Projected timeline of the Sun's life

In astronomy, stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from hundreds of thousands to billions of years. Image File history File links Download high-resolution version (872x210, 53 KB) Illustration of the life-cycle of the Sun. ... Image File history File links Download high-resolution version (872x210, 53 KB) Illustration of the life-cycle of the Sun. ... For other uses, see Astronomy (disambiguation). ... This article is about the astronomical object. ...

Stellar evolution is not studied by observing the life of a single star—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, each at a different point in its life, and simulating stellar structure with computer models. Spiral Galaxy ESO 269-57 Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, and chemical composition) of celestial objects such as stars, galaxies, and the interstellar medium, as well as their interactions. ... The simplest commonly used model of stellar structure is the spherically symmetric quasi-static model, which assumes that a star is very close to an equilibrium state, and that it is spherically symmetric. ... A computer simulation or a computer model is a computer program which attempts to simulate an abstract model of a particular system. ...

Birth

Main article: Star formation

NGC 604, a giant star-forming region in the Triangulum Galaxy

Hubble telescope image known as pillars of creation, where stars are forming in the Eagle Nebula

Stellar evolution begins with a giant molecular cloud (GMC), also known as a stellar nursery. Most of the 'empty' space inside a galaxy actually contains around 0.1 to 1 particle per cm³, but inside a GMC, the typical density is a few million particles per cm³. A GMC is between 50 and 300 light years across and contains 100,000 to 10,000,000 times as much mass as our Sun. Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ... Download high resolution version (1127x1201, 2479 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (1127x1201, 2479 KB) Wikipedia does not have an article with this exact name. ... The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a spiral galaxy about 3. ... Download high resolution version (935x920, 128 KB)Star forming pillars in the Eagle Nebula, as seen by the Hubble Space Telescopes WFPC2. ... Download high resolution version (935x920, 128 KB)Star forming pillars in the Eagle Nebula, as seen by the Hubble Space Telescopes WFPC2. ... The Hubble Space Telescope (HST) is a telescope orbiting the Earth at the outer edges of the atmosphere. ... The Eagle Nebula (also known as Messier Object 16, M16 or NGC 6611), perhaps one of the most famous and easily recognized space objects, is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Cheseaux in 1745-46. ... A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... A stellar nursery is a massive cosmic dust cloud in which microscopic particles may slowly aggregate due to gravitational attraction and eventually give rise to protostars and subsequently planetary systems, with one or more stars and planets. ... For other uses, see Galaxy (disambiguation). ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... Sol redirects here. ...

As a GMC orbits the galaxy, one of several events might occur to cause its gravitational collapse. GMCs may collide with each other, or pass through dense regions of spiral arms. A nearby supernova explosion can also be a trigger, sending shocked matter into the GMC at very high speeds. Finally, galactic collisions can trigger massive bursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces. This article or section does not cite its references or sources. ... A spiral galaxy presents a face-on view of its spiral arms. ... For other uses, see Supernova (disambiguation). ... The Andromeda Galaxy. ...

As it collapses, a GMC fragments, breaking 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, the fragments condense into rotating spheres of superhot gas known as protostars. Potential energy can be thought of as energy stored within a physical system. ... A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ...

This initial stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the GMC. Often, these star-forming cocoons can be seen in silhouette against bright emission from surrounding gas, and are then known as Bok globules. For other uses, see Silhouette (disambiguation). ... An image of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope A Bok globule is a dark cloud of dense dust and gas in which star formation is sometimes taking place. ...

Very small protostars never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs. The exact boundary between stars and brown dwarfs depends upon chemical composition; those with higher metallicity (relative abundance of elements heavier than hydrogen and helium) have a lower limit. For an object with a metallicity roughly equal to that of the Sun, the boundary is roughly 0.075 solar masses. Brown dwarfs heavier than 13 Jupiter masses (M_J) do fuse deuterium, and some astronomers prefer to call only these objects brown dwarfs, classifying anything larger than a planet but smaller than this a sub-stellar object. Both types, deuterium-burning or not, shine dimly and die away slowly, cooling gradually over hundreds of millions of years. The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ... The globular cluster M80. ... For other uses, see Jupiter (disambiguation). ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ...

