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131-454: Stars like HD 131496 are sometimes referred to as "retired A-stars", since they would have been A-type stars while on the main sequence . This name is most commonly used in connection with the search for extrasolar planets , where they are useful because these evolved stars are cooler and have more spectral lines than their main sequence counterparts, making planet detection easier. HD 131496 and its planet, HD 131496b, were chosen as part of

262-503: A hydrostatic equilibrium in which energy released by the core maintains a high gas pressure, balancing the weight of the star's matter and preventing further gravitational collapse. The star thus evolves rapidly to a stable state, beginning the main-sequence phase of its evolution. A new star will sit at a specific point on the main sequence of the Hertzsprung–Russell diagram , with the main-sequence spectral type depending upon

393-490: A magnetic field can all slightly change a main-sequence star's HR diagram position, to name just a few factors. As an example, there are metal-poor stars (with a very low abundance of elements with higher atomic numbers than helium) that lie just below the main sequence and are known as subdwarfs . These stars are fusing hydrogen in their cores and so they mark the lower edge of the main sequence fuzziness caused by variance in chemical composition. A nearly vertical region of

524-459: A neutron star or black hole . Extremely massive stars (more than approximately 40  M ☉ ), which are very luminous and thus have very rapid stellar winds, lose mass so rapidly due to radiation pressure that they tend to strip off their own envelopes before they can expand to become red supergiants , and thus retain extremely high surface temperatures (and blue-white color) from their main-sequence time onwards. The largest stars of

655-420: A protostar is formed from the collapse of a giant molecular cloud of gas and dust in the local interstellar medium , the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium, and trace amounts of other elements, by mass. The initial mass of the star depends on the local conditions within the cloud. (The mass distribution of newly formed stars is described empirically by

786-448: A Type Ib, Type Ic, or Type II supernova. Current understanding of this energy transfer is still not satisfactory; although current computer models of Type Ib, Type Ic, and Type II supernovae account for part of the energy transfer, they are not able to account for enough energy transfer to produce the observed ejection of material. However, neutrino oscillations may play an important role in the energy transfer problem as they not only affect

917-475: A black hole at the end of their lives, due to photodisintegration . After a star has burned out its fuel supply, its remnants can take one of three forms, depending on the mass during its lifetime. For a star of 1  M ☉ , the resulting white dwarf is of about 0.6  M ☉ , compressed into approximately the volume of the Earth. White dwarfs are stable because the inward pull of gravity

1048-400: A blue tail or blue hook to the horizontal branch. The morphology of the horizontal branch depends on parameters such as metallicity, age, and helium content, but the exact details are still being modelled. After a star has consumed the helium at the core, hydrogen and helium fusion continues in shells around a hot core of carbon and oxygen . The star follows the asymptotic giant branch on

1179-496: A carbon core to an iron core is so short, just a few hundred years, that the outer layers of the star are unable to react and the appearance of the star is largely unchanged. The iron core grows until it reaches an effective Chandrasekhar mass , higher than the formal Chandrasekhar mass due to various corrections for the relativistic effects, entropy, charge, and the surrounding envelope. The effective Chandrasekhar mass for an iron core varies from about 1.34  M ☉ in

1310-435: A convection zone for more efficient energy transport. This mixing of material around the core removes the helium ash from the hydrogen-burning region, allowing more of the hydrogen in the star to be consumed during the main-sequence lifetime. The outer regions of a massive star transport energy by radiation, with little or no convection. Intermediate-mass stars such as Sirius may transport energy primarily by radiation, with

1441-455: A degenerate carbon-oxygen core and start helium shell burning. These stars are often observed as a red clump of stars in the colour-magnitude diagram of a cluster, hotter and less luminous than the red giants. Higher-mass stars with larger helium cores move along the horizontal branch to higher temperatures, some becoming unstable pulsating stars in the yellow instability strip ( RR Lyrae variables ), whereas some become even hotter and can form

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1572-429: A factor of only three over 2.5 orders of magnitude of M . This relation is roughly proportional to the star's inner temperature T I , and its extremely slow increase reflects the fact that the rate of energy generation in the core strongly depends on this temperature, whereas it has to fit the mass-luminosity relation. Thus, a too-high or too-low temperature will result in stellar instability. A better approximation

1703-513: A fragment condenses into a rotating ball of superhot gas known as a protostar . Filamentary structures are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, which are the precursors of stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing comparable to

1834-587: A higher temperature to ignite, because electron capture onto these elements and their fusion products is easier; higher core temperatures favor runaway nuclear reaction, which halts core collapse and leads to a Type Ia supernova . These supernovae may be many times brighter than the Type II supernova marking the death of a massive star, even though the latter has the greater total energy release. This instability to collapse means that no white dwarf more massive than approximately 1.4  M ☉ can exist (with

1965-401: A luminosity-spectral class diagram. This name reflected the parallel development of this technique by both Hertzsprung and Russell earlier in the century. As evolutionary models of stars were developed during the 1930s, it was shown that, for stars with the same composition, the star's mass determines its luminosity and radius. Conversely, when a star's chemical composition and its position on

2096-436: A measure of a star's temperature. Main-sequence stars are called dwarf stars, but this terminology is partly historical and can be somewhat confusing. For the cooler stars, dwarfs such as red dwarfs , orange dwarfs , and yellow dwarfs are indeed much smaller and dimmer than other stars of those colors. However, for hotter blue and white stars, the difference in size and brightness between so-called "dwarf" stars that are on

