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68-406: Red giants vary in the way by which they generate energy: Many of the well-known bright stars are red giants because they are luminous and moderately common. The K0 RGB star Arcturus is 36 light-years away, and Gacrux is the nearest M-class giant at 88 light-years' distance. A red giant will usually produce a planetary nebula and become a white dwarf at the end of its life. A red giant

136-424: A habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional one billion years. Later studies have refined this scenario, showing how for a 1  M ☉ star

204-426: A helium-4 nucleus ) is the dominant process that generates energy in the cores of main-sequence stars. It is also called "hydrogen burning", which should not be confused with the chemical combustion of hydrogen in an oxidizing atmosphere. There are two predominant processes by which stellar hydrogen fusion occurs: proton–proton chain and the carbon–nitrogen–oxygen (CNO) cycle. Ninety percent of all stars, with

272-462: A planetary nebula with the core of the star exposed, ultimately becoming a white dwarf . The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution. The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is spent on the red-giant branch. The horizontal-branch and asymptotic-giant-branch phases proceed tens of times faster. If

340-402: A type II supernova . The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all. Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system , if present, uninhabitable, some research suggests that, during the evolution of a 1  M ☉ star along the red-giant branch, it could harbor

408-539: A white dwarf . [REDACTED] Media related to Red giants at Wikimedia Commons List of brightest stars This is a list of stars arranged by their apparent magnitude – their brightness as observed from Earth. It includes all stars brighter than magnitude +2.50 in visible light , measured using a V -band filter in the UBV photometric system . Stars in binary systems (or other multiples ) are listed by their total or combined brightness if they appear as

476-551: A flash and execute a blue loop before reaching the asymptotic giant branch . Such a star initially moves away from the AGB toward bluer colours, then loops back again to what is called the Hayashi track . An important consequence of blue loops is that they give rise to classical Cepheid variables , of central importance in determining distances in the Milky Way and to nearby galaxies. Despite

544-627: A heavily cited picture that gave promise of accounting for the observed relative abundances of the elements; but it did not itself enlarge Hoyle's 1954 picture for the origin of primary nuclei as much as many assumed, except in the understanding of nucleosynthesis of those elements heavier than iron by neutron capture. Significant improvements were made by Alastair G. W. Cameron and by Donald D. Clayton . In 1957 Cameron presented his own independent approach to nucleosynthesis, informed by Hoyle's example, and introduced computers into time-dependent calculations of evolution of nuclear systems. Clayton calculated

612-487: A huge factor when involving a beta decay , due to the relation between the intermediate bound state (e.g. diproton ) half-life and the beta decay half-life, as in the proton–proton chain reaction . Note that typical core temperatures in main-sequence stars give kT of the order of keV. Thus, the limiting reaction in the CNO cycle , proton capture by 7 N , has S ( E 0 ) ~ S (0) = 3.5   keV·b, while

680-438: A much larger effect would be Roche lobe overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet. (A similar process in multiple star systems is believed to be the cause of most novas and type Ia supernovas .) Many of the well-known bright stars are red giants, because they are luminous and moderately common. The red-giant branch variable star Gamma Crucis

748-528: A paper describing how advanced fusion stages within massive stars would synthesize the elements from carbon to iron in mass. Hoyle's theory was extended to other processes, beginning with the publication of the 1957 review paper "Synthesis of the Elements in Stars" by Burbidge , Burbidge , Fowler and Hoyle , more commonly referred to as the B FH paper . This review paper collected and refined earlier research into

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816-549: A result of hydrogen fusion, but the core does not become hot enough to initiate helium fusion. Helium fusion first begins when a star leaves the red giant branch after accumulating sufficient helium in its core to ignite it. In stars around the mass of the Sun, this begins at the tip of the red giant branch with a helium flash from a degenerate helium core, and the star moves to the horizontal branch where it burns helium in its core. More massive stars ignite helium in their core without

884-399: A second. This final burning in massive stars, called explosive nucleosynthesis or supernova nucleosynthesis , is the final epoch of stellar nucleosynthesis. A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe . The need for a physical description was already inspired by the relative abundances of

952-403: A shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch , a second red-giant phase. The helium fusion results in the build-up of a carbon–oxygen core. A star below about 8  M ☉ will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming

1020-413: A single star to the naked eye , or listed separately if they do not. As with all magnitude systems in astronomy , the scale is logarithmic and inverted i.e. lower/more negative numbers are brighter. Most stars on this list appear bright from Earth because they are nearby, not because they are intrinsically luminous . For a list which compensates for the distances, converting the apparent magnitude to