For a more massive protostar, the core temperature will eventually reach 10 megakelvins, initiating the proton-proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium. In stars of slightly over 1 solar mass, the CNO cycle contributes a considerable portion of the energy generation. The onset of nuclear fusion leads over a relatively short time to a hydrostatic equilibrium in which energy released by the core exerts a "radiation pressure" balancing the weight of the star's matter, preventing further gravitational collapse. The star thus evolves rapidly to a stable state. The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... Overveiw of the proton-proton chain. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... For other uses, see Helium (disambiguation). ... This article does not cite its references or sources. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ...

LH 95 stellar nursery in Large Magellanic Cloud

New stars come in a variety of sizes and colors. They range in spectral type from hot and blue to cool and red, and in mass from at least as low as 0.085 solar masses to more than 20 solar masses. The brightness and color of a star depend on its surface temperature, which in turn depends on its mass. Image File history File linksMetadata Download high-resolution version (1196x1280, 2043 KB) Videos of LH95 at ESA/Hubble Image of LH95 at ESA/Hubble LH 95 stellar nurserie in Large Magellanic Cloud source: http://hubblesite. ... Image File history File linksMetadata Download high-resolution version (1196x1280, 2043 KB) Videos of LH95 at ESA/Hubble Image of LH95 at ESA/Hubble LH 95 stellar nurserie in Large Magellanic Cloud source: http://hubblesite. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequenly refined in terms of other characteristics. ...

A new star will fall at a specific point on the main sequence of the Hertzsprung-Russell diagram. Small, cool red dwarfs burn hydrogen slowly and will remain on the main sequence for hundreds of billions of years, while massive hot supergiants will leave the main sequence after just a few million years. A mid-sized star like the Sun will remain on the main sequence for about 10 billion years. The Sun is thought to be in the middle of its lifespan; thus, it is on the main sequence. Once a star expends most of the hydrogen in its core, it moves off the main sequence. Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. ... For the type of star, see Red dwarf. ... Supergiants are the most massive stars. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ...

A dense starfield in Sagittarius

Image File history File links Sagittarius_Star_Cloud. ... Image File history File links Sagittarius_Star_Cloud. ... For other uses, see Sagittarius. ...

Maturity

After millions to billions of years, depending on the initial mass of the star, the continuous fusion of hydrogen into helium will cause a build-up of helium in the core. Larger and hotter stars produce helium more rapidly than cooler and less massive ones. The accumulation of helium, which is denser than hydrogen, in the core causes gravitational self-compression and a gradual increase in the rate of fusion. Higher temperatures must be attained to resist this increase in gravitational compression and to maintain a steady state.

Eventually, the core exhausts its supply of hydrogen, and without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity, it contracts until either electron degeneracy becomes sufficient to oppose gravity, or the core becomes hot enough (around 100 megakelvins) for helium fusion to begin. Which of these happens first depends upon the star's mass. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium. ...

Low-mass stars

What happens after a low-mass star ceases to produce energy through fusion is not directly known: the universe is thought to be around 13.7 billion years old, which is less time (by several orders of magnitude, in some cases) than it takes for the fusion to cease in such stars. Current theory is based on computer modelling. For other uses, see Universe (disambiguation). ...

A star of less than about 0.5 solar mass will never be able to fuse helium even after the core ceases hydrogen fusion. There simply is not a stellar envelope massive enough to bear down enough pressure on the core. These are the red dwarfs, such as Proxima Centauri, some of which will live thousands of times longer than the Sun. Recent astrophysical models suggest that red dwarfs of 0.1 solar masses may stay on the main sequence for almost six trillion years, and take several hundred billion more to slowly collapse into a white dwarf.^[1] If a star's core becomes stagnant (as is thought will be the case for the Sun), it will still be surrounded by layers of hydrogen which the star may subsequently draw upon. However, if the star is fully convective (as thought to be the case for the lowest-mass stars), it will not have such surrounding layers. If it does, it will develop into a red giant as described for mid-sized stars below, but never fuse helium as they do; otherwise, it will simply contract until electron degeneracy pressure halts its collapse, thus directly turning into a white dwarf. For the type of star, see Red dwarf. ... Proxima Centauri (Latin proximus, -a, -um: meaning next to or nearest to)[4] is a red dwarf star that is likely a part of the Alpha Centauri star system and is the nearest star to the Sun at a distance of 4. ... This article or section does not adequately cite its references or sources. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ...