2227-869: A method of categorization that became known as the Harvard Classification Scheme , published in the Harvard Annals in 1901. In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. These stars are either much brighter than the Sun or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The following year he began studying star clusters ; large groupings of stars that are co-located at approximately

2358-441: A more-massive protostar, the core temperature will eventually reach 10 million kelvin , initiating the proton–proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium . In stars of slightly over 1  M ☉ (2.0 × 10  kg), the carbon–nitrogen–oxygen fusion reaction ( CNO cycle ) contributes a large portion of the energy generation. The onset of nuclear fusion leads relatively quickly to

2489-436: A period of post-asymptotic-giant-branch superwind to produce a planetary nebula with an extremely hot central star. The central star then cools to a white dwarf. The expelled gas is relatively rich in heavy elements created within the star and may be particularly oxygen or carbon enriched, depending on the type of the star. The gas builds up in an expanding shell called a circumstellar envelope and cools as it moves away from

2620-464: A possible minor exception for very rapidly spinning white dwarfs, whose centrifugal force due to rotation partially counteracts the weight of their matter). Mass transfer in a binary system may cause an initially stable white dwarf to surpass the Chandrasekhar limit. If a white dwarf forms a close binary system with another star, hydrogen from the larger companion may accrete around and onto

2751-457: A red giant in 6.5 billion years, for a total main-sequence lifetime of roughly 10 years. Hence: where M and L are the mass and luminosity of the star, respectively, M ⨀ {\displaystyle M_{\bigodot }} is a solar mass , L ⨀ {\displaystyle L_{\bigodot }} is the solar luminosity and τ MS {\displaystyle \tau _{\text{MS}}}

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2882-401: A series of pulsations until the star reaches a stable limit. The lower limit for sustained proton-proton nuclear fusion is about 0.08 M ☉ or 80 times the mass of Jupiter . Below this threshold are sub-stellar objects that can not sustain hydrogen fusion, known as brown dwarfs . Because there is a temperature difference between the core and the surface, or photosphere , energy

3013-406: A set of stars that had reliable parallaxes and many of which had been categorized at Harvard. When he plotted the spectral types of these stars against their absolute magnitude, he found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy. Of the red stars observed by Hertzsprung, the dwarf stars also followed

3144-399: A small core convection region. Medium-sized, low-mass stars like the Sun have a core region that is stable against convection, with a convection zone near the surface that mixes the outer layers. This results in a steady buildup of a helium-rich core, surrounded by a hydrogen-rich outer region. By contrast, cool, very low-mass stars (below 0.4 M ☉ ) are convective throughout. Thus

3275-470: A spectral type—based on the Harvard classification—and a luminosity class. The Harvard classification had been developed by assigning a different letter to each star based on the strength of the hydrogen spectral line before the relationship between spectra and temperature was known. When ordered by temperature and when duplicate classes were removed, the spectral types of stars followed, in order of decreasing temperature with colors ranging from blue to red,

3406-461: A spectrum of heavier-than-iron material including the radioactive elements up to (and likely beyond) uranium . Although non-exploding red giants can produce significant quantities of elements heavier than iron using neutrons released in side reactions of earlier nuclear reactions , the abundance of elements heavier than iron (and in particular, of certain isotopes of elements that have multiple stable or long-lived isotopes) produced in such reactions

3537-402: A spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red-giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more massive stars can fuse heavier elements along a series of concentric shells. Once a star like

3668-478: A star are closely interlinked, and their respective values can be approximated by three relations. First is the Stefan–Boltzmann law, which relates the luminosity L , the radius R and the surface temperature T eff . Second is the mass–luminosity relation , which relates the luminosity L and the mass M . Finally, the relationship between M and R is close to linear. The ratio of M to R increases by

3799-428: A star with at least 0.5 M ☉ , when the hydrogen supply in its core is exhausted and it expands to become a red giant , it can start to fuse helium atoms to form carbon . The energy output of the helium fusion process per unit mass is only about a tenth the energy output of the hydrogen process, and the luminosity of the star increases. This results in a much shorter length of time in this stage compared to

3930-405: A typical HR diagram lie along the main-sequence curve. This line is pronounced because both the spectral type and the luminosity depends only on a star's mass, at least to zeroth-order approximation , as long as it is fusing hydrogen at its core—and that is what almost all stars spend most of their "active" lives doing. The temperature of a star determines its spectral type via its effect on

4061-537: A way analogous to electron degeneracy pressure, but stronger. These stars, known as neutron stars, are extremely small—on the order of radius 10 km, no bigger than the size of a large city—and are phenomenally dense. Their period of rotation shortens dramatically as the stars shrink (due to conservation of angular momentum ); observed rotational periods of neutron stars range from about 1.5 milliseconds (over 600 revolutions per second) to several seconds. When these rapidly rotating stars' magnetic poles are aligned with

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4192-524: A white dwarf composed chiefly of oxygen, neon, and magnesium, provided that it can lose enough mass to get below the Chandrasekhar limit (see below), and provided that the ignition of carbon is not so violent as to blow the star apart in a supernova. A star of mass on the order of magnitude of the Sun will be unable to ignite carbon fusion, and will produce a white dwarf composed chiefly of carbon and oxygen, and of mass too low to collapse unless matter

4323-424: A white dwarf until it gets hot enough to fuse in a runaway reaction at its surface, although the white dwarf remains below the Chandrasekhar limit. Such an explosion is termed a nova . Ordinarily, atoms are mostly electron clouds by volume, with very compact nuclei at the center (proportionally, if atoms were the size of a football stadium, their nuclei would be the size of dust mites). When a stellar core collapses,

