If the final element to be fused in a star is iron, where does everything after that come from?
In: 154
Iron is the final fusion event that **produces energy**. But further fusion does happen.
There are, broadly, two main processes that produce heavy elements.
—-
The first is the [*s-process*](https://en.wikipedia.org/wiki/S-process) (s for “slow”, in that it takes thousands of years).
Inside the core of an old star, there are a lot of extra neutrons flying around. Sometimes a nucleus absorbs one, raising its mass number by one. If the new nucleus is unstable, it undergoes beta decay, converting one of its neutrons into a proton and raising its atomic number by one. This new nucleus can then absorb another neutron and repeat the process.
This continues until you reach lead and bismuth. The next element, polonium, is so unstable that it decays faster than this process can proceed, so it does not make any heavier elements. (Specifically, polonium-210 is. Polonium-209 is more stable, but isn’t produced by the s-process.)
This process does not produce energy, however. In fact, it consumes a little bit of it, and these sorts of reactions drain the lifetime of the star and ultimately cause its collapse.
—–
The second is the [*r-process*](https://en.wikipedia.org/wiki/R-process) (for “rapid”). The r-process occurs in a matter of seconds in the extremely exotic environment of a supernova or neutron star merger. The exact degree to which each of those two events contributes is an active area of research, but current physics seems to be leaning towards neutron star mergers being more important than previously thought.
In the r-process, nuclei are bombarded with such an insane number of neutrons that there’s no time for beta decays on normal timescales. To put a sense of scale to this, you need an amount of free neutrons comparable to the density of water, which is absolutely insane – this is about 13 orders of magnitude more neutron bombardment than occurs inside an active nuclear reactor.
Rather than the slow absorb neutron -> beta decay -> absorb another neutron approach of the s-process, these nuclei absorb neutrons until they physically cannot absorb another (the “neutron drip line”, which is very far on the neutron-rich side, far beyond the point at which things are wildly unstable to beta decay under less exotic conditions). These wildly unstable nuclei undergo very rapid beta decay essentially as fast as new neutrons are added.
This process occurs so fast that even for the wildly unstable elements between lead and uranium, there’s no time for their typical forms of decay. A half-life of hours means nothing when you’re adding neutrons many times a second; you need extraordinarily unstable nuclei to get decays on those timescales. That means that the r-process can run up the periodic table incredibly fast, producing superheavy elements (possibly some beyond the reach of human particle accelerators).
Those superheavy elements rapidly decay in the aftermath of a supernova (or if blasted off during a neutron star merger), and that decay is responsible for a lot of the light produced by supernovae. None of the superheavy elements survive long enough to be incorporated into the next generation of stars or their planets, but some of their stable versions and decay products – like uranium – are long lived enough to do so.
So, the idea here is that iron (specifically iron-56) has the lowest energy of any atomic nucleus. In other words, trying to fuse iron will be endothermic (absorbing energy) rather than exothermic (releasing energy) as is the case for lighter elements. So, by the time a star reaches iron, its core is no longer able to produce energy and so it collapses.
It’s long been thought that supernovas produced elements heavier than iron. So, while producing these heavier elements does absorb energy, the supernova provides enough energy to do this. More recent discoveries have suggested that collisions between neutron stars, The remnants of stellar cores following a supernova, are responsible for many of the heavier elements in the universe. These collisions take place in binary system consisting of two neutron stars orbiting each other other. They slowly spiraling towards each other and eventually meet.
Iron is the final fusion event that **produces energy**. But further fusion does happen.
There are, broadly, two main processes that produce heavy elements.
—-
The first is the [*s-process*](https://en.wikipedia.org/wiki/S-process) (s for “slow”, in that it takes thousands of years).
Inside the core of an old star, there are a lot of extra neutrons flying around. Sometimes a nucleus absorbs one, raising its mass number by one. If the new nucleus is unstable, it undergoes beta decay, converting one of its neutrons into a proton and raising its atomic number by one. This new nucleus can then absorb another neutron and repeat the process.
This continues until you reach lead and bismuth. The next element, polonium, is so unstable that it decays faster than this process can proceed, so it does not make any heavier elements. (Specifically, polonium-210 is. Polonium-209 is more stable, but isn’t produced by the s-process.)
This process does not produce energy, however. In fact, it consumes a little bit of it, and these sorts of reactions drain the lifetime of the star and ultimately cause its collapse.