Mid-sized stars

The Cat's Eye Nebula, a planetary nebula formed by the death of a star with about the same mass as the sun

In either case, the accelerated fusion in the hydrogen-containing layer immediately over the core causes the star to expand. Since this lifts the outer layers away from the core, thus reducing the gravitational pull on them, they expand faster than the energy production increases, thus causing them to cool, and thus causing the star to become redder than when it was on the main sequence. Such stars are known as red giants. Download high resolution version (800x874, 64 KB)X-ray/optical composite image of NGC 6543, the Cats Eye Nebula (X-ray: NASA/UIUC/Y.Chu et al. ... Download high resolution version (800x874, 64 KB)X-ray/optical composite image of NGC 6543, the Cats Eye Nebula (X-ray: NASA/UIUC/Y.Chu et al. ... The Cats Eye Nebula (NGC 6543) is a planetary nebula in the constellation of Draco. ... NGC 6543, The Cats Eye Nebula NGC 6853, The Dumbbell Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ...

According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M. Examples include Aldebaran in the constellation Taurus and Arcturus in the constellation of Bootes. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... This article is about the astronomical object. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ... Aldebaran from the Arabic (الدبران al-dabarān) meaning the follower, (α Tau / α Tauri / Alpha Tauri) is the brightest star in the constellation Taurus and one of the brightest stars in the nighttime sky. ... Taurus (IPA: , Latin: , symbol , ) is one of the constellations of the zodiac. ... Arcturus (α Boo / α Boötis / Alpha Boötis) (IPA: ) is the brightest star in the constellation Boötes, and the third brightest star in the night sky, with a visual magnitude of −0. ... BOOTES, the Burst Observer and Optical Transient Exploring System, is located in Southern Spain and makes use of two sets of wide-field cameras, 240 km apart. ...

A star of up to a few solar masses will develop a helium core supported by electron degeneracy pressure, surrounded by layers which still contain hydrogen. Its gravity compresses the hydrogen in the layer immediately above it, thus causing it to fuse faster than hydrogen would fuse in a main-sequence star of the same mass. This in turn causes the star to become more luminous (from 1,000 – 10,000 times brighter) and expand; the degree of expansion outstrips the increase in luminosity, thus causing the effective temperature to decrease. For other uses, see Helium (disambiguation). ... The effective temperature of a star is the temperature of a black body with the same luminosity (L) as the star and is defined according to the Stefan-Boltzman law L = sigma T_{eff}^{4}. The effective temperature of our Sun is around 5,800 kelvins (K) and correspond to...

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 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, with lower ¹²C/¹³C ratios and altered proportions of carbon and nitrogen. These are detectable with spectroscopy, and have been measured for many evolved stars. Convection in the most general terms refers to the internal movement of currents within fluids (i. ... Isotopes are atoms of a chemical element whose nuclei have the same atomic number, Z, but different atomic weights, A. The word isotope, meaning at the same place, comes from the fact that isotopes are located at the same place on the periodic table. ... This article does not cite its references or sources. ... Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ...

Simplified illustration of the evolution of a star with the mass of the Sun. The star forms from a collapsing cloud of gas (1) and then undergoes a contraction period as a protostar (2) before joining the main sequence (3). Once the Hydrogen at the core is consumed it expands into a red giant (4), then sheds its envelope into a planetary nebula and degenerates into a white dwarf (5).

As the hydrogen around the core is consumed, the core absorbs the resulting helium, causing it to contract further, which in turn causes the remaining hydrogen to fuse even faster. This eventually leads to ignition of helium fusion (which includes the triple-alpha process) in the core. In stars of more than approximately 0.5 solar masses, electron degeneracy pressure may delay helium fusion for millions or tens of millions of years; in more massive stars, the combined weight of the helium core and the overlying layers means that such pressure is not sufficient to delay the process significantly. Image File history File links Stellar_evolution_sun. ... Image File history File links Stellar_evolution_sun. ... Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium. ... Overview of the Triple-alpha process. ...

