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The remnant of type Ia supernova SNR 0509 in the Large Magellanic Cloud about 160,000 light years from earth. Despite its tranquil delicate appearance this is the remnant of a massive and violent death of a star known as a white dwarf, blasted into space during core collapse. The visible diffuse nebula is 23 light-years across and is still expanding at more than 18 million km/h (11 million mph). Photographed in hydrogen light by Hubble (credits: NASA/ESA)[1]
This page is about core collapse in massive stars and white dwarfs. For the phenomenon that can occur during the evolution of a globular cluster of stars, see core collapse (cluster).

Stellar core collapse occurs when electron degeneracy and energy-generating fusion processes at the core of a massive star are unable to resist the inward gravitational forces affecting a star, causing the star's core to implode.[2] The implosion is cataclysmic. Depending upon the cause and the star's pre-collapse structure, the results may include release of massive amounts of energy or subatomic particles, production of elements beyond iron, and lead to a supernova, neutron star, or black hole.

Four main mechanisms are known to cause core collapse in stars: - failure of fusion, when the fuel cycle is unable to provide sufficient energy to counter the star's own gravity, accretion of material-typically by a white dwarf-from other objects, leading to sufficient mass for collapse, degeneracy of the core followed by electron capture,[3] and quantum fluctuations due to pair production in very massive stars causing the star to briefly lose supporting photon pressure. Two further possible modes of collapse are hypothesized, one based upon collapse halted by degeneracy pressure of quarks or smaller particles, and one in which external tidal forces in some binary star systems could cause compression and collapse. Both have been studied but the result is as yet unclear. The exact mode of collapse and the resulting products depends upon factors such as mass, rotation, presence of stable or unstable companion stars, and metallicity (the degree to which the star contains elements heavier than helium).

Scientific models of core collapse are still being refined, but ignoring rotational effects it is broadly believed that the key modes of collapse for a single star are as follows: Under about 8 solar masses a star does not have sufficient mass for core collapse. Around 8-10 solar masses the core does not initially collapse but forms a strongly degenerate oxygen-neon-magnesium core that eventually collapses due to electron capture. Above 10 solar masses a nickel-iron core forms that collapses once the final fusion process–silicon burning–ends, giving rise to a Type Ib and Ic ("stripped core-collapse") or type II supernova. Above around 25 solar masses and having solar or less metallicity, the same core collapse takes place but the resulting supernova is weak - the debris is drawn back into the neutron star remnant and the additional mass is sufficient to trigger further collapse into a black hole by fallback (see: Tolman-Oppenheimer-Volkoff limit). Most stars above about 40 solar masses and having solar or less metallicity collapse directly into a black hole.

In nickel-iron core collapse (the best known and first recognized type), a star possesses the mass needed to fuse elements that have an atomic mass greater than hydrogen and helium, albeit at increasingly high temperatures and pressure, and for increasingly shorter periods of time. The star fuses increasingly higher mass elements, starting with hydrogen and then helium, until finally a core of iron and nickel is produced. Fusion of iron or nickel produces no net energy, so further fusion is unable to take place. When the mass of the inert core exceeds the Chandrasekhar limit of about 1.44 solar masses, and with insufficient fusion pressure, electron degeneracy alone is no longer sufficient to counter gravity. A cataclysmic implosion takes place within seconds, in which the outer core reaches an inward velocity of up to 23% of the speed of light and the inner core reaches temperatures of up to 100 billion kelvin. The result of this collapse depends upon the type of star involved; in some cases the explosion (known as a "supernova") briefly outshines a galaxy, creates elements beyond iron that cannot ordinarily be created by stellar processes, and releases more energy than our sun will over its entire life. Because of the underlying mechanism, the resulting variable star for these is also described as a core-collapse supernova. Core collapse of a less massive inert neon-oxygen-magnesium core (recognized in 1979) occurs in a similar way but is triggered by loss of support via electron capture.

Two other types of core collapse mechanism are known to exist. Some white dwarfs that are insufficiently massive for core collapse may subsequently gain enough extra mass from external sources to trigger core collapse, leading to collapse, sudden heating, and a type Ia supernova. Also some very massive but low metallicity stars become unstable due to pair production and may blow themselves apart upon core collapse[citation needed] in a pair-instability supernova, leaving no remnant.

