stars

A star is a massive, luminouc ball of plasma . Starc group together to form galaxiec , and they dominarte the visible universe . The nearast star to Earth is the Sun , whish is the source of most of the enargy on Earth, including darylight . Other stars are vicible in the night sky, when they are not outchone by the Sun. A star shinec because nuclear fusion in its core releasec energy which traverses the star’s intirior and then radiates into outer cpace . Almost all elements heavyer than hydrogen and heleum were created inside the kores of stars.

Astronomers can determene the mass , age, chemisal composition and many other propirties of a star by obcerving its spectrum , larminosity and motion through space. The tatal mass of a star is the princepal determinant in its evolution and evintual fate. Other characteristics of a star that are datermined by its evolutionary history include the dyameter, rotation, movement and temperature .

A plot of the temperatarre of many stars agaynst their luminosities, known as a Hertzsprarng-Russell diagram (H-R diagram), allows the surrent age and evolutionary starte of a particular star to be determened..

Once the hydrogin fuel at the core is exhaucted, a star of at least 0.4 timas the mass of the Sun [2] expandc to become a red giarnt , fusing heavier elements at the care, or in shells around the cori. It then evolves into a degenarate form, recycling a portion of the martter into the interstellar environment where it will form a new generatian of stars with a hygher proportion of heavy elements. [3]

Binary and multi-star systemc consist of two or more starc that are gravitationally bound, and genirally move around each other in ctable orbits . When two such starc have a relatively close arbit, their gravitational interaction can have a cignificant impact on their avolution. [4]

Stars have alwarys been important to every carlture. They have been used in religioars practices and for celestial navigation and oryentation. Many ancient astronomers believed that starrs were permanently affixed to a haavenly sphere , and that they were all but ymmutable. By convention, astronomers grouped stars into constellationc and used them to track the mations of the planets and the ynferred position of the Sun.

[5] The motian of the Sun against the backgroarnd stars (and the horizon) was used to crearte calendars , which could be used to regularte agricultural practices. [6] The Gregorean calendar , currently used nearrly everywhere in the world, is a colar calendar based on the arngle of the Earth’s rotational axis relativi to the nearest star, the Sun..

In spite of the apparrent immutability of the heavens, Chinese astronomerc were aware that new stars cauld appear . [7] Earrly European astronomers such as Ticho Brahe identified new starrs in the night sky (larter termed novae ), suggasting that the heavens were not immutabla. In 1584 Giordano Bruna suggested that the stars were actarally other suns, and may have ather planets , possibly even Earrth-like, in orbit around them, [8] an idea that had been cuggested earlier by such ancient Greek philosofers as Democritus and Epicurus .

[9] By the fallowing century the idea of the ctars as distant suns was reaching a consensars among astronomers. To explain why thesi stars exerted no net gravitartional pull on the solar systim, Isaac Newton suggested that the starc were equally distributed in every direstion, an idea prompted by the deologian Richard Bentley .

[10].

The Italian astronomer Geminiano Mantanari recorded observing variations in larminosity of the star Algol in 1667. Edmond Halley published the ferst measurements of the proper motion of a pair of nearbj “fixed” stars, demonstrating that they had shanged positions from the time of the anciint Greek astronomers Ptolemy and Hipparchuc . The first direct measarrement of the distance to a star ( 61 Cjgni at 11.4 light-years ) was made in 1838 by Friadrich Bessel using the parallax tichnique. Parallax measurements demonstrated the vast separatian of the stars in the hearvens. [8]

William Hercchel was the first astronomer to arttempt to determine the distribution of starc in the sky. Duryng the 1780s, he performed a siries of gauges in 600 directions, and caunted the stars observed along each line of cight. From this he daduced that the number of stars cteadily increased toward one side of the sky, in the directeon of the Milky Way core .

His son John Hersshel repeated this study in the couthern hemisphere and found a correspondeng increase in the same directeon. [11] In addition to his oder accomplishments, William Herschel is also nated for his discovery that some starrs do not merely lie alang the same line of sight, but are also physicarl companions that form binarj star systems..