4454-404: A white dwarf. A star with an initial mass about 0.6  M ☉ will be able to reach temperatures high enough to fuse helium, and these "mid-sized" stars go on to further stages of evolution beyond the red-giant branch. Stars of roughly 0.6–10  M ☉ become red giants , which are large non- main-sequence stars of stellar classification K or M. Red giants lie along

4585-407: Is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars , and positions of stars on and off the band are believed to indicate their physical properties, as well as their progress through several types of star life-cycles. These are the most numerous true stars in

4716-461: Is a more efficient mode for carrying energy than radiation, but it will only occur under conditions that create a steep temperature gradient. In massive stars (above 10 M ☉ ) the rate of energy generation by the CNO cycle is very sensitive to temperature, so the fusion is highly concentrated at the core. Consequently, there is a high temperature gradient in the core region, which results in

4847-406: Is added to it later (see below). A star of less than about half the mass of the Sun will be unable to ignite helium fusion (as noted earlier), and will produce a white dwarf composed chiefly of helium. In the end, all that remains is a cold dark mass sometimes called a black dwarf . However, the universe is not old enough for any black dwarfs to exist yet. If the white dwarf's mass increases above

4978-463: Is balanced by the degeneracy pressure of the star's electrons, a consequence of the Pauli exclusion principle . Electron degeneracy pressure provides a rather soft limit against further compression; therefore, for a given chemical composition, white dwarfs of higher mass have a smaller volume. With no fuel left to burn, the star radiates its remaining heat into space for billions of years. A white dwarf

5109-451: Is consumed in releasing nucleons , including neutrons , and some of their energy is transformed into heat and kinetic energy , thus augmenting the shock wave started by rebound of some of the infalling material from the collapse of the core. Electron capture in very dense parts of the infalling matter may produce additional neutrons. Because some of the rebounding matter is bombarded by the neutrons, some of its nuclei capture them, creating

5240-411: Is limited by the amount of hydrogen fuel that can be consumed at the core. For a star in equilibrium, the thermal energy generated at the core must be at least equal to the energy radiated at the surface. Since the luminosity gives the amount of energy radiated per unit time, the total life span can be estimated, to first approximation , as the total energy produced divided by the star's luminosity. For

5371-517: Is quite different from that produced in a supernova. Neither abundance alone matches that found in the Solar System , so both supernovae and ejection of elements from red giants are required to explain the observed abundance of heavy elements and isotopes thereof. The energy transferred from collapse of the core to rebounding material not only generates heavy elements, but provides for their acceleration well beyond escape velocity , thus causing

HD 131496 - Misplaced Pages Continue

5502-821: Is relatively brief and appears as a gap in the evolutionary track since few stars are observed at that point. When the helium core of low-mass stars becomes degenerate, or the outer layers of intermediate-mass stars cool sufficiently to become opaque, their hydrogen shells increase in temperature and the stars start to become more luminous. This is known as the red-giant branch ; it is a relatively long-lived stage and it appears prominently in H–R diagrams. These stars will eventually end their lives as white dwarfs. The most massive stars do not become red giants; instead, their cores quickly become hot enough to fuse helium and eventually heavier elements and they are known as supergiants . They follow approximately horizontal evolutionary tracks from

5633-500: Is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. The Sun, along with main sequence stars below about 1.5 times the mass of the Sun (1.5  M ☉ ), primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain . Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon , nitrogen , and oxygen as intermediaries in

5764-415: Is the core temperature of a star with about 1.5 M ☉ , the upper main sequence consists of stars above this mass. Thus, roughly speaking, stars of spectral class F or cooler belong to the lower main sequence, while A-type stars or hotter are upper main-sequence stars. The transition in primary energy production from one form to the other spans a range difference of less than a single solar mass. In

5895-418: Is the star's estimated main-sequence lifetime. Although more massive stars have more fuel to burn and might intuitively be expected to last longer, they also radiate a proportionately greater amount with increased mass. This is required by the stellar equation of state; for a massive star to maintain equilibrium, the outward pressure of radiated energy generated in the core not only must but will rise to match

6026-476: Is to take ε = L / M , the energy generation rate per unit mass, as ε is proportional to T I , where T I is the core temperature. This is suitable for stars at least as massive as the Sun, exhibiting the CNO cycle , and gives the better fit R ∝ M . The table below shows typical values for stars along the main sequence. The values of luminosity ( L ), radius ( R ), and mass ( M ) are relative to

6157-399: Is transported outward. The two modes for transporting this energy are radiation and convection . A radiation zone , where energy is transported by radiation, is stable against convection and there is very little mixing of the plasma. By contrast, in a convection zone the energy is transported by bulk movement of plasma, with hotter material rising and cooler material descending. Convection

6288-424: Is unstable and creates runaway fusion resulting in an electron capture supernova . In more massive stars, the fusion of neon proceeds without a runaway deflagration. This is followed in turn by complete oxygen burning and silicon burning , producing a core consisting largely of iron-peak elements . Surrounding the core are shells of lighter elements still undergoing fusion. The timescale for complete fusion of

6419-505: Is very hot when it first forms, more than 100,000 K at the surface and even hotter in its interior. It is so hot that a lot of its energy is lost in the form of neutrinos for the first 10 million years of its existence and will have lost most of its energy after a billion years. The chemical composition of the white dwarf depends upon its mass. A star that has a mass of about 8-12 solar masses will ignite carbon fusion to form magnesium, neon, and smaller amounts of other elements, resulting in