1088-404: A situation that has been described as the mirror principle : when the core within the shell contracts, the layers of the star outside the shell must expand. The detailed physical processes that cause this are complex. Still, the behavior is necessary to satisfy simultaneous conservation of gravitational and thermal energy in a star with the shell structure. The core contracts and heats up due to

1156-400: A small range of luminosities around 75  L ☉ . Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red-giant branch, up to several times more luminous at the end of the thermal pulsing phase. Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to

1224-594: A star, helium fusion will continue in a shell around the carbon–oxygen core. In all cases, helium is fused to carbon via the triple-alpha process, i.e., three helium nuclei are transformed into carbon via Be . This can then form oxygen, neon, and heavier elements via the alpha process. In this way, the alpha process preferentially produces elements with even numbers of protons by the capture of helium nuclei. Elements with odd numbers of protons are formed by other fusion pathways. The reaction rate density between species A and B , having number densities n A , B ,

1292-400: A white dwarf. Very-low-mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and

1360-406: Is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun . However, their outer envelope is lower in temperature, giving them a yellowish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than

1428-487: Is called the horizontal branch in metal-poor stars , so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram. An analogous process occurs when the core helium is exhausted, and the star collapses once again, causing helium in a shell to begin fusing. At the same time, hydrogen may begin fusion in

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1496-473: Is driven by gravitational collapse and its associated heating, resulting in the subsequent burning of carbon , oxygen and silicon . However, most of the nucleosynthesis in the mass range A = 28–56 (from silicon to nickel) is actually caused by the upper layers of the star collapsing onto the core , creating a compressional shock wave rebounding outward. The shock front briefly raises temperatures by roughly 50%, thereby causing furious burning for about

1564-403: Is given by: r = n A n B k {\displaystyle r=n_{A}\,n_{B}\,k} where k is the reaction rate constant of each single elementary binary reaction composing the nuclear fusion process: k = ⟨ σ ( v ) v ⟩ {\displaystyle k=\langle \sigma (v)\,v\rangle } here, σ ( v )

1632-538: Is more important in more massive main-sequence stars. These works concerned the energy generation capable of keeping stars hot. A clear physical description of the proton–proton chain and of the CNO cycle appears in a 1968 textbook. Bethe's two papers did not address the creation of heavier nuclei, however. That theory was begun by Fred Hoyle in 1946 with his argument that a collection of very hot nuclei would assemble thermodynamically into iron . Hoyle followed that in 1954 with

1700-1588: Is the reduced mass . Since this integration has an exponential damping at high energies of the form ∼ e − E k T {\displaystyle \sim e^{-{\frac {E}{kT}}}} and at low energies from the Gamow factor, the integral almost vanished everywhere except around the peak, called Gamow peak , at E 0 , where: ∂ ∂ E ( − E G E − E k T ) = 0 {\displaystyle {\frac {\partial }{\partial E}}\left(-{\sqrt {\frac {E_{\text{G}}}{E}}}-{\frac {E}{kT}}\right)\,=\,0} Thus: E 0 = ( 1 2 k T E G ) 2 3 {\displaystyle E_{0}=\left({\frac {1}{2}}kT{\sqrt {E_{\text{G}}}}\right)^{\frac {2}{3}}} The exponent can then be approximated around E 0 as: e − E k T − E G E ≈ e − 3 E 0 k T exp ⁡ ( − ( E − E 0 ) 2 4 3 E 0 k T ) {\displaystyle e^{-{\frac {E}{kT}}-{\sqrt {\frac {E_{\text{G}}}{E}}}}\approx e^{-{\frac {3E_{0}}{kT}}}\exp \left(-{\frac {(E-E_{0})^{2}}{{\frac {4}{3}}E_{0}kT}}\right)} And

1768-400: Is the cross-section at relative velocity v , and averaging is performed over all velocities. Semi-classically, the cross section is proportional to π λ 2 {\displaystyle \pi \,\lambda ^{2}} , where λ = h / p {\displaystyle \lambda =h/p} is the de Broglie wavelength . Thus semi-classically

1836-481: Is the nearest M-class giant star at 88 light-years. The K1.5 red-giant branch star Arcturus is 36 light-years away. The Sun will exit the main sequence in approximately 5 billion years and start to turn into a red giant. As a red giant, the Sun will grow so large (over 200 times its present-day radius : ~ 215   R ☉ ; ~ 1  AU ) that it will engulf Mercury , Venus , and likely Earth. It will lose 38% of its mass growing, then will die into

