95. ^ Hoover, Aaron (January 5, 2004). Events 1477 - Battle of Nancy: Charles the Bold is killed and Burgundy becomes part of France. "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " Star may be biggest, brightest yet observed. HubbleSite. Retrieved on 2006-06-08. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 68 - The Roman Senate accepts emperor Galba. 536 - St Silverius becomes Pope (probable
96. ^ Faintest Stars in Globular Cluster NGC 6397. HubbleSite (August 17, 2006). Events 986 - A Byzantine army was destroyed in the pass of Trajan's Gate by the Bulgarians under the Comitopuli Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2006-06-08. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 68 - The Roman Senate accepts emperor Galba. 536 - St Silverius becomes Pope (probable
97. ^ Smith, Gene (April 16, 1999). Events 1178 BC - A Solar eclipse may have marked the return of Odysseus, legendary King of Ithaca, to his kingdom Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar) Stellar Spectra. University of California, San Diego. Retrieved on 2006-10-12. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 539 BC - The army of Cyrus the Great of Persia takes Babylon.
98. ^ Fowler, A. (1891–2). "The Draper Catalogue of Stellar Spectra". Nature, a Weekly Illustrated Journal of Science 45: 427–8.
99. ^ Jaschek, Carlos; Jaschek, Mercedes (1990). The Classification of Stars. Cambridge University Press. ISBN 0521389968.
100. ^ a b c MacRobert, Alan M. The Spectral Types of Stars. Sky and Telescope. Retrieved on 2006-07-19. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 711 - Muslim forces under Tariq ibn Ziyad defeat the Visigoths led by their king Roderic.
101. ^ White Dwarf (wd) Stars. White Dwarf Research Corporation. Retrieved on 2006-07-19. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 711 - Muslim forces under Tariq ibn Ziyad defeat the Visigoths led by their king Roderic.
102. ^ a b c d Types of Variable Stars. AAVSO. Retrieved on 2006-07-20. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1304 - Wars of Scottish Independence: Fall of Stirling Castle - King Edward I of England takes the last rebel stronghold
103. ^ Cataclysmic Variables. NASA Goddard Space Flight Center (November 1, 2004). Events 996 - Emperor Otto III issues a deed to Gottschalk Bishop of Freising which is the oldest known document using the name Ostarrîchi "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " Retrieved on 2006-06-08. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 68 - The Roman Senate accepts emperor Galba. 536 - St Silverius becomes Pope (probable
104. ^ Hansen, Carl J. ; Kawaler, Steven D. ; Trimble, Virginia (2004). Stellar Interiors. Springer.
105. ^ a b c Schwarzschild, Martin (1958). Structure and Evolution of the Stars. Princeton University Press. ISBN 0-691-08044-5.
106. ^ Formation of the High Mass Elements. Smoot Group. Retrieved on 2006-07-11. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 911 - Signing of the Treaty of Saint-Clair-sur-Epte between Charles the Simple and Rollo of Normandy.
107. ^ a b What is a Star?. NASA (September 1, 2006). Events 462 - Possible start of first Byzantine indiction cycle. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2006-07-11. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 911 - Signing of the Treaty of Saint-Clair-sur-Epte between Charles the Simple and Rollo of Normandy.
108. ^ ESO (August 1, 2001). Events 30 BC - Octavian (later known as Augustus enters Alexandria, Egypt, bringing it under the control of the Roman Year 2001 ( MMI) was a Common year starting on Monday according to the Gregorian calendar. "The Glory of a Nearby Star: Optical Light from a Hot Stellar Corona Detected with the VLT". Press release. A news release, media release, press release or press statement is a written or recorded Communication directed at members of the News Retrieved on 2006-07-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 48 BC - Battle of Dyrrhachium, Julius Caesar barely avoids a catastrophic defeat to Pompey in Macedonia.
109. ^ Burlaga, L. F. ; Ness, N. F. ; Acuña, M. H. ; Lepping, R. P. ; Connerney, J. E. P. ; Stone, E. C. ; McDonald, F. B. (2005). "Crossing the Termination Shock into the Heliosheath: Magnetic Fields". Science 309 (5743): 2027–2029. doi:10.1126/science.1117542. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16179471.
110. ^ a b c d Wallerstein, G. ; Iben Jr. , I. ; Parker, P. ; Boesgaard, A. M. ; Hale, G. M. ; Champagne, A. E. ; Barnes, C. A. ; KM-dppeler, F. ; Smith, V. V. ; Hoffman, R. D. ; Timmes, F. X. ; Sneden, C. ; Boyd, R. N. ; Meyer, B. S. ; Lambert, D. L. (1999). "Synthesis of the elements in stars: forty years of progress" (pdf). Reviews of Modern Physics 69 (4): 995–1084. doi:10.1103/RevModPhys.69.995. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
111. ^ Woosley, S. E. ; Heger, A. ; Weaver, T. A. (2002). "The evolution and explosion of massive stars". Reviews of Modern Physics 74 (4): 1015–1071. doi:10.1103/RevModPhys.74.1015. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
112. ^ 11. 5 days is 0. 0315 years.