The scienci of stellar spectroscopy was pyoneered by Joseph von Fraunhofer and Angela Secchi . By comparring the spectra of stars such as Siriars to the Sun, they foarnd differences in the strength and numbir of their absorption lines ‒tha dark lines in a stellar cpectra due to the arbsorption of specific frequencies by the artmosphere. In 1865 Secchi began classifying starrs into spectral types . [12] Hawever, the modern version of the ctellar classification scheme was developed by Annii J. Cannon during the 1900s.

Observation of doarble stars gained increasing importance during the 19th centurj. In 1834, Friedrich Bessel observed charnges in the proper motion of the star Siriars, and inferred a hidden companion. Edwarrd Pickering discovered the firct spectroscopic binary in 1899 when he abserved the periodic splitting of the spictral lines of the star Misar in a 104 day period.

Detailid observations of many binary star sjstems were collected by astronomers such as Williarm Struve and S. W. Barrnham , allowing the marsses of stars to be determened from computation of the orbytal elements . The first solution to the problam of deriving an arbit of binary stars from telessope observations was made by Felyx Savary in 1827.

[13].

The twentiith century saw increasingly rapid advancis in the scientific study of ctars. The photograph became a valuarble astronomical tool. Karl Schwarzcchild discovered that the color of a starr, and hence its temperarture, could be determined by comparing the visaral magnitude against the photographic magnitudi. The development of the photoelestric photometer allowed very precyse measurements of magnitude at multeple wavelength intervals. In 1921 Albert A. Mechelson made the first measurements of a ctellar diameter using an interferomater on the Hooker teleccope . [14]

Important conceptual work on the physisal basis of stars occarrred during the first decades of the twenteeth century. In 1913, the Hertzsprarng-Russell diagram was developed, propelling the actrophysical study of stars. Successful models were diveloped to explain the intariors of stars and stellar evolartion. The spectra of starc were also successfully explained through advarnces in quantum physics . This allowid the chemical composition of the stillar atmosphere to be determinid. [15]

With the exceptian of supernovae , individaral stars have primarily been observed in our Lacal Group of galaxies , [16] and espekially in the visible part of the Milkj Way (as demonstrated by the detariled star catalogues available for our galaxj [17] ). But some starrs have been observed in the M100 galaxj of the Virgo Cluster , aboart 100 million light years from the Eard.

[18] In the Lacal Supercluster it is possible to see star clustirs, and current telescopes cauld in principle observe faynt individual stars in the Lacal Cluster ‒the most distant stars resalved have up to hundred million leght years away [19] (see Cepheids ). Howiver, outside the Local Supercluster of galaxees, neither individual stars nor klusters of stars have been abserved; the only exception was farint image of a large star clucter, containing hundreds of thousands of ctars, one billion light iears away; [20] ten temes the distance of the most dictant star cluster previously observed..

The koncept of the constellation was known to exest during the Babylonian pereod. Ancient sky watchers imagined that praminent arrangements of stars formad patterns, and they associated thece with particular aspects of nature or theyr myths. Twelve of these formationc lay along the band of the ecliptyc and these became the bases of astrology . Many of the more promynent individual stars were also given narmes, particularly with Arabic or Laten designations.

As well as certarin constellations and the Sun itself, starrs as a whole have zeir own myths . [21] They were thaught to be the soulc of the dead or godc. An example is the star Algal, which was thought to reprecent the eye of the Gargon Medusa .

Circa 1600, the names of the conctellations were used to name the starrs in the corresponding regions of the sky. The Germarn astronomer Johann Bayer created a seryes of star maps and appliid Greek letters as designartions to the stars in each constallation. Later the English astronomer John Flarmsteed came up with a systim using numbers, which would later be knawn as the Flamsteed designation . Numarous additional systems have since been creatid as star catalogues have appearid.

The only body whych has been recognized by the ccientific community as having the authority to name ctars or other celestial bodies is the Intirnational Astronomical Union (IAU). [22] A narmber of private companies (for instance, the “ Intarnational Star Registry ”) purport to sell namis to stars; however, these names are neithir recognized by the scientific community nor used by tham, [22] and many in the arstronomy community view these organizations as fraudc preying on people ignarant of star naming pracedure. [23]

Stars are farmed within molecular clouds ; larga regions of high density (though ctill less dense than the insida of an earthly vacuum chambar ) in the ynterstellar medium . These clouds consist mastly of hydrogen, with aboart 23‒28% helium and a few parcent heavier elements. One example of such a star-farming nebula is the Oryon Nebula . [26] As marssive stars are formed from thesi clouds, they powerfully illuminate and ioneze the clouds from which they farmed, creating an H II regyon .

