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Why is the element iron responsible for supernova? Can any star create more element than iron within the span of its life?

I understand that when star dies due to supernova, other elements are created (Gold and other 92 elements) due to more heat generated than required not only for fusion of Iron element but also for the fusion of higher element with each other.

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  • $\begingroup$ You should definitely improve your way to ask questions :) It is very much related to your comprehension of the topic, and to the quality of the answers you will receive. For example: the iron is not "responsible" for supernova, it is the last element produced by nuclear fusion. So you could ask: According to this reference (reference), the iron is the last element produced within star's core, but I don't understand the reason, bla bla bla. $\endgroup$ – Py-ser Apr 22 '14 at 10:41
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    $\begingroup$ @Py-ser Iron is not the last element created by fusion. It is the last for which fusion produces energy. Further elements up to Uranium (last natural element) actually consume energy when creating. $\endgroup$ – Envite Apr 22 '14 at 11:09
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    $\begingroup$ Actually, isn't it Ni which is the last for which fusion produces instead of consumes energy? $\endgroup$ – Jeremy Apr 22 '14 at 11:40
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    $\begingroup$ @Envite astronomy.nju.edu.cn/~lixd/GA/AT4/AT421/HTML/AT42104.htm the alpha process continues up to Nickel-56 $\endgroup$ – Jeremy Apr 22 '14 at 21:32
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    $\begingroup$ So, 56-Ni is the last element created by fusion that produces energy instead of consuming it. This happens to decay to iron... but it is 56-Ni that is the last element produced by fusion producing energy. $\endgroup$ – Jeremy Apr 23 '14 at 10:07
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Your question is a bit oversimplified because there are many types of supernovae based on the size and configuration of the star. But I can answer your question about "why iron" by considering what keeps a star from exploding in the first place.

In the simplest terms of star formation, when material from an interstellar nebula starts to collapse under its own gravity, the pressure and temperatures involved will become great enough to eventually start fusing hydrogen into helium (it's a bit more complicated than that, but I'm speaking in generalities). If you were to consider the helium atoms created by that process, you'll notice that each helium atom weighs just a bit less than the two hydrogen atoms that formed it. That bit of extra mass is given off as energy which is produced in great quantities as the hydrogen continues to be fused into helium.

During the star's "main sequence", the release of energy by the hydrogen-helium fusion helps counteract the weight of the star's gasses pushing inward. Material presses in; energy pushes out in perfect balance. This balance of gravity and energy output continues until the star uses up most of its hydrogen.

It's at this this point (when there is no hydrogen left in the core of the star to fuse into helium) that the fusion reaction stops and gravity will resume to collapse the star further. As this star collapses, it will quickly become denser and hotter until the temperature and pressures of the interior are great enough to start fusing helium into heavier elements… and the process continues.

That is, until the star starts fusing elements into iron…

The fusion into iron is the first element that does not create more energy than it takes to produce. The effect is that there is no net energy being produced to counteract the gravity pushing inward. So the outer layers will quickly collapse into a much denser and smaller ball causing the remaining star material to undergo fusion all at once, causing the supernova.

So, in that sense, iron is not the cause of the supernova, but its presence marks the inevitable end of this star's life cycle… in this particular scenario.

But understand that this is an oversimplification to illustrate the process you asked about. There are many other sequences of a star's life cycle. Our sun, for example, does not have sufficient mass to keep collapsing down with sufficient pressures to fuse heavier elements into iron. Without getting into other pathways for the production of heavy elements (even in smaller stars like our sun) — once our sun starts creating carbon and oxygen, the fuel starts to run out and the core will simply collapse and rebound as it swells up into a red giant, before losing its outer layers as a planetary nebula while the core shrinks to become a white dwarf (and eventually cooling into a black dwarf).