When the temperature and pressure in the core become sufficient to ignite helium fusion in the core, a helium flash will occur if the core is largely supported by electron degeneracy pressure; in more massive stars, whose core is not overwhelmingly supported by electron degeneracy pressure, the ignition of helium fusion occurs relatively quietly. Even if a helium flash occurs, the time of very rapid energy release (on the order of 10⁸ Suns) is brief, so that the visible outer layers of the star are relatively undisturbed.^[2] The energy released by helium fusion causes the core to expand, so that hydrogen fusion in the overlying layers slows, and thus total energy generation decreases. Therefore, the star contracts, although not all the way to the main sequence; it thus migrates to the horizontal branch on the HR-diagram, gradually shrinking in radius and increasing its surface temperature. A Helium flash is the sudden beginning of helium burning in the core of intermediate mass stars, or on the surface of an accreting white dwarf star. ... The horizontal branch (HB) is a stage of stellar evolution which immediately follows the red giant branch. ...

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star follows the Asymptotic Giant Branch on the HR-diagram, paralleling the original red giant evolution, but with even faster energy generation (which thus lasts for a shorter time).^[3] A period of Stellar evolution undertaken by all low to intermediate mass stars (0. ...

Changes in the energy output cause the star to change in size and temperature for certain periods. The energy output itself is shifted to lower frequency emission. This is accompanied by increased mass loss through powerful stellar winds and violent pulsations. Stars in this phase of life are called Late type stars, OH_{-IR stars} or Mira-type stars, depending on their exact characteristics. 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 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. Chandra X-ray Image of Mira Mira (ο Cet / 68 Ceti / HD14386 / HIP10826 / ADS 1778 AP / Omicron Ceti) is a binary star in the constellation Cetus consisting of the red giant, Mira A or just Mira, and a white dwarf, Mira B or VZ Ceti. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colorless (gas) pale blue (liquid) Standard atomic weight 15. ... For other uses, see Carbon (disambiguation). ... An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. ...

Helium burning reactions are extremely sensitive to temperature, which causes great instability. Huge pulsations build up, which eventually give the outer layers of the star enough kinetic energy to be ejected, potentially forming a planetary nebula. At the center of the nebula remains the core of the star, which cools down to become a small but dense white dwarf. The kinetic energy of an object is the extra energy which it possesses due to its motion. ... NGC 6543, The Cats Eye Nebula NGC 6853, The Dumbbell Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... This article or section does not adequately cite its references or sources. ...

Massive stars

The Crab Nebula, the shattered remnants of a star which exploded as a supernova almost 1000 years ago

In massive stars, the core is already large enough at the onset of hydrogen shell burning 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 much as lower mass stars; however, they were much brighter than lower mass stars to begin with, and are thus still brighter than the red giants formed from less massive stars. These stars are known as red supergiants. Image File history File linksMetadata Download high resolution version (2224x2212, 3149 KB) Summary Image: A Giant Hubble Mosaic of the Crab Nebula Source: http://hubblesite. ... Image File history File linksMetadata Download high resolution version (2224x2212, 3149 KB) Summary Image: A Giant Hubble Mosaic of the Crab Nebula Source: http://hubblesite. ... The Crab Nebula (catalogue designations M 1, NGC 1952, Taurus A) is a supernova remnant in the constellation of Taurus. ... Supergiants are the most massive stars. ...

Extremely massive stars (more than approximately 40 solar masses), 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. 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 the way from the core to the surface, resulting in the absence of a separate core and envelope due to thorough mixing.^[4]

The core grows hotter and denser as it gains material from fusion of hydrogen at the base of the envelope. In a massive star, electron degeneracy pressure is insufficient to halt collapse by itself, so as each major element is consumed in the center, progressively heavier elements ignite, temporarily halting collapse. If the core of the star is not too massive (less than approximately 1.4 solar masses, taking into account mass loss that has occurred by this time), it may then form a white dwarf (possibly surrounded by a planetary nebula) as described above for less massive stars, with the difference that the white dwarf is composed chiefly of oxygen, neon, and magnesium.

The onion-like layers of a massive, evolved star just prior to core collapse. (Not to scale.)

