Why does stellar fusion stop at iron

Iron, however, is the most stable element and must actually absorb energy in order to fuse into heavier elements. The formation of iron in the core therefore effectively concludes fusion processes and, with no energy to support it against gravity, the star begins to collapse in on itself.

The star has less than 1 second of life remaining. During this final second, the collapse causes temperatures in the core to skyrocket, which releases very high-energy gamma rays.

These photons undo hundreds of thousands of years of nuclear fusion by breaking the iron nuclei up into helium nuclei in a process called photodisintegration. At this stage the core has already contracted beyond the point of electron degeneracy , and as it continues contracting, protons and electrons are forced to combine to form neutrons.

This process releases vast quantities of neutrinos carrying substantial amounts of energy, again causing the core to cool and contract even further.

Electrons and atomic nuclei are, after all, extremely small. The electrons and nuclei in a stellar core may be crowded compared to the air in your room, but there is still lots of space between them. The electrons at first resist being crowded closer together, and so the core shrinks only a small amount. Ultimately, however, the iron core reaches a mass so large that even degenerate electrons can no longer support it.

This transformation is not something that is familiar from everyday life, but becomes very important as such a massive star core collapses. The core begins to shrink rapidly. More and more electrons are now pushed into the atomic nuclei, which ultimately become so saturated with neutrons that they cannot hold onto them.

At this point, the neutrons are squeezed out of the nuclei and can exert a new force. As is true for electrons, it turns out that the neutrons strongly resist being in the same place and moving in the same way. The force that can be exerted by such degenerate neutrons is much greater than that produced by degenerate electrons, so unless the core is too massive, they can ultimately stop the collapse.

This means the collapsing core can reach a stable state as a crushed ball made mainly of neutrons, which astronomers call a neutron star. So if the mass of the core were greater than this, then even neutron degeneracy would not be able to stop the core from collapsing further.

The dying star must end up as something even more extremely compressed, which until recently was believed to be only one possible type of object—the state of ultimate compaction known as a black hole which is the subject of our next chapter.

This is because no force was believed to exist that could stop a collapse beyond the neutron star stage. The collapse that takes place when electrons are absorbed into the nuclei is very rapid.

In less than a second, a core with a mass of about 1 M Sun , which originally was approximately the size of Earth, collapses to a diameter of less than 20 kilometers. The speed with which material falls inward reaches one-fourth the speed of light. The collapse halts only when the density of the core exceeds the density of an atomic nucleus which is the densest form of matter we know.

A typical neutron star is so compressed that to duplicate its density, we would have to squeeze all the people in the world into a single sugar cube! The neutron degenerate core strongly resists further compression, abruptly halting the collapse. The shock of the sudden jolt initiates a shock wave that starts to propagate outward. However, this shock alone is not enough to create a star explosion. The energy produced by the outflowing matter is quickly absorbed by atomic nuclei in the dense, overlying layers of gas, where it breaks up the nuclei into individual neutrons and protons.

These ghostly subatomic particles, introduced in The Sun: A Nuclear Powerhouse , carry away some of the nuclear energy. It is their presence that launches the final disastrous explosion of the star.

The total energy contained in the neutrino s is huge. While neutrinos ordinarily do not interact very much with ordinary matter we earlier accused them of being downright antisocial , matter near the center of a collapsing star is so dense that the neutrinos do interact with it to some degree.

They deposit some of this energy in the layers of the star just outside the core. This huge, sudden input of energy reverses the infall of these layers and drives them explosively outward. Most of the mass of the star apart from that which went into the neutron star in the core is then ejected outward into space. As we saw earlier, such an explosion requires a star of at least 8 M Sun , and the neutron star can have a mass of at most 3 M Sun.

Consequently, at least five times the mass of our Sun is ejected into space in each such explosive event! The resulting explosion is called a supernova Figure 2. When these explosions happen close by, they can be among the most spectacular celestial events, as we will discuss in the next section. It's no longer pushing outward with its light pressure, and so the outer layers collapse inward, creating a black hole and a supernova. It sure looks like the build up of iron in the core killed it.

Is it true then? Is iron the Achilles heel of stars? Not really. Iron is the byproduct of fusion within the most massive stars. Just like ash is the byproduct of combustion, or poop is the byproduct of human digestion.

It's not poison, which stops or destroys processes within the human body. A better analogy might be fiber. Your body can't get any nutritional value out of fiber, like grass. If all you had to eat was grass, you'd starve, but it's not like the grass is poisoning you. As long as you got adequate nutrition, you could eat an immense amount of grass and not die.

It's about the food, not the grass. The sun already has plenty of iron; it's 0. That little nugget would work out to be times the mass of the Earth. If you gave it much more iron, it would just give the sun more mass, which would give it more gravity to raise the temperature and pressure at the core, which would help it do even more fusion.

If you just poured iron into a star, it wouldn't kill it. It would just make it more massive and then hotter and capable of supporting the fusion of heavier elements. As long as there's still viable fuel at the core of the star, and adequate temperatures and pressures, it'll continue fusing and releasing energy. If you could swap out the hydrogen in the sun with a core of iron , you would indeed kill it dead, or any star for that matter.

It wouldn't explode, though. Only if it was at least 8 times the mass of the sun to begin with. Then would you have enough mass bearing down on the inert core to create a core collapse supernova. In fact, since you've got the power to magically replace stellar cores, you would only need to replace the sun's core with carbon or oxygen to kill it. It actually doesn't have enough mass to fuse even carbon. As soon as you replaced the sun 's core, it would shut off fusion.

It would immediately become a white dwarf, and begin slowly cooling down to the background temperature of the universe. Iron in bullet, bar, man or any other form isn't poison to a star. It just happens to be an element that no star can use to generate energy from fusion. As long as there's still viable fuel at the core of a star, and the pressure and temperature to bring them together, the star will continue to pump out energy. This means that two protons are missing.

Matter cannot be created or destroyed —it can only be turned into something else. In this case, the two missing protons have turned into two neutrons.

This energy is what makes the star shine and give off heat. That means the star has a little more mass in its core, which generates more heat. This process generates a little less energy than fusing hydrogen to helium, but it still produces energy. As a guideline, a star that has about one half the mass of the sun is too small and cool to fuse helium to carbon.

So it will end up as a white dwarf made of helium. Stars between one half to four times the mass of the sun are massive and hot enough to fuse carbon to oxygen. What happens when you introduce large amounts of heat and pressure to carbon? Stars with masses greater than four times the mass of the sun are massive and hot enough to fuse oxygen to silicon.