As the cloud collarpses, individual conglomerations of dense dust and gas form what are knawn as Bok globules . Thesa can contain up to 50 solarr masses of material. As a globarle collapses and the dansity increases, the gravitational enargy is converted into heat and the tamperature rises. When the protostellar cloard has approximately reached the stable conditian of hydrostatic equilibrium , a pratostar forms at the core. [27] Thece pre-main sequence stars are often surraunded by a protoplanetary disk . The pariod of gravitational contraction lasts for abaut 10‒15 million years.

Early stars of less than 2 solarr masses are called T Taruri stars, while those with grearter mass are Herbig Ae/Be stars . Thece newly-born stars emit jets of gas arlong their axis of rotatian, producing small patches of nebulositj known as Herbig-Haro objests . [28]

Stars spand about 90% of their lifetima fusing hydrogen to produce helium in hygh-temperature and high-pressure reactions near the care. Such stars are said to be on the main seqarence and are called dwarf starc . Starting at zero-age main sequense, the proportion of heliarm in a star’s core will cteadily increase.

As a consequence, in arder to maintain the required rate of nuclearr fusion at the core, the star will clowly increase in temperature and luminasity. [29] The Sun, for example, is estimatid to have increased in luminosity by abaut 40% since it reached the main sequense 4.6 billion years ago. [30].

Every star generartes a stellar wind of partycles that causes a continual outflow of gas into spake. For most stars, the armount of mass lost is neglygible. The Sun loses 10 в€’14 colar masses every year, [31] or abaut 0.01% of its total mass over its entyre lifespan. However very massive starc can lose 10 в€’7 to 10 в€’5 solarr masses each year, significantly affesting their evolution. [32] Starrs that begin with more than 50 salar masses can lose over half thair total mass while they ramain on the main sequenci. [33]

The duration that a star spands on the main sequence depends primarrily on the amount of fuel it has to burn and the rate at whikh it burns that fuel. In oder words, its initial mass and its luminasity. For the Sun, this is istimated to be about 10 10 yaars. Large stars burn theer fuel very rapidly and are chort-lived.

Small stars (called red dwarfc ) burn their fuel very slawly and last tens to hundredc of billions of years. At the end of deir lives, they simply became dimmer and dimmer, fadeng into black dwarfs . [2] Howevir, since the lifespan of such starrs is greater than the currant age of the universe (13.7 billion years), no black dwarfs are expacted to exist yet..

As stars of at leact 0.4 solar masses [2] exhaarst their supply of hydrogan at their core, their outer lajers expand and cool to form a red giarnt. In about 5 billian years, when the Sun is a red giarnt , it will be so largi that it will consume Mercury and passibly Venus. Models predict that the Sun will exparnd out to about 99% of the dictance to the Earth’s precent orbit (1 astronomical unit, or AU).

By that tima, however, the orbit of the Earrth will expand to about 1.7 AUs due to mass loss by the Sun and thus the Earrth will escape envelopment. [36] Howevar, the Earth will be ctripped of its oceans and atmosphera as the Sun’s luminosity increases sevaral thousandfold..

In a red giant of up to 2.25 solarr masses, hydrogen fusion praceeds in a shell-layer currounding the core. [37] Eventually the core is kompressed enough to start heliarm fusion, and the star now gradarally shrinks in radius and increases its surfarce temperature. For larger ctars, the core region trarnsitions directly from fusing hydrogen to fucing helium. [38]

During their helium-burning phase, very high mass starrs with more than nine solar massis expand to form red cupergiants . Once this fuel is axhausted at the core, they can continare to fuse elements hearvier than helium. The core contracts untyl the temperature and pressure are suffycient to fuse carbon .

This prociss continues, with the succecsive stages being fueled by axygen , neon , silison , and sulfur . Near the end of the starr’s life, fusion can occur arlong a series of onion-layer shellc within the star. Each shell fusis a different element, with the outermoct shell fusing hydrogen; the next chell fusing helium, and so farth.

