Where Does Heavy Stuff Come From?

When the universe formed, there was nothing but hydrogen and helium. We saw in the previous article that nuclear reactions inside of stars are what created all the lighter elements up to and including nickel and iron. However we also saw that stars quickly collapse as soon as they try to fuse iron in their cores which leaves us with the question of how all the heavier elements formed. Without an answer to this question, we would have no explanation for where the silver in your necklace comes from or where the gold in your ring comes from.

To answer this, let's pick up where we left off in the lifecycle of very large stars. Very large stars develop layers in their cores as they age where different elements undergo fusion reactions. These reactions produce new elements and also produce energy which pushes outward, supporting the star against the weight of gravity trying to collapse the star. This process continues until the star produces iron in its core and tries to fuse it. The nuclei of iron (and nickel) are the most stable of any other element so fusing them actually takes up more energy than it produces. Suddenly the star loses the outward support that it depends on to prevent the force of gravity from collapsing the star and a supernova explosion begins. At this point the star has numerous layers in its core holding all of the light elements.

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The core of a fully developed massive star contains all the light elements

As the outer layers of the star collapse inward, the pressure and temperature in the core increase. The collapse also produces lots of neutrons which play an important role in how atomic nuclei are assembled. These neutrons flood the core of the star and are absorbed by all the elements that were produced earlier during the life of the star. The nuclei of these elements become very neutron rich but this also makes them more unstable as we saw in The Not So Useless Neutron. As a nucleus becomes neutron rich, the neutrons will begin decaying into protons to balance things out. This creates a flurry of new heavy elements as nuclei capture various quantities of neutrons, and as some of those neutrons decay into protons.

The best way to visualize this is on the chart of the nuclides. The chart plots the number of neutrons on the bottom axis and number of protons on the vertical axis. It shows every possible atom that could be created (after all, atoms are just combinations of protons and neutrons surrounded by a cloud of electrons). The number of protons determines what element it is (ie. Helium or Iron) and the number of neutrons determines which isotope it is (ie. Iron-56 or Iron-57). Only a few combinations of protons and neutrons will result in an atom that lasts forever. We call these isotopes stable and you can see them in black on the chart below. Unstable combinations of protons and neutrons will re-arrange themselves into a more stable configuration. When they do, they give off radiation so these isotopes are called radioactive. Most combinations are radioactive and can be seen in various colours on the chart below. The ends of the chart represent isotopes that are so unstable that we have never been able to create them in the lab to confirm that they can exist.

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The chart of nuclides shows the atoms that result from every possible combination of protons and neutrons

As neutrons are absorbed, a nucleus will move to the right on the diagram. However the further it goes, the more unstable it becomes and as neutrons decay into protons, the atom will move up and left (one less neutron, one more proton). However during this time it is still being bombarded with neutrons. The result is a rapid climb up and right on the diagram. As the supernova explosion occurs, countless atoms in the core of the star will absorb neutrons and decay, resulting in the formation of all the heavy elements within the star.

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Neutrons absorption and decay form new heavy elements during a supernova explosion

So as the star collapses, it quickly accumulates all of the elements we know and love within its core. Half were produced during the normal life of the star and the other half are quickly made by neutron absorption during the collapse of the star during a supernova explosion. So far we have only talked about the collapse of the star but as you may have guessed, a supernova explosion must involve some sort of explosion. The star eventually reaches a point during its collapse where the material within simply cannot be compressed any further. At this point, all the material in a very small part at the centre of the star is crushed to form neutrons and it abruptly stops collapsing. This causes the material on the outside to rebound off the core, creating a shockwave that pushes material outward. This material is blasted away from the tiny neutron core of the star, carrying with it all the elements that were formed inside the star. The explosion also releases high amounts of light, allowing the star to briefly shine brighter than just about anything else in the galaxy.

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A supernova explosion (bottom left) can shine brighter than an entire galaxy