1904-581: The absolute magnitude , see the list of most luminous stars . The Sun is the brightest star as viewed from Earth , at −26.78 mag. The second brightest is Sirius at −1.46 mag. For comparison, the brightest non-stellar objects in the Solar System have maximum brightnesses of: Any exact order of the visual brightness of stars is not perfectly defined for four reasons: All of these stars have multiple valid names or catalogue designations . The table lists their Bayer designation and

1972-414: The main sequence and will not have become giants yet) and more massive stars are expected to have more massive planets. However, the masses of the planets that have been found around giant stars do not correlate with the masses of the stars; therefore, the planets could be growing in mass during the stars' red giant phase. The growth in planet mass could be partly due to accretion from stellar wind, although

2040-421: The strong nuclear force which is effective only at very short distances. In the following decade the Gamow factor was used by Atkinson and Houtermans and later by Edward Teller and Gamow himself to derive the rate at which nuclear reactions would occur at the high temperatures believed to exist in stellar interiors. In 1939, in a Nobel lecture entitled "Energy Production in Stars", Hans Bethe analyzed

2108-412: The triple-alpha process . Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash . In more-massive stars, the collapsing core will reach these temperatures before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash. The core helium fusing phase of a star's life

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2176-469: The CNO cycle becomes the dominant source of energy. This temperature is achieved in the cores of main-sequence stars with at least 1.3 times the mass of the Sun . The Sun itself has a core temperature of about 1.57 × 10  K . As a main-sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its CNO cycle. Main sequence stars accumulate helium in their cores as

2244-430: The CNO cycle contributes more than 20% of the total energy. As the star ages and the core temperature increases, the region occupied by the convection zone slowly shrinks from 20% of the mass down to the inner 8% of the mass. The Sun produces on the order of 1% of its energy from the CNO cycle. The type of hydrogen fusion process that dominates in a star is determined by the temperature dependency differences between

2312-474: The CNO cycle energy generation occurs within the inner 15% of the star's mass, hence it is strongly concentrated at the core. This results in such an intense outward energy flux that convective energy transfer becomes more important than does radiative transfer . As a result, the core region becomes a convection zone , which stirs the hydrogen fusion region and keeps it well mixed with the surrounding proton-rich region. This core convection occurs in stars where

2380-422: The Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun ( L ☉ ); spectral types of K or M have surface temperatures of 3,000–4,000  K (compared with the Sun's photosphere temperature of nearly 6,000 K ) and radii up to about 200 times the Sun ( R ☉ ). Stars on the horizontal branch are hotter, with only

2448-404: The Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs. Very-high-mass stars develop into supergiants that follow an evolutionary track that takes them back and forth horizontally over the H–R diagram, at the right end constituting red supergiants . These usually end their life as

2516-422: The chemical elements in the solar system. Those abundances, when plotted on a graph as a function of the atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions (see history of nucleosynthesis theory ). This suggested a natural process that is not random. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it

2584-549: The cross section is proportional to m E {\textstyle {\frac {m}{E}}} . However, since the reaction involves quantum tunneling , there is an exponential damping at low energies that depends on Gamow factor E G , giving an Arrhenius equation : σ ( E ) = S ( E ) E e − E G E {\displaystyle \sigma (E)={\frac {S(E)}{E}}e^{-{\sqrt {\frac {E_{\text{G}}}{E}}}}} where S ( E ) depends on

2652-1113: The details of the nuclear interaction, and has the dimension of an energy multiplied for a cross section. One then integrates over all energies to get the total reaction rate, using the Maxwell–Boltzmann distribution and the relation: r V = n A n B ∫ 0 ∞ S ( E ) E e − E G E 2 E π ( k T ) 3 e − E k T 2 E m R d E {\displaystyle {\frac {r}{V}}=n_{A}n_{B}\int _{0}^{\infty }{\frac {S(E)}{E}}\,e^{-{\sqrt {\frac {E_{\text{G}}}{E}}}}2{\sqrt {\frac {E}{\pi (kT)^{3}}}}e^{-{\frac {E}{kT}}}\,{\sqrt {\frac {2E}{m_{\text{R}}}}}dE} where m R = m 1 m 2 m 1 + m 2 {\displaystyle m_{\text{R}}={\frac {m_{1}m_{2}}{m_{1}+m_{2}}}}

2720-423: The different possibilities for reactions by which hydrogen is fused into helium. He defined two processes that he believed to be the sources of energy in stars. The first one, the proton–proton chain reaction , is the dominant energy source in stars with masses up to about the mass of the Sun. The second process, the carbon–nitrogen–oxygen cycle , which was also considered by Carl Friedrich von Weizsäcker in 1938,