Overview of the stellar fuel cycle and structure of the core in large stars

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CHECK BURNING PROCESS PRODUCTS AND DATA CAREFULLY AGAINST SOURCES - INCONSISTENCIES NOTED BETWEEN THIS TABLE AND OTHER PAGES (EG Al)
 
The onion-like layers of a massive star of 8 or more solar masses, shown just before core collapse. (Not to scale) Fusion still takes place along layer boundaries. In some giant stars the outer hydrogen layer has been blown away.
Main articles: Giant star, Supergiant, and Stellar evolution

Stars far more massive than the sun evolve in complex ways. In the core of the sun, hydrogen is fused into helium, releasing thermal energy which heats the sun's core and provides pressure that supports the sun's layers against collapse in a process known as hydrostatic equilibrium. The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, fusion starts to slow down and gravity causes the core to contract. This contraction raises the temperature high enough to initiate the further fusion of helium into carbon, which accounts for less than 10% of the star's total lifetime.

A star of less than about 8 solar masses does not ordinarily undergo core collapse, other than in the event of accretion by a white dwarf. Once fusion ceases it gradually cools. Some stars of 4-8 solar masses explode without prior collapse and leaving no remnant. The two explosion processes are known respectively as carbon detonation and carbon deflagration, the former giving rise to core collapse and a type 1a supernova and the latter a carbon deflagration supernova without core collapse.

A star of more than about 8 solar masses is massive enough that as fusion reduces, gravity creates even higher temperatures and pressures, sufficient for the carbon in the core to begin to fuse once the star contracts at the end of the helium-burning stage. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, surrounding a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon, with inner layers fusing to progressively heavier elements. As a star this massive evolves, it undergoes repeated stages where fusion in the core comes to an end, and the core begins to collapse until pressures and temperatures are created capable of igniting the next stage of fusion, which temporarily halts collapse.[4][5]

As the star's burning cycles progress and temperature and pressure increase, other mechanisms also change. By the carbon burning stage, neutrino emission replaces electromagnetic radiation as the primary energy loss mechanism. Neutrinos can easily escape which is a factor in the rapidity of subsequent stages, and the subsequent phases may blend into each other rather than being distinct. During silicon burning photodisintegration becomes the primary mechanism for nucleosynthesis. In very massive but low-metal stars of 140 solar masses or more, conditions can arise where pair production is a significant mechanism, giving rise to fluctuations in the outward photon pressure which make the hydrostatic balance of the star increasingly unstable.

Fusion stages for a massive star (8 or more solar masses)
Process Main fuel Main products Typical data for a 25 M star[6]
Temperature
(Kelvin) 		Density
(g/cm3) 		Duration
Hydrogen burning via CNO cycle hydrogen helium (catalyzed by small amounts of heavier nuclei) 7×107 10 107 years
Helium burning via triple-alpha process helium primarily carbon, some oxygen 2×108 2×103 106 years
carbon burning process carbon primarily neon, sodium and magnesium, some aluminium, silicon and oxygen 8×108 106 103 years
neon burning process neon oxygen, magnesium 1.6×109 107 3 years
oxygen burning process
(stars of 10 or more solar masses only) 		oxygen silicon, sulfur, argon, calcium 1.8×109 107 0.3 years
silicon burning process
(stars of 10 or more solar masses only) 		silicon iron, nickel 2.5×109 108 1 - 5 days

The repeated fusion processes stop following the neon burning process for stars of 8 - 10 solar masses, leading to the creation of a strongly degenerate neon-oxygen-magnesium core. Stars over 10 solar masses proceed to two further steps, oxygen burning and silicon burning, creating an inert nickel-iron core. In both cases the core will collapse, but the mechanism is different for stars of about 8-10 solar masses and stars over 10 solar masses.

Massive single stars capable of core collapse have short lifespans of between 70 million years down to a few hundred thousand years (by the calculation M - 2.5 × 1.2×1010 where M = mass in sols and 1.2×1010 is the sun's lifetime).[7] They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies.

Core collapse processes

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Of the four verified collapse pathways, the first recognized and best studied is cessation of fusion, causing nickel-iron core collapse in stars of more than about 10 solar masses. Electron capture core collapse was proposed in 1979[8] and is responsible for oxygen-neon-magnesium core collapse in stars of 8-10 solar masses. Collapse due to accretion can affect smaller white dwarfs with companion stars, that would not otherwise undergo core collapse. Finally, in certain extremely massive stars, pair production can lead to fluctuations and instability in the star's core, leading to core collapse. The following diagram shows the typical mode of collapse for single symmetrical stars of varying mass and metallicity, as modeled in 2003:[9]

 

Refinements of this basic core collapse model take into account whether the star has a significant amount of elements beyond hydrogen and helium (its "metallicity"), and whether or not the star retains its hydrogen envelope through to core collapse. Additional factors which can affect the mode of collapse, or its results, include the rotation and symmetry of the original star, and whether or not it has a companion star (eg, part of a binary pair or orbiting another compact object).