Above a certain mass (estimated at approximately 2.5 solar masses, within a star originally of around 10 solar masses), the core will reach the temperature (approximately 1.1 gigakelvins) at which neon partially breaks down to form oxygen and helium, the latter of which immediately fuses with some of the remaining neon to form magnesium; then oxygen fuses to form sulfur, silicon, and smaller amounts of other elements. Finally, the temperature gets high enough that any nucleus can be partially broken down, most commonly releasing an alpha particle (helium nucleus) which immediately fuses with another nucleus, so that several nuclei are effectively rearranged into a smaller number of heavier nuclei, with net release of energy because the addition of fragments to nuclei exceeds the energy required to break them off the parent nuclei. Image File history File links Evolved_star_fusion_shells. ... Image File history File links Evolved_star_fusion_shells. ... Neon burning process is a set of nuclear fusion reactions that take place in massive stars (at least 8 MSun). ... The oxygen burning process is a nuclear fusion reaction that occurs in massive stars that have used up the lighter elements in their cores. ...

A star with a core mass too great to form a white dwarf but insufficient to achieve sustained conversion of neon to oxygen and magnesium will undergo core collapse (due to electron capture, as described above) before achieving fusion of the heavier elements.^[5] Both heating and cooling caused by electron capture onto minor constituent elements (such as aluminum and sodium) prior to collapse caused by electron capture onto major constituent elements may have a significant impact on total energy generation within the star shortly before collapse.^[6] This may produce a noticeable effect on the abundance of elements and isotopes ejected in the subsequent supernova.

Once the nucleosynthesis process arrives at iron-56, the continuation of this process consumes energy (the addition of fragments to nuclei releases less energy than required to break them off the parent nuclei). If the mass of the core exceeds the Chandrasekhar limit, electron degeneracy pressure will be unable to support its weight against the force of gravity, and the core will undergo sudden, catastrophic collapse to form a neutron star or (in 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 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. As some of the rebounding matter is bombarded by the neutrons, some of its nuclei capture them, creating a spectrum of heavier-than-iron material including the radioactive elements up to (and likely beyond) uranium.^[7] Although non-exploding red giant stars 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 is quite different from that produced in a supernova. Neither abundance alone matches that found in our solar system, so both supernovae and ejection of elements from red giant stars are required to explain the observed abundance of heavy elements and isotopes thereof. Cross section of a red giant showing nucleosynthesis and elements formed Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the heavier elements. ... For other uses, see Iron (disambiguation). ... The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... This article is in need of attention from an expert on the subject. ... For other uses, see Black hole (disambiguation). ... Potential energy can be thought of as energy stored within a physical system. ... For other uses, see Supernova (disambiguation). ... For other uses, see Neutrino (disambiguation). ... Beaded ring brightens from 2003 and 2005 SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. ... Nucleon is the common name used in nuclear chemistry to refer to a neutron or a proton, the components of an atoms nucleus. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 940 MeV/c² (1. ... Introduction The shock wave is one of several different ways in which a gas in a supersonic flow can be compressed. ... General Name, symbol, number uranium, U, 92 Chemical series actinides Group, period, block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ... This article is about the Solar System. ...

The energy transferred from collapse of the core to rebounding material not only generates heavy elements, but (by a mechanism which is not fully understood) provides for their acceleration well beyond escape velocity, thus causing a Type Ib, Type Ic, or Type II supernova. Note that 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.^[8] 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 the collapse of an iron core.^[9]

Stellar remnants

After a star has burned out its fuel supply, its remnants can take one of three forms, depending on the mass during its lifetime.

White dwarfs

Main article: White dwarf

For a star of 1 solar mass, the resulting white dwarf is of about 0.6 solar masses, compressed into approximately the volume of the Earth. White dwarfs are stable because the inward pull of gravity is balanced by the degeneracy pressure of the star's electrons. (This is 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. This article or section does not adequately cite its references or sources. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. ...

The chemical composition of the white dwarf depends upon its mass. A star of a few solar masses will ignite carbon fusion to form magnesium, neon, and smaller amounts of other elements, resulting in 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 apart the star in a supernova.^[10] 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 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. The carbon burning process is a nuclear fusion reaction that occurs in massive stars (at least 4 MSun at birth) that have used up the lighter elements in their cores. ... The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ...

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 dwarf stars to exist yet. A black dwarf is a hypothetical astronomical object: a white dwarf so old that it has cooled down so that it no longer emits significant heat or light. ...