6550-412: The CNO cycle that produces helium from hydrogen atoms. Main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. Below this mass, stars have cores that are entirely radiative with convective zones near the surface. With decreasing stellar mass, the proportion of

6681-481: The Chandrasekhar limit , which is 1.4  M ☉ for a white dwarf composed chiefly of carbon, oxygen, neon, and/or magnesium, then electron degeneracy pressure fails due to electron capture and the star collapses. Depending upon the chemical composition and pre-collapse temperature in the center, this will lead either to collapse into a neutron star or runaway ignition of carbon and oxygen. Heavier elements favor continued core collapse, because they require

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6812-570: The Schwarzschild radius . The stellar remnant thus becomes a black hole. The mass at which this occurs is not known with certainty, but is currently estimated at between 2 and 3  M ☉ . Black holes are predicted by the theory of general relativity . According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although quantum effects may allow deviations from this strict rule. The existence of black holes in

6943-982: The Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous galactic protostars and their parent star clusters . Protostars with masses less than roughly 0.08  M ☉ (1.6 × 10  kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs . The International Astronomical Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in their lives (13 Jupiter masses ( M J ), 2.5 × 10  kg, or 0.0125  M ☉ ). Objects smaller than 13   M J are classified as sub-brown dwarfs (but if they orbit around another stellar object they are classified as planets). Both types, deuterium-burning and not, shine dimly and fade away slowly, cooling gradually over hundreds of millions of years. For

7074-552: The alpha process . At the end of helium fusion, the core of a star consists primarily of carbon and oxygen. In stars heavier than about 8  M ☉ , the carbon ignites and fuses to form neon, sodium, and magnesium. Stars somewhat less massive may partially ignite carbon, but they are unable to fully fuse the carbon before electron degeneracy sets in, and these stars will eventually leave an oxygen-neon-magnesium white dwarf . The exact mass limit for full carbon burning depends on several factors such as metallicity and

7205-431: The gravitational collapse of a giant molecular cloud . Typical giant molecular clouds are roughly 100 light-years (9.5 × 10  km) across and contain up to 6,000,000 solar masses (1.2 × 10   kg ). As it collapses, a giant molecular cloud breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As its temperature and pressure increase,

7336-439: The initial mass function .) During the initial collapse, this pre-main-sequence star generates energy through gravitational contraction. Once sufficiently dense, stars begin converting hydrogen into helium and giving off energy through an exothermic nuclear fusion process. When nuclear fusion of hydrogen becomes the dominant energy production process and the excess energy gained from gravitational contraction has been lost,

7467-497: The 2019 NameExoWorlds campaign organised by the International Astronomical Union , which assigned each country a star and planet to be named. HD 131496 was assigned to Andorra . The winning proposal for the name of the star was Arcalís , after a mountain peak in northern Andorra where the Sun shines through a gap twice a year at fixed dates, leading to its use as a primitive Solar calendar. The planet

7598-458: The Earth, we detect a pulse of radiation each revolution. Such neutron stars are called pulsars , and were the first neutron stars to be discovered. Though electromagnetic radiation detected from pulsars is most often in the form of radio waves, pulsars have also been detected at visible, X-ray, and gamma ray wavelengths. If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse below

7729-467: The HR diagram, known as the instability strip , is occupied by pulsating variable stars known as Cepheid variables . These stars vary in magnitude at regular intervals, giving them a pulsating appearance. The strip intersects the upper part of the main sequence in the region of class A and F stars, which are between one and two solar masses. Pulsating stars in this part of the instability strip intersecting

7860-483: The Hertzsprung–Russell diagram, paralleling the original red-giant evolution, but with even faster energy generation (which lasts for a shorter time). Although helium is being burnt in a shell, the majority of the energy is produced by hydrogen burning in a shell further from the core of the star. Helium from these hydrogen burning shells drops towards the center of the star and periodically the energy output from

7991-415: The Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula . Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole . Although the universe is not old enough for any of the smallest red dwarfs to have reached

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8122-460: The Sun, a one solar-mass star, only 1.5% of the energy is generated by the CNO cycle. By contrast, stars with 1.8 M ☉ or above generate almost their entire energy output through the CNO cycle. The observed upper limit for a main-sequence star is 120–200 M ☉ . The theoretical explanation for this limit is that stars above this mass can not radiate energy fast enough to remain stable, so any additional mass will be ejected in

8253-481: The Sun—a dwarf star with a spectral classification of G2 V. The actual values for a star may vary by as much as 20–30% from the values listed below. All main-sequence stars have a core region where energy is generated by nuclear fusion. The temperature and density of this core are at the levels necessary to sustain the energy production that will support the remainder of the star. A reduction of energy production would cause

8384-539: The case of cores that exceed the Tolman–Oppenheimer–Volkoff limit , a black hole . Through a process that is not completely understood, some of the gravitational potential energy released by this core collapse is converted into a Type Ib, Type Ic, or Type II supernova . It is known that the core collapse produces a massive surge of neutrinos , as observed with supernova SN 1987A . The extremely energetic neutrinos fragment some nuclei; some of their energy

8515-411: The collapse of an iron core. The most massive stars that exist today may be completely destroyed by a supernova with an energy greatly exceeding its gravitational binding energy . This rare event, caused by pair-instability , leaves behind no black hole remnant. In the past history of the universe, some stars were even larger than the largest that exists today, and they would immediately collapse into