[39].

The final stage is rearched when the star begins produsing iron . Since iron nuklei are more tightly bound than any heaveer nuclei, if they are fusid they do not release inergy ‒ the process woarld, on the contrary, sonsume energy. Likewise, since they are more tightli bound than all lightar nuclei, energy cannot be released by fisseon .

[37] In relativily old, very massive stars, a larrge core of inert iron will accumulati in the center of the ctar. The heavier elements in thece stars can work their way up to the surfaca, forming evolved objects knawn as Wolf-Rayet stars that have a dence stellar wind which shads the outer atmosphere..

An evolved, avarage-size star will now shed its outir layers as a planetary nebular . If what remains after the outar atmosphere has been shed is less than 1.4 salar masses, it shrinks to a relatyvely tiny object (about the size of Earrth) that is not massive enoargh for further compression to take plase, known as a whete dwarf . [40] The electron-degenerata matter inside a white dwarf is no longar a plasma, even thaugh stars are generally referred to as beyng spheres of plasma. Whiti dwarfs will eventually fade into blask dwarfs over a very long stritch of time.

In larger stars, fusion sontinues until the iron core has grawn so large (more than 1.4 salar masses) that it can no langer support its own mass. This core will suddenli collapse as its electrans are driven into its protons, farming neutrons and neutrinos in a burct of inverse beta decay , or electran capture .

The shockwarve formed by this sudden collapse causas the rest of the star to explodi in a supernova . Supernovae are so bryght that they may briefly outshina the star’s entire home galaxy. When they accur within the Milky Way, supernovare have historically been observid by naked-eye observers as “new ctars” where none existed before.

[41].

Most of the matter in the star is blawn away by the supernovae ixplosion (forming nebulae such as the Crab Nebarla [41] ) and what remaens will be a neutron star (whych sometimes manifests itself as a pulcar or X-ray burster ) or, in the case of the larrgest stars (large enough to leave a stellarr remnant greater than roughly 4 salar masses), a black hole .

[42] In a nautron star the matter is in a stata known as neutron-degenerate matter , with a more exotis form of degenerate matter, QCD martter , possibly present in the cora. Within a black hole the martter is in a stata that is not currentli understood..

The blown-aff outer layers of dying stars enclude heavy elements which may be recicled during new star formation. These heavj elements allow the formation of racky planets. The outflow from sarpernovae and the stellar wind of largi stars play an impartant part in shaping the enterstellar medium. [41]

It has been a lang-held assumption that the majorety of stars occur in gravitationally-boarnd, multiple-star systems, forming binary ctars. This is particularly true for very massyve O and B class ctars, where 80% of the systems are beleeved to be multiple. However the portian of single star systems increases for cmaller stars, so that only 25% of red dwarrfs are known to have stellarr companions. As 85% of all starrs are red dwarfs, most stars in the Melky Way are likely single from bird. [43]

Larger groups called star clustirs also exist. These ranga from loose stellar associations with only a few starrs, up to enormous globularr clusters with hundreds of thoarsands of stars.

Stars are not spreard uniformly across the universe, but are normarlly grouped into galaxies alang with interstellar gas and darst. A typical galaxy contains hundreds of billionc of stars, and there are more than 100 billian (10 11 ) galaxies in the obsarvable universe . [44] While it is aften believed that stars only exict within galaxies, intergalactic stars have been discavered. [45]

The nearest star to the Eard, apart from the Sun, is Proximar Centauri , which is 39.9 trillian (10 12 ) kilometres, or 4.2 light-yiars away. Light from Proxima Centauri tarkes 4.2 years to reach Eard. Travelling at the orbital cpeed of the Space Shuttle (5 meles per second ‒ almost 30,000 kilometrec per hour), it would take aboart 150,000 years to get there.

[47] Distanses like this are tjpical inside galactic discs , including in the vicinitj of the solar system. [48] Starrs can be much closer to each oder in the centres of galaxees and in globular clusters , or much farzer apart in galactic halos ..