If the white dwarf's mass increases above the Chandrasekhar limit, which is 1.4 solar masses 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 either lead to collapse into a neutron star or runaway ignition of carbon and oxygen. Heavier elements favor continued core collapse, because they require 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.^[11] 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 solar masses can exist (with 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. The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ... Electron capture is a decay mode for isotopes that will occur when there are too many protons in the nucleus of an atom, and there isnt enough energy to emit a positron; however, it continues to be a viable decay mode for radioactive isotopes that can decay by positron... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... Centrifugal force (from Latin centrum centre and fugere to flee) is a term which may refer to two different forces which are related to rotation. ... A binary system is an astronomy term referring to two objects in space, usually stars, which are so close that their gravitational forces attract one another into a mutual orbit. ...

If a white dwarf forms a close binary system with another star, hydrogen from the larger companion may accrete around and onto a white dwarf until it gets hot enough to fuse in a runaway reaction, although the white dwarf remains below the Chandrasekhar limit. Such an explosion is termed a nova. Artists conception of a white dwarf star accreting hydrogen from a larger companion A nova (pl. ...

Neutron stars

Bubble-like shock wave still expanding from a supernova explosion 15,000 years ago (view larger image).

Main article: Neutron star

When a stellar core collapses, the pressure causes electron capture, thus converting the great majority of the protons into neutrons. The electromagnetic forces keeping separate nuclei apart are gone (proportionally, if nuclei were the size of dust motes, atoms would be as large as football stadiums), and most of the core of the star becomes a dense ball of contiguous neutrons (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 a way analogous to electron degeneracy pressure, but stronger. Cygnus Loop Small, NASA Copyright released [1] File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Cygnus Loop Small, NASA Copyright released [1] File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... For other uses, see Proton (disambiguation). ... This article or section does not adequately cite its references or sources. ...

These stars, known as neutron stars, are extremely small—on the order of radius 10km, no bigger than the size of a large city—and are phenomenally dense. Their period of revolution shortens dramatically as the star shrinks (due to conservation of angular momentum); some spin at over 600 revolutions per second. When these rapidly rotating stars' magnetic poles are aligned with the Earth, a pulse of radiation is received each revolution. Such neutron stars are called pulsars, and were the first neutron stars to be discovered. In physics, angular momentum intuitively measures how much the linear momentum is directed around a certain point called the origin; the moment of momentum. ... It has been suggested that Radio pulsar be merged into this article or section. ...

Black holes

Main article: Black hole

If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse below 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 solar masses. For other uses, see Black hole (disambiguation). ... The Schwarzschild radius (sometimes inappropriately referred to as the gravitational radius[1]) is a characteristic radius associated with every mass. ...

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 the universe is well supported, both theoretically and by astronomical observation. For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ...

Since the core collapse supernova mechanism itself is imperfectly 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; 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.

References

Additional References

• Astronomy 606 (Stellar Structure and Evolution) lecture notes, Cole Miller, Department of Astronomy, University of Maryland
• Astronomy 162, Unit 2 (The Structure & Evolution of Stars) lecture notes, Richard W. Pogge, Department of Astronomy, Ohio State University

The University of Maryland, College Park (also known as UM, UMD, or UMCP) is a public university located in the city of College Park, in Prince Georges County, Maryland, just outside Washington, D.C., in the United States. ... The Ohio State University (OSU) is a coeducational public research university in the state of Ohio. ...

See also

• Nucleosynthesis
• Timeline of stellar astronomy
• Stellar engineering
• Galaxy_formation_and_evolution

Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). ... Timeline of stellar astronomy 134 BC - Hipparchus creates the magnitude scale of stellar apparent luminosities 1596 - David Fabricius notices that Miras brightness varies 1672 - Geminiano Montanari notices that Algols brightness varies 1686 - Gottfried Kirch notices that Chi Cygnis brightness varies 1718 - Edmund Halley discovers stellar proper motions... Stellar engineering is the practice of creating or modifying stars through artificial means. ... In astrophysics, the questions of galaxy formation and evolution are: How, from a homogeneous universe, did we obtain the very heterogeneous one we live in? How did galaxies form? How do galaxies change over time? A spectacular head-on collision between two galaxies is seen in this NASA Hubble Space...