—–
The second is the [*r-process*](https://en.wikipedia.org/wiki/R-process) (for “rapid”). The r-process occurs in a matter of seconds in the extremely exotic environment of a supernova or neutron star merger. The exact degree to which each of those two events contributes is an active area of research, but current physics seems to be leaning towards neutron star mergers being more important than previously thought.
In the r-process, nuclei are bombarded with such an insane number of neutrons that there’s no time for beta decays on normal timescales. To put a sense of scale to this, you need an amount of free neutrons comparable to the density of water, which is absolutely insane – this is about 13 orders of magnitude more neutron bombardment than occurs inside an active nuclear reactor.
Rather than the slow absorb neutron -> beta decay -> absorb another neutron approach of the s-process, these nuclei absorb neutrons until they physically cannot absorb another (the “neutron drip line”, which is very far on the neutron-rich side, far beyond the point at which things are wildly unstable to beta decay under less exotic conditions). These wildly unstable nuclei undergo very rapid beta decay essentially as fast as new neutrons are added.
This process occurs so fast that even for the wildly unstable elements between lead and uranium, there’s no time for their typical forms of decay. A half-life of hours means nothing when you’re adding neutrons many times a second; you need extraordinarily unstable nuclei to get decays on those timescales. That means that the r-process can run up the periodic table incredibly fast, producing superheavy elements (possibly some beyond the reach of human particle accelerators).
Those superheavy elements rapidly decay in the aftermath of a supernova (or if blasted off during a neutron star merger), and that decay is responsible for a lot of the light produced by supernovae. None of the superheavy elements survive long enough to be incorporated into the next generation of stars or their planets, but some of their stable versions and decay products – like uranium – are long lived enough to do so.
Iron is the final fusion event that **produces energy**. But further fusion does happen.
There are, broadly, two main processes that produce heavy elements.
—-
The first is the [*s-process*](https://en.wikipedia.org/wiki/S-process) (s for “slow”, in that it takes thousands of years).
Inside the core of an old star, there are a lot of extra neutrons flying around. Sometimes a nucleus absorbs one, raising its mass number by one. If the new nucleus is unstable, it undergoes beta decay, converting one of its neutrons into a proton and raising its atomic number by one. This new nucleus can then absorb another neutron and repeat the process.
This continues until you reach lead and bismuth. The next element, polonium, is so unstable that it decays faster than this process can proceed, so it does not make any heavier elements. (Specifically, polonium-210 is. Polonium-209 is more stable, but isn’t produced by the s-process.)
This process does not produce energy, however. In fact, it consumes a little bit of it, and these sorts of reactions drain the lifetime of the star and ultimately cause its collapse.
—–
The second is the [*r-process*](https://en.wikipedia.org/wiki/R-process) (for “rapid”). The r-process occurs in a matter of seconds in the extremely exotic environment of a supernova or neutron star merger. The exact degree to which each of those two events contributes is an active area of research, but current physics seems to be leaning towards neutron star mergers being more important than previously thought.
In the r-process, nuclei are bombarded with such an insane number of neutrons that there’s no time for beta decays on normal timescales. To put a sense of scale to this, you need an amount of free neutrons comparable to the density of water, which is absolutely insane – this is about 13 orders of magnitude more neutron bombardment than occurs inside an active nuclear reactor.
Rather than the slow absorb neutron -> beta decay -> absorb another neutron approach of the s-process, these nuclei absorb neutrons until they physically cannot absorb another (the “neutron drip line”, which is very far on the neutron-rich side, far beyond the point at which things are wildly unstable to beta decay under less exotic conditions). These wildly unstable nuclei undergo very rapid beta decay essentially as fast as new neutrons are added.
This process occurs so fast that even for the wildly unstable elements between lead and uranium, there’s no time for their typical forms of decay. A half-life of hours means nothing when you’re adding neutrons many times a second; you need extraordinarily unstable nuclei to get decays on those timescales. That means that the r-process can run up the periodic table incredibly fast, producing superheavy elements (possibly some beyond the reach of human particle accelerators).
Those superheavy elements rapidly decay in the aftermath of a supernova (or if blasted off during a neutron star merger), and that decay is responsible for a lot of the light produced by supernovae. None of the superheavy elements survive long enough to be incorporated into the next generation of stars or their planets, but some of their stable versions and decay products – like uranium – are long lived enough to do so.
So, the idea here is that iron (specifically iron-56) has the lowest energy of any atomic nucleus. In other words, trying to fuse iron will be endothermic (absorbing energy) rather than exothermic (releasing energy) as is the case for lighter elements. So, by the time a star reaches iron, its core is no longer able to produce energy and so it collapses.