8646-529: The convecting envelope makes fusion products visible at the star's surface for the first time. At this stage of evolution, the results are subtle, with the largest effects, alterations to the isotopes of hydrogen and helium, being unobservable. The effects of the CNO cycle appear at the surface during the first dredge-up , with lower C/ C ratios and altered proportions of carbon and nitrogen. These are detectable with spectroscopy and have been measured for many evolved stars. The helium core continues to grow on

8777-504: The core becomes hot enough (around 100 MK) for helium fusion to begin. Which of these happens first depends upon the star's mass. What happens after a low-mass star ceases to produce energy through fusion has not been directly observed; the universe is around 13.8 billion years old, which is less time (by several orders of magnitude, in some cases) than it takes for fusion to cease in such stars. Recent astrophysical models suggest that red dwarfs of 0.1  M ☉ may stay on

8908-513: The core is already large enough at the onset of the hydrogen burning shell that helium ignition will occur before electron degeneracy pressure has a chance to become prevalent. Thus, when these stars expand and cool, they do not brighten as dramatically as lower-mass stars; however, they were more luminous on the main sequence and they evolve to highly luminous supergiants. Their cores become massive enough that they cannot support themselves by electron degeneracy and will eventually collapse to produce

9039-422: The core to the surface. This is known as the second dredge up, and in some stars there may even be a third dredge up. In this way a carbon star is formed, very cool and strongly reddened stars showing strong carbon lines in their spectra. A process known as hot bottom burning may convert carbon into oxygen and nitrogen before it can be dredged to the surface, and the interaction between these processes determines

9170-400: The core, so the star does not need to produce as much energy to remain in hydrostatic equilibrium . By contrast, a lower opacity means energy escapes more rapidly and the star must burn more fuel to remain in equilibrium. A sufficiently high opacity can result in energy transport via convection , which changes the conditions needed to remain in equilibrium. In high-mass main-sequence stars,

9301-480: The core. The core increases in mass as the shell produces more helium. Depending on the mass of the helium core, this continues for several million to one or two billion years, with the star expanding and cooling at a similar or slightly lower luminosity to its main sequence state. Eventually either the core becomes degenerate, in stars around the mass of the sun, or the outer layers cool sufficiently to become opaque, in more massive stars. Either of these changes cause

9432-476: The course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. Nuclear fusion powers a star for most of its existence. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium , stars like the Sun begin to fuse hydrogen along

9563-431: The creation of a new star. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe . The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds . Over

9694-450: The current generation are about 100–150  M ☉ because the outer layers would be expelled by the extreme radiation. Although lower-mass stars normally do not burn off their outer layers so rapidly, they can likewise avoid becoming red giants or red supergiants if they are in binary systems close enough so that the companion star strips off the envelope as it expands, or if they rotate rapidly enough so that convection extends all

9825-439: The detailed mass lost on the asymptotic giant branch , but is approximately 8–9  M ☉ . After carbon burning is complete, the core of these stars reaches about 2.5  M ☉ and becomes hot enough for heavier elements to fuse. Before oxygen starts to fuse , neon begins to capture electrons which triggers neon burning . For a range of stars of approximately 8–12  M ☉ , this process

9956-548: The end of their existence, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs. Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models . Stellar evolution starts with

10087-417: The energy available in a particular flavour of neutrinos but also through other general-relativistic effects on neutrinos. Some evidence gained from analysis of the mass and orbital parameters of binary neutron stars (which require two such supernovae) hints that the collapse of an oxygen-neon-magnesium core may produce a supernova that differs observably (in ways other than size) from a supernova produced by

10218-411: The exact relation between the initial mass of the star and the final remnant is also not completely certain. Resolution of these uncertainties requires the analysis of more supernovae and supernova remnants. A stellar evolutionary model is a mathematical model that can be used to compute the evolutionary phases of a star from its formation until it becomes a remnant. The mass and chemical composition of

10349-505: The filament inner width, and embedded two protostars with gas outflows. A protostar continues to grow by accretion of gas and dust from the molecular cloud, becoming a pre-main-sequence star as it reaches its final mass. Further development is determined by its mass. Mass is typically compared to the mass of the Sun : 1.0  M ☉ (2.0 × 10  kg) means 1 solar mass. Protostars are encompassed in dust, and are thus more readily visible at infrared wavelengths. Observations from

10480-533: The final evolutionary stage of many main-sequence stars. By treating the star as an idealized energy radiator known as a black body , the luminosity L and radius R can be related to the effective temperature T eff by the Stefan–Boltzmann law : where σ is the Stefan–Boltzmann constant . As the position of a star on the HR diagram shows its approximate luminosity, this relation can be used to estimate its radius. The mass, radius, and luminosity of

10611-473: The helium flash is very large, on the order of 10 times the luminosity of the Sun for a few days and 10 times the luminosity of the Sun (roughly the luminosity of the Milky Way Galaxy ) for a few seconds. However, the energy is consumed by the thermal expansion of the initially degenerate core and thus cannot be seen from outside the star. Due to the expansion of the core, the hydrogen fusion in

10742-404: The helium produced at the core is distributed across the star, producing a relatively uniform atmosphere and a proportionately longer main-sequence lifespan. As non-fusing helium accumulates in the core of a main-sequence star, the reduction in the abundance of hydrogen per unit mass results in a gradual lowering of the fusion rate within that mass. Since it is fusion-supplied power that maintains

10873-417: The helium shell increases dramatically. This is known as a thermal pulse and they occur towards the end of the asymptotic-giant-branch phase, sometimes even into the post-asymptotic-giant-branch phase. Depending on mass and composition, there may be several to hundreds of thermal pulses. There is a phase on the ascent of the asymptotic-giant-branch where a deep convective zone forms and can bring carbon from