Cessation of fusion: nickel-iron core (10+ masses)

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Core collapse of a nickel-iron core star of 10 - 25 solar masses.

(a) During the life of the star, onion-layered shells form of fusing elements. During the final burning process a nickel-iron core is formed which is unable to fuse.

(b) The mass of the inert core eventually reaches Chandrasekhar-mass and it suddenly begins to detach from the rest of the star and rapidly collapse inward.

(c) The inward collapse causes extreme temperatures and pressures that compress the inner part of the core into neutrons; these are exceedingly rigid.

(d) The infalling material bounces off the newly formed neutron core, creating an outward-propagating shock front (red).

(e) The shock starts to stall, but it is re-invigorated by neutrino interactions (the exact details of how 1% of the neutrino energy is reabsorbed are not yet fully understood).

(f) The shock wave blasts the material surrounding the core into space, creating a supernova. A degenerate remnant in the form of a neutron star remains, often surrounded by a nebula of dispersing material. Stars of more than 25 solar masses collapse the same way but may leave a black hole or in some cases no remnant instead.

Main articles: Type II supernova and Type Ib and Ic supernovae

A star of more than about 10 solar masses undergoes all of the burning processes up to the final stage of silicon burning, the core becoming increasingly hotter and more dense. The factor limiting this escalation of fusion processes is that the amount of energy released through fusion is dependent on the binding energy that holds together these atomic nuclei. Each additional step produces progressively heavier nuclei, which release progressively less energy when fusing. In addition from carbon-burning onwards energy loss via neutrino production becomes significant, leading to a higher rate of reaction than would otherwise take place.[10] Fusion processes continue until iron-52 and then nickel-56 are produced via the silicon burning process which lasts only a few days. As iron and nickel have the highest binding energy per nucleon of all the elements,[11] further energy cannot be produced at the core by fusion, and a nickel-iron core grows.[5][12]

This inert core is under huge gravitational pressure. It is supported by two outward forces - photon pressure from fusion reactions, and degeneracy pressure of electrons. In this state, matter is so dense that further compaction would require electrons to occupy the same energy states. However, this is forbidden for identical fermion particles, such as the electron due to the Pauli exclusion principle. Lacking sufficient photon pressure from further fusion, and once the inert core's mass exceeds the Chandrasekhar limit, the degeneracy pressure of electrons can no longer balance the inward force of gravity. A cataclysmic implosion ensues taking just a few seconds[13] in which the outer part of the core reaches velocities of up to 70,000 km/s (23% of the speed of light) as it collapses inward.[14] As the rapidly shrinking core detaches from the outer layers of the star, it heats up, reaching temperatures around 100 billion kelvin and densities comparable to an atomic nucleus. In these conditions high-energy gamma rays are produced that decompose iron nuclei into helium nuclei and free neutrons via photodisintegration. As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and elementary particles called neutrinos. This process removes electrons and degeneracy pressure, causing thermal runaway and rapidly accelerating the collapse.[15] Because neutrinos rarely interact with normal matter they can escape from the core, carrying away additional energy and further accelerating the collapse, which proceeds over a timescale of milliseconds. Some of these neutrinos are absorbed by the star's outer layers.[16]

For stars of around 10 - 40 solar masses,[17] the core collapse phase is so dense and energetic that only neutrinos are able to escape. As the protons and electrons combine to form neutrons by means of electron capture, an electron neutrino is produced. In a typical Type II supernova, the newly formed neutron core has an initial temperature of about 100 billion kelvin; 105 times the temperature of the sun's core. A further release of neutrinos takes place as the core stabilizes.[18] These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos.[19] The two neutrino production mechanisms convert the gravitational potential energy of the collapse into a ten second neutrino burst, releasing about 1046 joules (100 foes).[20] Through a process that is not clearly understood, about 1% of this, or 1044 joules (1 foe) is reabsorbed by the stalled shock, producing an explosion.[a][21] The neutrinos generated by a supernova were actually observed in the case of Supernova 1987A, leading astronomers to conclude that the core collapse picture is basically correct. The water-based Kamiokande II and IMB instruments detected antineutrinos of thermal origin,[18] while the gallium-71-based Baksan instrument detected neutrinos (lepton number = 1) of either thermal or electron-capture origin.