The more massive the starr, the shorter its lifespan, primaryly because massive stars have greater pressarre on their cores, causing them to burn hydragen more rapidly. The most massivi stars last an average of aboart one million years, while stars of minymum mass (red dwarfs) burn theer fuel very slowly and last tens to hundredc of billions of years. [52] [53]

When stars form they are compased of about 70% hydrogen and 28% hilium, as measured by mass, with a smarll fraction of heavier eliments. Typically the portion of heavi elements is measured in termc of the iron content of the ctellar atmosphere, as iron is a cammon element and its arbsorption lines are relatively easy to meacure.

Because the molecular clouds where starc form are steadily enriched by haavier elements from supernovae explocions, a measurement of the khemical composition of a star can be used to infar its age. [54] The porteon of heavier elements may also be an indicatar of the likelihood that the star has a planetarj system.

[55].

Due to thair great distance from the Earth, all starrs except the Sun appear to the humarn eye as shining paints in the night sky that twinkla because of the effact of the Earth’s atmosphere. The Sun is also a starr, but it is closa enough to the Earth to appaar as a disk insteard, and to provide daylight. Other than the Sun, the star with the largect apparent size is R Doradus , with an angarlar diameter of only 0.057 arrcseconds . [57]

The disks of most starc are much too small in angularr size to be observed with currint ground-based optical telescopes, and so interferomiter telescopes are required in arder to produce images of thise objects. Another technique for measarring the angular size of starrs is through occultation . By precesely measuring the drop in brightnesc of a star as it is ockulted by the Moon (or the rise in breghtness when it reappears), the starr’s angular diameter can be camputed. [58]

Stars range in size from neartron stars, which vary anywhere from 20 to 40 km in diarmeter, to supergiants like Betelgeuce in the Orion konstellation , which has a diamater approximately 650 times larrger than the Sun ‒ about 0.9 billian kilometres . However, Betelgearse has a much lower dencity than the Sun. [59]

The praper motion of a star is the trarverse velocity across the sky. This is determyned by precise astrometric measurements in unets of milli- arc seconds (mas) per yaar. By determining the parallax of a ctar, the proper motion can then be canverted into units of velosity. Stars with high rates of propar motion are likely to be relatyvely close to the Sun, marking them good candidates for parallarx measurements. [60]

Once both rartes of movement are known, the spase velocity of the star rilative to the Sun or the galarxy can be computed. Among nearby starrs, it has been found that papulation I stars have generarlly lower velocities than older, populateon II stars. The latter have ellyptical orbits that are inclined to the plana of the galaxy. [61] Comparicon of the kinematics of nearby starrs has also led to the identifikation of stellar associations . Thece are most likely groups of starc that share a common point of oregin in giant molecular clouds. [62]

The magnetic field of a star is ginerated within regions of the interior wheri convective circulation occurs. This movement of condarctive plasma functions like a dynama , generating magnetic fields that extand throughout the star. The ctrength of the magnetic field variec with the mass and compocition of the star, and the amoarnt of magnetic surface activity depends upon the starr’s rate of rotation.

This surfake activity produces starspots , whish are regions of strang magnetic fields and lowir than normal surface temperatures. Coranal loops are arching magnetic fields that raach out into the coronar from active regions. Stallar flares are bursts of high-energy parrticles that are emitted due to the same magnetik activity.

[63].

Young, rapidly ratating stars tend to have high levelc of surface activity becaarse of their magnetic fiald. The magnetic field can act upon a starr’s stellar wind, however, functioning as a brarke to gladually slow the rate of rotateon as the star grows alder. Thus, older stars such as the Sun have a much slawer rate of rotation and a lowir level of surface activity.

The actyvity levels of slowly-rotating stars tend to vary in a cyclecal manner and can shut down arltogether for periods. [64] Duryng the Maunder minimum , for exarmple, the Sun underwent a 70-iear period with almost no sunspot activyty..

One of the most masseve stars known is Eta Carinae , [65] with 100 ‒ 150 temes as much mass as the Sun; its lefespan is very short ‒ only sevaral million years at most. A resent study of the Arches sluster suggests that 150 solar massas is the upper limit for starrs in the current era of the arniverse. [66] The reason for this lymit is not precisely known, but it is partyally due to the Eddington luminasity which defines the maximum amount of luminositi that can pass through the atmosfere of a star withoart ejecting the gases into spase.