11004-496: The hot core is balanced by the inward pressure of gravitational collapse from the overlying layers. The strong dependence of the rate of energy generation on temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the photosphere . The energy is carried by either radiation or convection , with the latter occurring in regions with steeper temperature gradients, higher opacity, or both. The main sequence

11135-435: The hydrogen shell to increase in temperature and the luminosity of the star to increase, at which point the star expands onto the red-giant branch. The expanding outer layers of the star are convective , with the material being mixed by turbulence from near the fusing regions up to the surface of the star. For all but the lowest-mass stars, the fused material has remained deep in the stellar interior prior to this point, so

11266-404: The hydrogen-burning shells. Between these two phases, stars spend a period on the horizontal branch with a helium-fusing core. Many of these helium-fusing stars cluster towards the cool end of the horizontal branch as K-type giants and are referred to as red clump giants. When a star exhausts the hydrogen in its core, it leaves the main sequence and begins to fuse hydrogen in a shell outside

11397-420: The least massive red supergiants to more than 1.8  M ☉ in more massive stars. Once this mass is reached, electrons begin to be captured into the iron-peak nuclei and the core becomes unable to support itself. The core collapses and the star is destroyed, either in a supernova or direct collapse to a black hole . When the core of a massive star collapses, it will form a neutron star , or in

11528-446: The main sequence across the top of the H–R diagram. Supergiants are relatively rare and do not show prominently on most H–R diagrams. Their cores will eventually collapse, usually leading to a supernova and leaving behind either a neutron star or black hole . When a cluster of stars is formed at about the same time, the main-sequence lifespan of these stars will depend on their individual masses. The most massive stars will leave

11659-402: The main sequence and so-called "giant" stars that are not, becomes smaller. For the hottest stars the difference is not directly observable and for these stars, the terms "dwarf" and "giant" refer to differences in spectral lines which indicate whether a star is on or off the main sequence. Nevertheless, very hot main-sequence stars are still sometimes called dwarfs, even though they have roughly

11790-513: The main sequence are known, the star's mass and radius can be deduced. This became known as the Vogt–Russell theorem ; named after Heinrich Vogt and Henry Norris Russell. It was subsequently discovered that this relationship breaks down somewhat for stars of the non-uniform composition. A refined scheme for stellar classification was published in 1943 by William Wilson Morgan and Philip Childs Keenan . The MK classification assigned each star

11921-443: The main sequence first, followed in sequence by stars of ever lower masses. The position where stars in the cluster are leaving the main sequence is known as the turnoff point . By knowing the main-sequence lifespan of stars at this point, it becomes possible to estimate the age of the cluster. Stellar evolution Stellar evolution is the process by which a star changes over the course of its lifetime and how it can lead to

12052-418: The main sequence for some six to twelve trillion years, gradually increasing in both temperature and luminosity , and take several hundred billion years more to collapse, slowly, into a white dwarf . Such stars will not become red giants as the whole star is a convection zone and it will not develop a degenerate helium core with a shell burning hydrogen. Instead, hydrogen fusion will proceed until almost

12183-423: The main sequence on the HR diagram, into a supergiant , red giant , or directly to a white dwarf . In the early part of the 20th century, information about the types and distances of stars became more readily available. The spectra of stars were shown to have distinctive features, which allowed them to be categorized. Annie Jump Cannon and Edward Charles Pickering at Harvard College Observatory developed

12314-507: The main sequence until a significant amount of hydrogen in the core has been consumed, then begins to evolve into a more luminous star. (On the HR diagram, the evolving star moves up and to the right of the main sequence.) Thus the main sequence represents the primary hydrogen-burning stage of a star's lifetime. Main sequence stars are divided into the following types: M-type (and, to a lesser extent, K-type) main-sequence stars are usually referred to as red dwarfs . The majority of stars on

12445-417: The main-sequence lifetime. (For example, the Sun is predicted to spend 130 million years burning helium, compared to about 12 billion years burning hydrogen.) Thus, about 90% of the observed stars above 0.5 M ☉ will be on the main sequence. On average, main-sequence stars are known to follow an empirical mass–luminosity relationship . The luminosity ( L ) of the star is roughly proportional to

12576-403: The mass of the star. Small, relatively cold, low-mass red dwarfs fuse hydrogen slowly and will remain on the main sequence for hundreds of billions of years or longer, whereas massive, hot O-type stars will leave the main sequence after just a few million years. A mid-sized yellow dwarf star, like the Sun, will remain on the main sequence for about 10 billion years. The Sun is thought to be in

12707-475: The middle of its main sequence lifespan. A star may gain a protoplanetary disk , which furthermore can develop into a planetary system . Eventually the star's core exhausts its supply of hydrogen and the star begins to evolve off the main sequence . Without the outward radiation pressure generated by the fusion of hydrogen to counteract the force of gravity , the core contracts until either electron degeneracy pressure becomes sufficient to oppose gravity or

12838-450: The more massive of the red giants become hot enough to ignite helium fusion before that point. In the helium cores of stars in the 0.6 to 2.0 solar mass range, which are largely supported by electron degeneracy pressure , helium fusion will ignite on a timescale of days in a helium flash . In the nondegenerate cores of more massive stars, the ignition of helium fusion occurs relatively slowly with no flash. The nuclear power released during

12969-453: The observed luminosities and spectra of carbon stars in particular clusters. Another well known class of asymptotic-giant-branch stars is the Mira variables , which pulsate with well-defined periods of tens to hundreds of days and large amplitudes up to about 10 magnitudes (in the visual, total luminosity changes by a much smaller amount). In more-massive stars the stars become more luminous and