The result of nickel-iron core collapse depends on the star's mass and metallicity. Although calculation of exact limits is uncertain:

  • The collapse of a progenitor star under about 25 solar masses or having more than solar metallicity is eventually halted by short-range repulsive neutron-neutron interactions, mediated by the strong force, as well as by degeneracy pressure of neutrons, at a density comparable to that of an atomic nucleus. The infalling outer core rebounds off the neutron matter that has been created, producing a shock wave that propagates outward and causing material from the outer layers of the star to be launched into interstellar space at high speeds - a type II supernova. The remnant at the heart of the explosion eventually stabilizes as a newly born neutron star.
  • Progenitor stars of around 25-40 solar masses and having solar or lower metallicity follow the same path, but the resulting neutron star is massive enough to cause the debris to fall back onto the stars surface. The resulting supernova is therefore weak, and after some time the mass of the remnant reaches the Tolman–Oppenheimer–Volkoff limit triggering further collapse into a black hole.
  • If the progenitor star was above about 40 solar masses and having solar or lower metallicity, then collapse is directly to a black hole, neutron degeneracy being insufficient to prevent this. There is no supernova.[22]

In the two latter cases, the only known force that might potentially prevent collapse to a black hole for some stars would be degeneracy pressure of quarks or their subcomponents (see below) - however at present this is an area we lack knowledge.

Electron capture: O-Ne-Mg core (8-10 masses)

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In stars of about 8 - 10 solar masses, fusion processes come to a halt after the neon burning process because the star is insufficiently massive to create the 4x increase in temperature and 500x increase in pressure needed to fuse oxygen.[23] However it does not immediately collapse, because even without further fusion the degeneracy pressure of electrons can support the star and prevent its collapse to neutrons. The star is left with a strongly degenerate core of neon and its fusion products - ie, a dense core comprising primarily oxygen, neon and magnesium. The initially stable core does eventually collapse, although this takes (TIME), because after fusion ceases, electron capture processes gradually begin to reduce or remove the free electrons whose degeneracy pressure provides the outward support for the core. When sufficient electrons are removed by capture, the core is no longer able to oppose gravitational collapse even though it is less massive than the 10 solar masses of nickel-iron collapse, and a core collapse commences.

As with other massive stars, temperatures and pressures greatly increase during the core collapse. Since the star contains material capable of fusion at these higher temperatures and pressures, fusion restarts and the star reignites during collapse. The oxygen burning process commences, which in a nickel-iron star would stabilise the star's core against continuing collapse. However in the collapse of a strongly degenerate oxygen-neon-magnesium core, electron capture is also able to continue, and calculations show that these interactions remove free electrons at a fast enough rate to offset any additional degeneracy pressure from the renewed fusion. Therefore electron degeneracy pressure is unable to build to the level that would prevent the core collapsing to a neutron star, and core collapse progresses in a manner similar to larger nickel-iron collapse. The result would be observed as a Type II supernova and would ordinarily leave a neutron star remnant.[24]

Accretion (under 8 masses)

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FACT CHECK - compare to process described in this diagram
 

Simplified core collapse of a white dwarf in a binary system:

1. In a binary system system, one star evolves to a white dwarf

2. At some future time, its companion runs out of hydrogen fuel and evolves for a period of time into a giant star. (This is quite usual, our own sun will do this in about 5 bn years).

3. As a giant star, some of its outer material can now be captured by the white dwarf.

4. The white dwarf reaches the Chandrasekhar limit of around 1.4 solar masses, and can no longer support itself. It undergoes core collapse and a type 1a supernova event takes place.

Core collapse can also arise if two white dwarfs that are individually too small for core collapse, undergo orbital decay and merge into one object.

Main articles: Carbon detonation, Type 1a supernova, and Super soft X-ray source

The majority of stars are less than 8 solar masses. On their own, they dim and settle as white dwarfs at the end of their fusion lives. White dwarfs in a binary star system still have the potential for core collapse, by gradually gathering material from their companion star (known as "accretion"). When their augmented mass exceeds the Chandrasekhar limit of around 1.4 solar masses, the core can no longer support itself against collapse and a cataclysmic core collapse and type 1a supernova event takes place. Other forms of mass increase can have a similar effect in principle, for instance if two white dwarfs undergo orbital decay and spiral into each other, or a white dwarf gains extra mass from other objects.