2788-504: The elements. It explains why the observed abundances of elements change over time and why some elements and their isotopes are much more abundant than others. The theory was initially proposed by Fred Hoyle in 1946, who later refined it in 1954. Further advances were made, especially to nucleosynthesis by neutron capture of the elements heavier than iron , by Margaret and Geoffrey Burbidge , William Alfred Fowler and Fred Hoyle in their famous 1957 B FH paper , which became one of

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2856-428: The end produces a helium nucleus as with the proton–proton chain. During a complete CNO cycle, 25.0 MeV of energy is released. The difference in energy production of this cycle, compared to the proton–proton chain reaction, is accounted for by the energy lost through neutrino emission. The CNO cycle is very temperature sensitive, a 10% rise of temperature would produce a 350% rise in energy production. About 90% of

2924-467: The exception of white dwarfs , are fusing hydrogen by these two processes. In the cores of lower-mass main-sequence stars such as the Sun , the dominant energy production process is the proton–proton chain reaction . This creates a helium-4 nucleus through a sequence of reactions that begin with the fusion of two protons to form a deuterium nucleus (one proton plus one neutron) along with an ejected positron and neutrino. In each complete fusion cycle,

2992-406: The first time-dependent models of the s -process in 1961 and of the r -process in 1965, as well as of the burning of silicon into the abundant alpha-particle nuclei and iron-group elements in 1968, and discovered radiogenic chronologies for determining the age of the elements. The most important reactions in stellar nucleosynthesis: Hydrogen fusion (nuclear fusion of four protons to form

3060-438: The habitable zone for 5.8 billion years and 2.1 billion years, respectively; for stars more massive than the Sun, the times are considerably shorter. As of 2023, several hundred giant planets have been discovered around giant stars. However, these giant planets are more massive than the giant planets found around solar-type stars. This could be because giant stars are more massive than the Sun (less massive stars will still be on

3128-408: The habitable zone lasts from 100 million years for a planet with an orbit similar to that of Mars to 210 million years for one that orbits at Saturn 's distance to the Sun, the maximum time (370 million years) corresponding for planets orbiting at the distance of Jupiter . However, planets orbiting a 0.5  M ☉ star in equivalent orbits to those of Jupiter and Saturn would be in

3196-502: The heating mechanisms for the chromospheres to form requires 3D simulations of red giants. Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells ( solar granules ), red-giant photospheres, as well as those of red supergiants , have just a few large cells, the features of which cause the variations of brightness so common on both types of stars. Red giants are evolved from main-sequence stars with masses in

3264-438: The hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars. When the star has mostly exhausted the hydrogen fuel in its core, the core's rate of nuclear reactions declines, and thus so do the radiation and thermal pressure the core generates, which are what support

3332-449: The individual articles. Hydrogen burning In astrophysics , stellar nucleosynthesis is the creation of chemical elements by nuclear fusion reactions within stars . Stellar nucleosynthesis has occurred since the original creation of hydrogen , helium and lithium during the Big Bang . As a predictive theory , it yields accurate estimates of the observed abundances of

3400-503: The lack of fusion, and so the outer layers of the star expand greatly, absorbing most of the extra energy from shell fusion. This process of cooling and expanding is the subgiant stage. When the envelope of the star cools sufficiently it becomes convective , the star stops expanding, its luminosity starts to increase, and the star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram . The evolutionary path

3468-464: The luminosity by around 10 times. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as

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3536-456: The main sequence when its core reaches a temperature (several million kelvins ) high enough to begin fusing hydrogen-1 (the predominant isotope), and establishes hydrostatic equilibrium . (In astrophysics, stellar fusion is often referred to as "burning", with hydrogen fusion sometimes termed " hydrogen burning ".) Over its main sequence life, the star slowly fuses the hydrogen in the core into helium; its main-sequence life ends when nearly all

3604-575: The most common proper name . Most of the proper names have been approved by the Working Group on Star Names of the International Astronomical Union (IAU). Popular names which have not been approved by the IAU are omitted. The source of magnitudes cited in this list is the linked Misplaced Pages articles. This basic list is a catalog of what Misplaced Pages itself documents. References can be found in

3672-401: The most heavily cited papers in astrophysics history. Stars evolve because of changes in their composition (the abundance of their constituent elements) over their lifespans, first by burning hydrogen ( main sequence star), then helium ( horizontal branch star), and progressively burning higher elements . However, this does not by itself significantly alter the abundances of elements in