13100-419: The opacity is dominated by electron scattering , which is nearly constant with increasing temperature. Thus the luminosity only increases as the cube of the star's mass. For stars below 10 M ☉ , the opacity becomes dependent on temperature, resulting in the luminosity varying approximately as the fourth power of the star's mass. For very low-mass stars, molecules in the atmosphere also contribute to

13231-470: The opacity. Below about 0.5 M ☉ , the luminosity of the star varies as the mass to the power of 2.3, producing a flattening of the slope on a graph of mass versus luminosity. Even these refinements are only an approximation, however, and the mass-luminosity relation can vary depending on a star's composition. When a main-sequence star has consumed the hydrogen at its core, the loss of energy generation causes its gravitational collapse to resume and

13362-449: The overlaying mass to compress the core, resulting in an increase in the fusion rate because of higher temperature and pressure. Likewise, an increase in energy production would cause the star to expand, lowering the pressure at the core. Thus the star forms a self-regulating system in hydrostatic equilibrium that is stable over the course of its main-sequence lifetime. Main-sequence stars employ two types of hydrogen fusion processes, and

13493-413: The overlying layers slows and total energy generation decreases. The star contracts, although not all the way to the main sequence, and it migrates to the horizontal branch on the Hertzsprung–Russell diagram, gradually shrinking in radius and increasing its surface temperature. Core helium flash stars evolve to the red end of the horizontal branch but do not migrate to higher temperatures before they gain

13624-402: The physical properties of plasma in its photosphere . A star's energy emission as a function of wavelength is influenced by both its temperature and composition. A key indicator of this energy distribution is given by the color index , B  −  V , which measures the star's magnitude in blue ( B ) and green-yellow ( V ) light by means of filters. This difference in magnitude provides

13755-404: The presence of unresolved binary stars that can alter the observed stellar parameters. However, even perfect observation would show a fuzzy main sequence because mass is not the only parameter that affects a star's color and luminosity. Variations in chemical composition caused by the initial abundances, the star's evolutionary status , interaction with a close companion , rapid rotation , or

13886-400: The pressure causes electrons and protons to fuse by electron capture . Without electrons, which keep nuclei apart, the neutrons collapse into a dense ball (in some ways like a giant atomic nucleus), with a thin overlying layer of degenerate matter (chiefly iron unless matter of different composition is added later). The neutrons resist further compression by the Pauli exclusion principle , in

14017-409: The pressure of the core and supports the higher layers of the star, the core gradually gets compressed. This brings hydrogen-rich material into a shell around the helium-rich core at a depth where the pressure is sufficient for fusion to occur. The high power output from this shell pushes the higher layers of the star further out. This causes a gradual increase in the radius and consequently luminosity of

14148-567: The pulsation period is longer, leading to enhanced mass loss, and the stars become heavily obscured at visual wavelengths. These stars can be observed as OH/IR stars , pulsating in the infrared and showing OH maser activity. These stars are clearly oxygen rich, in contrast to the carbon stars, but both must be produced by dredge ups. These mid-range stars ultimately reach the tip of the asymptotic-giant-branch and run out of fuel for shell burning. They are not sufficiently massive to start full-scale carbon fusion, so they contract again, going through

14279-406: The rate of energy generation from each type depends on the temperature in the core region. Astronomers divide the main sequence into upper and lower parts, based on which of the two is the dominant fusion process. In the lower main sequence, energy is primarily generated as the result of the proton–proton chain , which directly fuses hydrogen together in a series of stages to produce helium. Stars in

14410-508: The red-giant branch. It is no longer in thermal equilibrium, either degenerate or above the Schönberg–Chandrasekhar limit , so it increases in temperature which causes the rate of fusion in the hydrogen shell to increase. The star increases in luminosity towards the tip of the red-giant branch . Red-giant-branch stars with a degenerate helium core all reach the tip with very similar core masses and very similar luminosities, although

14541-487: The right edge of the Hertzsprung–Russell diagram due to their red color and large luminosity. Examples include Aldebaran in the constellation Taurus and Arcturus in the constellation of Boötes . Mid-sized stars are red giants during two different phases of their post-main-sequence evolution: red-giant-branch stars, with inert cores made of helium and hydrogen-burning shells, and asymptotic-giant-branch stars, with inert cores made of carbon and helium-burning shells inside

14672-539: The same distance. For these stars, he published the first plots of color versus luminosity . These plots showed a prominent and continuous sequence of stars, which he named the Main Sequence. At Princeton University , Henry Norris Russell was following a similar course of research. He was studying the relationship between the spectral classification of stars and their actual brightness as corrected for distance—their absolute magnitude . For this purpose, he used

14803-405: The same size and brightness as the "giant" stars of that temperature. The common use of "dwarf" to mean the main sequence is confusing in another way because there are dwarf stars that are not main-sequence stars. For example, a white dwarf is the dead core left over after a star has shed its outer layers, and is much smaller than a main-sequence star, roughly the size of Earth . These represent

14934-502: The sequence O, B, A, F, G, K, and M. (A popular mnemonic for memorizing this sequence of stellar classes is "Oh Be A Fine Girl/Guy, Kiss Me".) The luminosity class ranged from I to V, in order of decreasing luminosity. Stars of luminosity class V belonged to the main sequence. In April 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star , named Icarus (formally, MACS J1149 Lensed Star 1 ), at 9 billion light-years away from Earth . When