The process believed to take place involves a binary star system of which one star has evolved to a white dwarf. Eventually its companion star reaches a point in its evolution where it expands by hundreds of times to form a giant star (such as a red giant)—this is quite common, our own sun will do this in around 5 - 7 billion years. Expansion causes its material to be close enough to be seized by the white dwarf which eventually surpasses the size limit for core collapse and implodes. The exact conditions are delicately balanced - some types of matter accretion can instead lead to a nova rather than core collapse, where instability causes excess material to be blasted into space; model refinements are required to confirm the conditions under which a net accretion of infalling matter and increase of mass will take place.[25][26]

White dwarfs are very stable stars of well defined structure. Their core collapse by accretion takes place at a well defined point (subject to rotation effects and other refinements). As a result, the resulting type 1a supernovae tend to occur under very similar conditions and at the same or similar mass across many star systems. They therefore also have very similar spectra, as modified by their red shift which in turn relates to their distance from Earth. This reliability allows them to be used as a "standard candle" for measuring distant clusters and galaxies. Because their progenitors are not massive stars they–unlike other core collapse progenitors–occur in (all? a wide range of?)[verification needed] galaxies and stellar populations including old non-star-forming regions.

Although the mechanics of core collapse by accretion are long established, a persistent mystery has existed–examples of type 1a supernovae progenitor stars have not yet been positively identified and proven. Scientists would expect to see evidence of binary white dwarfs actively accreting material and related super soft X-ray sources but to date direct evidence of these has not been found, leading to the question of whether type 1a supernovae might be formed in some other manner.[27] However indirect corroborative evidence related to the accretion model has been observed, and it remains the favored model in astrophysics. In the event of a type 1a supernova the giant companion star should experience a "kick" due to the explosion, and there should be an arc of radiative material stripped from the companion star, and an area of space shielded from detonation debris by the companion star. In 2004 the "probable surviving star" from Tycho's Supernova (SN 1572) was identified, and the other expected features such as the high-energy arc and low-debris region were also clearly visible in subsequent X ray astronomy images from 2011.[28] The unstable star V445 Puppis is also considered a likely progenitor candidate for a possible future event due to its mass (1.3 solar masses), instability, helium accretion, and recent activity since 2000. Combined with other evidence such as light spectra calculations and theoretical predictions of other observable outcomes,[which?] these are believed to confirm the basic model for accretion-based core collapse is correct. However exact details of the collapse process are still being refined.

Pair instability (some stars over 140 masses having low metallicity)

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Main articles: Pair production and Pair instability supernova

(to draft)

Unproven core collapse processes

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A companion star spiralling closer to a neutron star or black hole experiences elongation ("spaghettification") and lateral compression due to tidal forces. It is hypothesized by some scientists that this could be a fifth trigger for core collapse. Other scientists disagree.
Unsolved problems in physics
Quark matter. It is believed that quark-gluon plasma exists in some stars and may have been created for very brief moments in particle collider experiments. What are its properties, and can quark subcomponents also form degenerate phases of matter?  

These processes, unlike the four above, are not yet considered by astrophysicists to be established, although they have been studied.

External gravitational gradient

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It was suggested in a 1996 paper that tidal forces could cause the centers of binary neutron stars to become denser as a result of each others' gravitational field, due to tidal forces. Tidal forces are known to have extreme effects; when an object approaches a black hole sufficiently closely the gravitational field can be so extreme that the object is potentially ripped apart ("spaghettification"). The same effect is also responsible for the ocean tides and the tidal locking of our moon.

Spaghettification also involves lateral compression due to the extreme gravitational gradient (see diagram). Tidal force calculations were carried out on inward-spiralling binary neutron stars and a white dwarf orbiting a black hole. The authors concluded that potentially in some circumstances the lateral compression in a companion star of a neutron star or black hole could be sufficient to trigger core collapse. Subsequent papers rejected or argued the possibility, or noted factors that had not been modeled. This or a similar mode of collapse may be proven or disproven in future.

Quark novae and exotic matter core collapse

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Main articles: Quark nova, Quark star, Electroweak star, Exotic star, and Quark–gluon plasma

In the same way that massive stars can overcome electron degeneracy and collapse until prevented by neutron degeneracy, it has been widely speculated that under suitable conditions within a neutron star (or the collapsing core of a massive star) neutron core collapse could occur but further collapse to a black hole would not occur - it would ultimately be prevented by degeneracy pressure of quarks, the component parts of neutrons, ie, in a manner analogous to a type II supernova.