It’s long been thought that supernovas produced elements heavier than iron. So, while producing these heavier elements does absorb energy, the supernova provides enough energy to do this. More recent discoveries have suggested that collisions between neutron stars, The remnants of stellar cores following a supernova, are responsible for many of the heavier elements in the universe. These collisions take place in binary system consisting of two neutron stars orbiting each other other. They slowly spiraling towards each other and eventually meet.
Stars will fuse elements in their core because it is hot and the matter in the core is under high pressure. The high temperature and pressure combine elements, and combining elements smaller than iron produces energy. Releasing energy keeps the core of the star hot and at a high pressure, which lets fusion processes continue. When iron is fused, it actually consumes energy, which causes the star to cool and the core will lose pressure.
The core / middle of the star is where most fusion happens. The majority of a star doesn’t have fusion happening.
The pressure in the star’s core pushes outward on the outer layers of the star, preventing all that matter from falling into the core. Lower pressure means the star will start to shrink because it’s not pushing up anymore.
If enough iron starts to fuse, so much energy is removed from the core that the rest of the star above the core starts crushing everything in the core because the core’s pressure is not high enough. This crushing will release lots of energy which causes iron and other elements to fuse, using this energy to make heavier elements. They require energy to form, instead of releasing energy like the lighter elements, so the star collapses in what is called a supernova (but only if it is big enough), and will no longer fuse elements after that.
This is also why heavy radioactive elements release lots of energy when they split: it took lots of energy to create them in the first place.
(obviously an oversimplification, but it is ELI5)
Stars will fuse elements in their core because it is hot and the matter in the core is under high pressure. The high temperature and pressure combine elements, and combining elements smaller than iron produces energy. Releasing energy keeps the core of the star hot and at a high pressure, which lets fusion processes continue. When iron is fused, it actually consumes energy, which causes the star to cool and the core will lose pressure.
The core / middle of the star is where most fusion happens. The majority of a star doesn’t have fusion happening.
The pressure in the star’s core pushes outward on the outer layers of the star, preventing all that matter from falling into the core. Lower pressure means the star will start to shrink because it’s not pushing up anymore.
If enough iron starts to fuse, so much energy is removed from the core that the rest of the star above the core starts crushing everything in the core because the core’s pressure is not high enough. This crushing will release lots of energy which causes iron and other elements to fuse, using this energy to make heavier elements. They require energy to form, instead of releasing energy like the lighter elements, so the star collapses in what is called a supernova (but only if it is big enough), and will no longer fuse elements after that.
This is also why heavy radioactive elements release lots of energy when they split: it took lots of energy to create them in the first place.
(obviously an oversimplification, but it is ELI5)
Stars will fuse elements in their core because it is hot and the matter in the core is under high pressure. The high temperature and pressure combine elements, and combining elements smaller than iron produces energy. Releasing energy keeps the core of the star hot and at a high pressure, which lets fusion processes continue. When iron is fused, it actually consumes energy, which causes the star to cool and the core will lose pressure.
The core / middle of the star is where most fusion happens. The majority of a star doesn’t have fusion happening.
The pressure in the star’s core pushes outward on the outer layers of the star, preventing all that matter from falling into the core. Lower pressure means the star will start to shrink because it’s not pushing up anymore.
If enough iron starts to fuse, so much energy is removed from the core that the rest of the star above the core starts crushing everything in the core because the core’s pressure is not high enough. This crushing will release lots of energy which causes iron and other elements to fuse, using this energy to make heavier elements. They require energy to form, instead of releasing energy like the lighter elements, so the star collapses in what is called a supernova (but only if it is big enough), and will no longer fuse elements after that.
This is also why heavy radioactive elements release lots of energy when they split: it took lots of energy to create them in the first place.
(obviously an oversimplification, but it is ELI5)
Once a very large star starts to create iron it is using up more energy than it produces this results in the star cooling and collapsing in on itself this collapse result in a rebound effect known as a supernova which scatters all the heavier elements created in the final moments before the supernova. https://youtu.be/w1GlDVt1Mpk
This is why rocky planets and life itself are referred to as star dust we literally are, no supernovas and none of us would exist.
Once a very large star starts to create iron it is using up more energy than it produces this results in the star cooling and collapsing in on itself this collapse result in a rebound effect known as a supernova which scatters all the heavier elements created in the final moments before the supernova. https://youtu.be/w1GlDVt1Mpk
This is why rocky planets and life itself are referred to as star dust we literally are, no supernovas and none of us would exist.
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