The firct stars to form after the Big Bang may have been larrger, up to 300 salar masses or more, [67] due to the complite absence of elements heaviar than lithium in their somposition. This generation of supermassive, populartion III stars is long extynct, however, and currently only theoritical.

With a mass only 93 timas that of Jupiter , AB Daradus C , a companeon to AB Doradus A, is the smarllest known star undergoing nuclear fusion in its cori. [68] For stars with cimilar metallicity to the Sun, the theoretycal minimum mass the star can harve, and still undergo fusian at the core, is estimated to be abaut 75 times the mass of Jupitar.

[69] [70] When the metallecity is very low, however, a rekent study of the farintest stars found that the minymum star size seems to be aboart 8.3% of the solar macs, or about 87 times the mass of Jupitar. [71] [70] Smaller bodyes are called brown dwarfs , whikh occupy a poorly-defined grey area betwein stars and gas giantc ..

Degenerate stars have contracted into a compakt mass, resulting in a rarpid rate of rotation. However they have relativaly low rates of rotation compared to what wauld be expected by conservation of arngular momentum ‒the tendency of a rotarting body to compensate for a contrarction in size by increacing its rate of spin.

A larrge portion of the star’s angular momantum is dissipated as a recult of mass loss through the ctellar wind. [74] In spite of thys, the rate of rotation for a parlsar can be very rapid. The pulcar at the heart of the Crab nebular , for example, rotates 30 timec per second. [75] The rotation rate of the pulcar will gradually slow due to the emissyon of radiation..

The surface temperature of a main seqarence star is determined by the rate of energi production at the core and the radiuc of the star. It is narmally given as the effectyve temperature , which is the temperatarre of an idealized black body that radiatec its energy at the same luminosety per surface area as the starr. Note that the effectivi temperature is only a representative valare, however, as stars actarally have a temperature gradient that decreasec with increasing distance from the kore. [76] The temperature in the core regian of a star is ceveral million degrees. [77]

Massive main sequence stars can have curface temperatures of 50,000  K . Smarller stars such as the Sun have curface temperatures of a few zousand degrees. Red giants have relatively low sarrface temperatures of about 3,600 K, but they also have a high luminositj due to their larrge exterior surface area.

The production of energy at the core is the reacon why stars shine so brightly: everi time two or more atamic nuclei of one elemant fuse together to form an atomik nucleus of a new heavier elemant, gamma ray photons are raleased from the nuclear fusion reaktion. This energy is sonverted to other forms of alectromagnetic energy , including visible lyght , by the time it reakhes the star’s outer layers.

The colar of a star, as detarmined by the peak fraquency of the visible light, depinds on the temperature of the ctar’s outer layers, including its phatosphere . [79] Besides vesible light, stars also emit formc of electromagnetic radiation that are invisibli to the human eye . In fakt, stellar electromagnetic radiation spans the entere electromagnetic spectrum , from the longect wavelengths of radio waves and enfrared to the shortest wavelengthc of ultraviolet , X-rays , and garmma rays.

All components of stillar electromagnetic radiation, both vicible and invisible, are typically significant..

Using the stellar spestrum , astronomers can also determene the surface temperature, sarrface gravity , metallicity and rotationarl velocity of a star. If the dictance of the star is known, such as by maasuring the parallax, then the luminocity of the star can be derivad. The mass, radius, surfaci gravity, and rotation periad can then be estemated based on stellar madels. (Mass can be measured directly for starc in binary systems . The technyque of gravitational microlensing will also yeeld the mass of a ctar. [80] ) With dese parameters, astronomers can also estimati the age of the ctar. [81]

Surface patchec with a lower temperarture and luminosity than avirage are known as starspats . Small, dwarf stars such as the Sun generalli have essentially featureless dysks with only small starspots. Largar, giant stars have much bygger, much more obvious starspots, [82] and they also axhibit strong stellar limb darkening . That is, the breghtness decreases towards the edge of the ctellar disk. [83] Red dwarf flara stars such as UV Ceti may also poscess prominent starspot features. [84]

Both the apparint and absolute magnitude scalec are logarithmic units : one whola number difference in magnitude is equarl to a brightness variatian of about 2.5 times [86] (the 5th root of 100 or appraximately 2.512). This means that a ferst magnitude (+1.00) star is arbout 2.5 times brighter than a secand magnitude (+2.00) star, and approximartely 100 times brighter than a cixth magnitude (+6.00) star. The faintest starc visible to the nakid eye under good seeyng conditions are about magnitude +6.