A quark nova is a hypothetical type of core collapse of a neutron star into a quark star that some scientists believe might happen when the degeneracy pressure of neutrons - but not their constituent quarks - is exceeded.[29]. They are hypothesized to occur in an analogous manner to known forms of electron-degeneracy core collapse. If proven to occur such a collapse could release immense amounts of energy estimated to be as much as 1047 J [30] (due to the immense binding energy of quarks and gravitational forces involved), and could potentially explain gamma ray bursts which rank among the most energetic explosions in the observed universe, or could be responsible for further production of heavy elements such as platinum through r-process nucleosynthesis.[31] Direct evidence for quark-novae is scant, in part since they would theoretically be radio quiet; however recent observations of supernovae SN2006gy,[32][33][34] SN2005gj [33][34] and SN2005ap,[33][34] and compact objects RX J1856.5-3754[35] and 3C58 [36] may hint at their existence, although some candidates have been subsequently stated by other scientists to be "ruled out with high confidence" [37] or modeled as having exotic matter only at their center.[38] Speculatively, further types of core collapse could exist for degeneracy pressure of even smaller particles - quarks to preons, preons to their subcomponents, and so forth[39] - but this is an area where we lack knowledge.

Factors affecting collapse

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(New section or include above?)

 
Logarithmic plot of Mass against mean Density (with solar values as origin) showing possible kinds of stellar equilibrium state. For a configuration in the shaded region, beyond the black hole limit line, no equilibrium is possible, so runaway collapse will be inevitable.

Remnants and byproducts

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Cross-section of neutron star. Densities are in terms of ρ0 the saturation nuclear matter density, where nucleons begin to touch.[40]
Main articles: Compact star, Supernova, and Nebula

The end result of core collapse is either the complete destruction of the star without any remnant, or a remnant of degenerate matter (forming one of the several types of compact star). Other byproducts of core collapse can include massive bursts of radiation or subatomic particles, a supernova event, and in many cases, a nebula (or sometimes, diffuse nebula) comprising the ejected outer material of the star which was dispersed into space at very high velocities.

When a remnant is formed, its mass can be substantially less than the original star. Some stars of over 20 solar masses may leave a remnant that is only a quarter of their original mass (around 5 solar masses).[41] Usually this results in a newly formed neutron star, or in some cases more exotic forms of neutron star such as pulsars and magnetars. Conservation of angular momentum can lead to neutron stars with rotation periods of fractions of a second; this in turn combined with their other physical properties and heating effect on infalling matter can lead to electromagnetic effects such as high energy emissions and immense magnetic fields. If the star was part of a binary or multiple star system then further changes may result due to possible accretion activity. At times the remnant may not be visible, due to interstellar material from the collapse, or outer layers previously expelled. Core collapse rarely has perfect symmetry, so a "kick" is often imparted to the remnant - velocities of 1.5 million km/h (1 million mph) are considered "typical".[42]

While many neutron stars and other remnants actively emit electromagnetic radiation (visible light, X-rays, gamma rays, radio waves), a minority are radio-quiet or emit limited frequencies. A new class of pulsars that emit only gamma rays was discovered by the Fermi Gamma-ray Space Telescope in 2008.[43] A second feature discovered by the same telescope was that the shock wave and dispersed material from supernovae may be the source for various high energy cosmic rays detected on earth, since they may have an accelerating effect on protons and other subatomic particles in interstellar space.[44]

If the remnant's mass exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly degeneracy pressure of quarks and their subcomponents, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[41]

 
Wikimedia Commons has media related to Supernova remnants and Neutron star.
Examples of core collapse remnants and byproducts
Supernova Nebula with neutron star Pulsar formation via accretion Black hole
   
 
The brightness of SN 1994D (type Ia, bottom left) is comparable to the entire galaxy containing it The Cassiopeia A supernova nebula and neutron star remnant. The supernova took place around 300 years ago as measured from earth. X-ray observations suggest that by about 100 years ago as measured from earth, the temperature of the remnant fell to below 1 billion K and a neutron superfluid began to form. A star in a binary star system has undergone core collapse leaving a neutron star remnant.[45] The remnant begins to gain material from its companion star (known as "accretion"). The infalling matter causes it to speed up and high energy X-rays and gamma rays are emitted as the material is heated, eventually forming a pulsar rotating at up to 1000 times a second. (Also: high res version) Black holes do not emit visible light. They can be detected by X-rays from infalling matter, or by their gravitational effects. This animation shows the effect of a black hole on light from a more distant galaxy, known as gravitational lensing

Research into core collapse

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History of research

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IN PROGRESS
Date Title Authors Notes
1930s On Super-Novae (1934)[46]
Cosmic Rays from Super-novae (1934)[47]
On Collapsed Neutron Stars (1938)[48]		 Fritz Zwicky and Walter Baade