3740-400: The name, stars on a blue loop from the red giant branch are typically not blue in colour but are rather yellow giants, possibly Cepheid variables. They fuse helium until the core is largely carbon and oxygen . The most massive stars become supergiants when they leave the main sequence and quickly start helium fusion as they become red supergiants . After the helium is exhausted in the core of

3808-473: The possibility that the heavier elements are produced in stars. This was a preliminary step toward the idea of stellar nucleosynthesis. In 1928 George Gamow derived what is now called the Gamow factor , a quantum-mechanical formula yielding the probability for two contiguous nuclei to overcome the electrostatic Coulomb barrier between them and approach each other closely enough to undergo nuclear reaction due to

3876-408: The proton–proton chain reaction releases about 26.2 MeV. The proton–proton chain reaction cycle is relatively insensitive to temperature; a 10% rise of temperature would increase energy production by this method by 46%, hence, this hydrogen fusion process can occur in up to a third of the star's radius and occupy half the star's mass. For stars above 35% of the Sun's mass, the energy flux toward

3944-416: The range from about 0.3  M ☉ to around 8  M ☉ . When a star initially forms from a collapsing molecular cloud in the interstellar medium , it contains primarily hydrogen and helium, with trace amounts of " metals " (in astrophysics, this refers to all elements heavier than hydrogen and helium). These elements are all uniformly mixed throughout the star. The star "enters"

4012-593: The reaction rate is approximated as: r V ≈ n A n B 4 2 3 m R E 0 S ( E 0 ) k T e − 3 E 0 k T {\displaystyle {\frac {r}{V}}\approx n_{A}\,n_{B}\,{\frac {4{\sqrt {2}}}{\sqrt {3m_{\text{R}}}}}\,{\sqrt {E_{0}}}{\frac {S(E_{0})}{kT}}e^{-{\frac {3E_{0}}{kT}}}} Values of S ( E 0 ) are typically 10 – 10 keV · b , but are damped by

4080-434: The star against gravitational contraction . The star further contracts, increasing the pressures and thus temperatures inside the star (as described by the ideal gas law ). Eventually a "shell" layer around the core reaches temperatures sufficient to fuse hydrogen and thus generate its own radiation and thermal pressure, which "re-inflates" the star's outer layers and causes them to expand. The hydrogen-burning shell results in

4148-444: The star has about 0.2 to 0.5  M ☉ , it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium. These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become

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4216-426: The star takes as it moves along the red-giant branch depends on the mass of the star. For the Sun and stars of less than about 2  M ☉ the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate , it will continue to heat until it reaches a temperature of roughly 1 × 10 K , hot enough to begin fusing helium to carbon via

4284-487: The surface in what is called a dredge-up . The first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge-up occurs during helium shell burning on the asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars. The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations. Rather, due to

4352-435: The surface is sufficiently low and energy transfer from the core region remains by radiative heat transfer , rather than by convective heat transfer . As a result, there is little mixing of fresh hydrogen into the core or fusion products outward. In higher-mass stars, the dominant energy production process is the CNO cycle , which is a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries and in

4420-415: The two reactions. The proton–proton chain reaction starts at temperatures about 4 × 10   K , making it the dominant fusion mechanism in smaller stars. A self-maintaining CNO chain requires a higher temperature of approximately 1.6 × 10  K , but thereafter it increases more rapidly in efficiency as the temperature rises, than does the proton–proton reaction. Above approximately 1.7 × 10  K ,

4488-452: The universe as the elements are contained within the star. Later in its life, a low-mass star will slowly eject its atmosphere via stellar wind , forming a planetary nebula , while a higher–mass star will eject mass via a sudden catastrophic event called a supernova . The term supernova nucleosynthesis is used to describe the creation of elements during the explosion of a massive star or white dwarf . The advanced sequence of burning fuels

4556-435: The very low mass density of the envelope, such stars lack a well-defined photosphere , and the body of the star gradually transitions into a ' corona '. The coolest red giants have complex spectra, with molecular lines , emission features, and sometimes masers , particularly from thermally pulsing AGB stars. Observations have also provided evidence of a hot chromosphere above the photosphere of red giants, where investigating

4624-400: Was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light. In 1920, Arthur Eddington , on the basis of the precise measurements of atomic masses by F.W. Aston and a preliminary suggestion by Jean Perrin , proposed that stars obtained their energy from nuclear fusion of hydrogen to form helium and raised

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