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  • $\begingroup$ @RobertCartaino perhaps you can include some references in your answer? For instance I am interested in a reference supporting the statement The fusion into iron is the first element that does not create more energy than it takes to produce $\endgroup$ – Jeremy Apr 22 '14 at 21:37
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    $\begingroup$ @Jeremy First a small clarification, iron is the last element that produces a net release of energy by nuclear fusion. Any fusion of/with iron into heavier elements consumes more energy than the process releases. Some sources: NASA Universe 101 on The Life and Death of Stars or Wikipedia on Supernova nucleosynthesis (where you might find other sources in references and suggested reading sections). $\endgroup$ – TildalWave Apr 23 '14 at 0:10
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    $\begingroup$ @TidalWave yup, so two things here: it does not require more energy to create Fe by fusion than is released (as stated by Robert), and also that wikipedia link you provide says: The second, and more common, cause is when a massive star, usually a red giant, reaches nickel-56 in its nuclear fusion (or burning) processes. This isotope undergoes radioactive decay into iron-56 - so it is Nickel, not Iron, that is the last element that produces a net release of energy by fusion (and it happens to decay to Fe). $\endgroup$ – Jeremy Apr 23 '14 at 10:11
  • $\begingroup$ As @Robert Cartaino noted, his explanation was condensed for comprehensibility. If you actually trace the chain of nucleosynthesis, (e.g., p22 of as.utexas.edu/astronomy/education/fall10/scalo/secure/…) the penultimate species is Fe52 which absorbs one more alpha particle to become Ni56 yielding a tiny amount of energy. Any more alpha absorptions cost energy. Ni56 is unstable, decaying to the lower-energy Fe56 with a half-life of about 6 days. When the SN goes bang, there will always be quite a lot of Ni56 that has not yet turned into Fe56. $\endgroup$ – Mark Olson Jan 23 at 13:35
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The binding energy per nucleon is among the highest for iron-56. Therefore nuclear fusion as well as fission/photodisintegration of iron-56 consumes energy.

Heat production is needed to prevent a star from collapsing to a much denser state. Iron-56 provides no way to produce heat by nuclear reactions. Hence core collapse is unavoidable.

If the star isn't large enough, nuclear fusion may stop, before the core of the star is fused to iron, since very high temperature (more than 2 billion Kelvin) and pressure are needed to fuse silicon to iron. This way the core may collapse earlier.

The collapse releases energy responsible for much of the luminosity of the supernova. Some additional (exothermic) fusion in outer layers of the exploding star may be triggered by the collapse during the explosion.

Unlike most supernovae, type Ia supernovae explode by runaway nuclear fusion, not mainly by core collapse.

A hypernova may form either due to lack of nuclear fuel, the core of the star first collapsing to a neutron star, followed by a collapse to a black hole, or by pair instability leaving no dense remnant.

The cause for the pair instability supernovae isn't lack of nuclear fuel, but the heat of the core. Due to Planck's law the energy of the radiated photons increases with temperature. As soon as they reach the energy needed to form electron-positron pairs, at least 1.02 MeV, heat energy is transformed to mass. This way pressure needed to keep the core stable, gets lost.

Pair-instability supernovae and -hypernovae are indicated by high amounts of released nickel-56, which will decay to iron-56.

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When a star has exhausted its supply of elements below iron the fusion process is no longer able to prevent the outer layers from collapsing in towards the core. If the star is sufficiently massive it will then eject the outer layers explosively as a supernova. So iron is not the cause, just the point in the fusion process at which this occurs.

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    $\begingroup$ Adding references to support your statements is a good way to make your answers really valuable $\endgroup$ – Jeremy Apr 23 '14 at 21:11
  • $\begingroup$ @Jeremy: thanks, I posted on my phone during a break at work, no time to find references. I'll try to improve post quality in future :-) $\endgroup$ – ntremble Apr 26 '14 at 11:04
  • $\begingroup$ Just looking to support new user's introduction to the site :-) $\endgroup$ – Jeremy Apr 26 '14 at 11:12
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The most important reason that core collapse happens is that particles moving close to the speed of light are notoriously gravitationally unstable. That's because when particles moving slower than that are compressed by gravity without energy escape, the gravitational energy released by gravity causes the particles to move faster, and this raises the pressure more than the increase in gravity (due to getting smaller), which allows the object to bounce back when compressed. But if the particles are already moving close to c, then they do not speed up enough, and even though the pressure does increase, so does the gravity, and there is no tendency for the star to bounce back from the contraction. Tack on escape of energy, and the pressure is incapable of rising as much as gravity does. This leads to a runaway, where a little escape of energy leads to a large amount of contraction of the core. This causes "collapse."

The cure for this is to prevent net loss of heat. That can be done in two ways-- either by release of heat from fusion (self-regulated to occur at whatever rate heat is escaping), or the particles responsible for the pressure can be close enough to their quantum mechanical ground state that they are disallowed from giving up any more heat (even in the absence of any fusion).

So core collapse is a kind of race between whether the electrons (which provide the pressure as they approach their quantum mechanical ground state because that raises their specific heat and they become gluttons for kinetic energy and therefore pressure) can reach their ground state and be disallowed from giving up any more heat, or if they reach close to the speed of light and suffer from the abovementioned instability as they suffer net heat loss. The winner of this race is determined by the amount of mass in the core-- if the mass is high, the speed of light is reached prior to the quantum mechanical ground state, and if the mass is low, the opposite occurs. There is nothing about iron that leads to this, except that iron cannot be a source of nuclear energy so does not provide the other type of protection from core collapse.

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