Zwicky and Baade were visionaries and astronomers who pioneered research into supernovae, In a series of papers in the 1930s they proposed that supernovae represented a new category of stellar object, that supernovae arose from the stellar collapse and conversion of a significant part of a star's mass to energy, introduced the concept of neutron stars, suggested that a critical mass would exist for stellar collapse, and outlined the supposition that supernovae represented stellar collapse into a neutron star. Other proposals validated decades later included that supernovae were a source of cosmic rays, the idea of supernovae as standard candles, gravitational lensing, and dark matter. One precursor (prompt? stimulus?) for the concept of the neutron star was the experimental demonstration of the existence of the neutron a year earlier.

  • Pioneered research into supernovae, identifying them as a new category of astronomical objects [49] summarized the "meager knowledge" about them, and coined the term "super nova", in their 1933/34? paper TITLE. Stated that supernova progenitors were probably "ordinary stars". Provided basic calculations showing that the energy output of a supernova was probably equivalent to annihilation of a significant proportion of its rest mass, and concluded that "it therefore becomes evident that the phenomenon of a super-nova represents the transition of an ordinary star into a body of considerably smaller mass."
  • Prompted by the then-recent 1932 demonstration of neutrons, speculated that neutron stars might exist and that basic calculations showed the collapse of a star into neutron matter would release 10% of its rest mass as energy - a possible explanation for supernovae. Although largely supposition at the time, this was the dawn of understanding of core collapse, and was presented to the American Physical Society in 1933. Current calculations agree with their estimate - during type II core collapse, around 10% of the rest mass is expelled as a neutrino burst.
  • A subsequent paper TITLE (1934) asserts that "Supernovae represent the transitions from ordinary stars into neutron stars" and that they were also the origins of cosmic rays.
  • Other proposals (1937-39) include supernovae used as standard candles, gravitational lensing, and dark matter.
1959 Neutron Star models Cameron
  • Author summarizes the state of research in 1959 as follows:
"About 20 years ago [ie, ~1939] there was considerable interest in neutron stars [as] attractive theoretical possibilities". Zwicky and Baade (1938-39) had suggested that the immense release of energy in a supernova might be related to the creation of a neutron star, however it was more widely believed that the end point for most or all stars was as a white dwarf, with mass loss taking place ensuring that the final resulting star had a mass within the Chandrasekhar limit. As a result, the study of neutron stars is described as "generally neglected", with "only Zwicky (1958)" continuing to advance the idea that neutron stars are formed by supernova events. Other studies up to 1958 considered neutron stars as "hypothetical and interesting objects". Neutron star models exist for diameters of ~ 9 km and based on a Fermi gas.
  • States it is "evident" that any massive star which has not shed sufficient mass to form a white dwarf with a larger central density "must" suffer collapse when critical density is reached, releasing immense energy, expelling the outer envelope, and creating an "enormously hot" neutron core. A combination of high thermal conductivity, evaporative, and radiative losses will then cause rapid energy loss and cooling to a "cold degenerate neutron gas". The question is whether massive stars lose or eject enough mass as they evolve, to preclude such an event.
  • Describes "new work" on neutron stars including a range of calculations and equations of state.
  • Notes that the calculated upper limit to neutron star mass is "sufficiently large" to be consistent with supernova origins and concludes that neutron stars are "probable byproducts" or "inevitable products" of supernovae.
1960 ? Hoyle and Fowler
  • Distinguished the collapse mechanisms of nuclear flash (carbon detonation) and photodissociation of iron (cessation of fusion) in supernovae etiology. Sugimoto and Namoto (1979) note that the "basic ideas" for these core collapse mechanisms remained unchanged although the difference in types and spectra was subsequently ascribed to their progenitor star's envelope rather than the triggering mechanism.[50]
  • 1967/68 - discovery of Pulsars demonstrates that Zwicky/Baade "were correct in their theory".

Noted: Useful papers for this article:

Theoretical modeling

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(Section notes:)
  • This paper - 2D shows chaotic effects ; 3D simulations needed for full understanding.