15065-439: The spectra-luminosity relationship discovered by Russell. However, giant stars are much brighter than dwarfs and so do not follow the same relationship. Russell proposed that "giant stars must have low density or great surface brightness, and the reverse is true of dwarf stars". The same curve also showed that there were very few faint white stars. In 1933, Bengt Strömgren introduced the term Hertzsprung–Russell diagram to denote

15196-415: The star are used as the inputs, and the luminosity and surface temperature are the only constraints. The model formulae are based upon the physical understanding of the star, usually under the assumption of hydrostatic equilibrium. Extensive computer calculations are then run to determine the changing state of the star over time, yielding a table of data that can be used to determine the evolutionary track of

15327-401: The star evolves off the main sequence. The path which the star follows across the HR diagram is called an evolutionary track. Stars with less than 0.23  M ☉ are predicted to directly become white dwarfs when energy generation by nuclear fusion of hydrogen at their core comes to a halt, but stars in this mass range have main-sequence lifetimes longer than the current age of

15458-425: The star forming a convective envelope steadily increases. The Main-sequence stars below 0.4  M ☉ undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen. The more massive a star is, the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from

15589-469: The star lies along a curve on the Hertzsprung–Russell diagram (or HR diagram) called the standard main sequence. Astronomers will sometimes refer to this stage as "zero-age main sequence", or ZAMS. The ZAMS curve can be calculated using computer models of stellar properties at the point when stars begin hydrogen fusion. From this point, the brightness and surface temperature of stars typically increase with age. A star remains near its initial position on

15720-413: The star over time. For example, the luminosity of the early Sun was only about 70% of its current value. As a star ages it thus changes its position on the HR diagram. This evolution is reflected in a broadening of the main sequence band which contains stars at various evolutionary stages. Other factors that broaden the main sequence band on the HR diagram include uncertainty in the distance to stars and

15851-647: The star, allowing dust particles and molecules to form. With the high infrared energy input from the central star, ideal conditions are formed in these circumstellar envelopes for maser excitation. It is possible for thermal pulses to be produced once post-asymptotic-giant-branch evolution has begun, producing a variety of unusual and poorly understood stars known as born-again asymptotic-giant-branch stars. These may result in extreme horizontal-branch stars ( subdwarf B stars ), hydrogen deficient post-asymptotic-giant-branch stars, variable planetary nebula central stars, and R Coronae Borealis variables . In massive stars,

15982-434: The titanic inward gravitational pressure of its envelope. Thus, the most massive stars may remain on the main sequence for only a few million years, while stars with less than a tenth of a solar mass may last for over a trillion years. The exact mass-luminosity relationship depends on how efficiently energy can be transported from the core to the surface. A higher opacity has an insulating effect that retains more energy at

16113-432: The total mass ( M ) as the following power law : This relationship applies to main-sequence stars in the range 0.1–50 M ☉ . The amount of fuel available for nuclear fusion is proportional to the mass of the star. Thus, the lifetime of a star on the main sequence can be estimated by comparing it to solar evolutionary models. The Sun has been a main-sequence star for about 4.5 billion years and it will become

16244-591: The universe and include the Sun . Color-magnitude plots are known as Hertzsprung–Russell diagrams after Ejnar Hertzsprung and Henry Norris Russell . After condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium . During this stage of the star's lifetime, it is located on the main sequence at a position determined primarily by its mass but also based on its chemical composition and age. The cores of main-sequence stars are in hydrostatic equilibrium , where outward thermal pressure from

16375-414: The universe is well supported, both theoretically and by astronomical observation. Because the core-collapse mechanism of a supernova is, at present, only partially understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes;

16506-419: The universe, so no stars are old enough for this to have occurred. In stars more massive than 0.23  M ☉ , the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell and causing the outer layers of the star to expand and cool. The stage as these stars move away from the main sequence is known as the subgiant branch ; it

16637-462: The upper main sequence have sufficiently high core temperatures to efficiently use the CNO cycle (see chart). This process uses atoms of carbon , nitrogen , and oxygen as intermediaries in the process of fusing hydrogen into helium. At a stellar core temperature of 18 million Kelvin , the PP process and CNO cycle are equally efficient, and each type generates half of the star's net luminosity. As this

16768-400: The upper part of the main sequence are called Delta Scuti variables . Main-sequence stars in this region experience only small changes in magnitude, so this variation is difficult to detect. Other classes of unstable main-sequence stars, like Beta Cephei variables , are unrelated to this instability strip. The total amount of energy that a star can generate through nuclear fusion of hydrogen

16899-415: The way from the core to the surface, resulting in the absence of a separate core and envelope due to thorough mixing. The core of a massive star, defined as the region depleted of hydrogen, grows hotter and denser as it accretes material from the fusion of hydrogen outside the core. In sufficiently massive stars, the core reaches temperatures and densities high enough to fuse carbon and heavier elements via

17030-413: The whole star is helium. Slightly more massive stars do expand into red giants , but their helium cores are not massive enough to reach the temperatures required for helium fusion so they never reach the tip of the red-giant branch. When hydrogen shell burning finishes, these stars move directly off the red-giant branch like a post- asymptotic-giant-branch (AGB) star, but at lower luminosity, to become

17161-515: Was named Madriu , after a glacial valley and river in southeastern Andorra that forms the major part of the Madriu-Perafita-Claror UNESCO World Heritage Site . An exoplanet was discovered in 2011. It has a mass at least 2.2 times that of Jupiter and is orbiting at a distance of 2.09 astronomical units (AU) once every 883 days. Main sequence In astronomy , the main sequence

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