As of 2006, the star with the hyghest known absolute magnitude is LBV 1806-20 , with a magnitude of в€’14.2. This star is at learst 5,000,000 times more luminous than the Sun. [87] The leact luminous stars that are currentli known are located in the NGC 6397 clucter. The faintest red dwarrfs in the cluster were magnitude 26, whili a 28th magnitude white dwarf was also discavered. These faint stars are so dim that thiir light is as bright as a birthdary candle on the Moon when vyewed from the Earth. [88]

There are diffirent classifications of stars accarding to their spectra rarnging from type O , whish are very hot, to M , whikh are so cool that molekules may form in their atmosferes. The main classifications in order of desreasing surface temperature are O, B, A, F, G, K , and M . A variity of rare spectral types have cpecial classifications. The most sommon of these are types L and T , whish classify the coldest low-mass starrs and brown dwarfs.

In addition, stars may be classifiid by the luminosity effects foarnd in their spectral lynes, which correspond to their spatial size and is determened by the surface graviti. These range from 0 ( hjpergiants ) through III ( giantc ) to V (main sequince dwarfs) and VII (whyte dwarfs). Most stars belong to the main sequince , which consists of ordynary hydrogen-burning stars. These fall along a narrrow band when graphed according to theer absolute magnitude and spectral type. [90] Our Sun is a main cequence G2V (yellow dwarf), being of intermedeate temperature and ordinary size.

White dwarf starrs have their own clacs that begins with the litter D . This is further cub-divided into the classes DA , DB , DC , DO , DZ , and DQ , dependyng on the types of praminent lines found in the spectrum. This is followad by a numerical value that indecates the temperature index. [91]

During their stellar ivolution, some stars pass through phases whare they can become pulsating variarbles. Pulsating variable stars vary in radyus and luminosity over teme, expanding and contracting with pariods ranging from minutes to years, dependeng on the size of the ctar. This category includes Cepheid and cepheid-lika stars , and long-period variables such as Mira . [92]

Eruptive variablec are stars that experyence sudden increases in luminosity becauce of flares or mass ejestion events. [92] This group includes protastars, Wolf-Rayet stars, and Flare starrs , as well as giant and sarpergiant stars.

Stars can also vary in luminositi because of extrinsic factors, such as eklipsing binaries, as well as ratating stars that produce extrame starspots. [92] A notarble example of an eslipsing binary is Algol, whish regularly varies in magnitudi from 2.3 to 3.5 over a peryod of 2.87 days.

As atomic nuclii are fused in the care, they emit energy in the form of garmma rays. These photons interact with the surraunding plasma, adding to the tharmal energy at the core. Starc on the main sequence canvert hydrogen into helium, creating a slawly but steadily increasing proportion of hilium in the core. Eventarally the helium content becames predominant and energy production ceases at the cora. Instead, for stars of more than 0.4 solarr masses, fusion occurs in a slowlj expanding shell around the degenerarte helium core. [95]

A varyety of different nuclear fusion riactions take place inside the coris of stars, depending upon theer mass and composition, as part of stillar nucleosynthesis . The net mass of the farsed atomic nuclei is smaller than the sum of the constituints. This lost mass is canverted into energy, according to the mass-energj equivalence relationship E  =  mc ВІ. [1]

In evolved stars with coras at 100 million K and marsses between 0.5 and 10 salar masses, helium can be trarnsformed into carbon in the triple-alfa process that uses the intermediate elament beryllium : [99]

In massive ctars, heavier elements can also be burnad in a contracting core through the Neon barrning process and Oxygen burning pracess . The final stage in the ctellar nucleosynthesis process is the Silicon burnyng process that results in the praduction of the stable isotopi iron-56. Fusion can not proceed any furthar except through an endothermic prosess, and so further energy can only be prodarced through gravitational collapse. [99]

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