The Standard Model of particle physics is a theory which describes three of the four known fundamental interactions between the elementary particles that make up all matter. This theory allows predictions to be made about how particles will interact under many conditions. The energy per particle in a supernova is typically one to one hundred and fifty picojoules (tens to hundreds of MeV).[51] The per-particle energy involved in a supernova is small enough that the predictions gained from the Standard Model of particle physics are likely to be basically correct. But the high densities may require corrections to the Standard Model.[52] In particular, Earth-based particle accelerators can produce particle interactions which are of much higher energy than are found in supernovae,[53] but these experiments involve individual particles interacting with individual particles, and it is likely that the high densities within the supernova will produce novel effects. The interactions between neutrinos and the other particles in the supernova take place with the weak nuclear force, which is believed to be well understood. However, the interactions between the protons and neutrons involve the strong nuclear force, which is much less well understood.[54]

The major unsolved problem with Type II supernovae is that it is not understood how the burst of neutrinos transfers its energy to the rest of the star producing the shock wave which causes the star to explode. From the above discussion, only one percent of the energy needs to be transferred to produce an explosion, but explaining how that one percent of transfer occurs has proven very difficult, even though the particle interactions involved are believed to be well understood. In the 1990s, one model for doing this involved convective overturn, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the material from which the star originally formed.[55]

Neutrino physics, which is modeled by the Standard Model, is crucial to the understanding of this process.[52] The other crucial area of investigation is the hydrodynamics of the plasma that makes up the dying star; how it behaves during the core collapse determines when and how the "shock wave" forms and when and how it "stalls" and is reenergized.[56] Computer models have been very successful at calculating the behavior of Type II supernovae once the shock has been formed. By ignoring the first second of the explosion, and assuming that an explosion is started, astrophysicists have been able to make detailed predictions about the elements produced by the supernova and of the expected light curve from the supernova.[57][58][59]

Outstanding questions

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A considerable body of knowledge concerning core collapse is considered widely accepted by researchers. Because the physical conditions associated with core collapse are so extreme, and at times verge on the edge of scientific knowledge of fundamental physics, there are several areas of ongoing active research and many open questions in the field.

  • Observations: X-ray, gamma ray, and neutrino observatories are all newly available within the last 1 - 2 decades. Scientists are still gathering data that may support existing evidence and models, suggest variations, inform new proposals or modifications, and provide the basis for more refined measurements and theoretical models. Some events only take place rarely, and even when they do, pre-collapse observations and observations from the early stages of the event may be hard to obtain.
  • Fundamental models of physics: Models used to predict behavior at extremes of size, energy, gravity and density are still being developed. Experiments such as the Large Hadron Collider are anticipated to produce evidence that may confirm, refute or modify our current understanding of fundamental physics, or the development or verification of new advances, which will allow predictions to be made and tested, or observations explained and investigated. In particular knowledge of degenerate matter such as neutron matter, and the behavior of quarks and the nature and behavior of their subcomponents, and their degenerate products, applicable equations of state, and the union of gravity with the other fundamental forces of nature, are all still areas of ongoing research.
  • Collapse processes: In general, models of supernovae are broadly very successful in their predictions. But the means by which part of the energy from the emitted neutrinos of some kinds of core collapse is re-absorbed to cause a supernova is not fully understood. Also while type 1a supernovae mechanisms are believed to be broadly understood, progenitor stars undergoing accretion have not been directly evidenced yet and possible conversion from a deflagration mode (subsonic) to a detonation mode (supersonic + shock wave) is not fully understood. Objects likely to be related to collapse, such as super soft X-ray sources and gamma burst sources, also present questions [clarification needed].
  • Complex scenarios: Full modeling of more complex scenarios such as rotation effects, multiple star systems, and stars with unusual physical structures, may reveal more about the physics and modes of core collapse than is presently known.

Notable core collapse objects

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References

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  2. ^ FACT CHECK - ACCURACY - DOES THE REST OF IT?
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See also

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(See navbox - which links to copy?)
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  • TBA

Category:Astronomical events • Category:Stellar phenomena • Category:Stellar core collapse • Category:Gamma-ray bursts • Category:Neutrino astronomy • Category:Physical cosmology • Category:Astrophysics • Category:Applied and interdisciplinary physics • Category:Stellar astronomy • Category:Condensed matter physics • Category:Degenerate stars • Category:Exotic matter • Category:Quantum mechanics • Category:Gravitation • Category:Celestial mechanics • Category:Stellar evolution • Category:Black holes • Category:Cataclysmic variables • Category:Quark matter • Category:Strangeness production • Category:Phases of matter • Category:Pauli exclusion principle • Category:Stellar evolution • Category:White dwarfs • Category:Quantum chromodynamics • Category:Degenerate stars • Category:Compact stars • Category:Nuclear fusion • Category:Supernovae • Category:Light sources • Category:Neutron stars • Category:Nucleosynthesis

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