I get that it creates heavier atoms by fusing lighter ones, and it happens during a supernova. But, what I don’t get is the process of it. And the teacher didn’t help either as they literally just read the module verbatim.
Thanks!
In: Chemistry
Stars fuse lower mass elements into higher mass. H > He, 3 x HE to Carbon and higher elements. There is a slow process by which elements are created by slow capture of neutrons. This means that the nucleus captures a free neutron and then undergoes decay which increases the atom number. Slow neutron capture might be once in 100 years.
As the lower mass fuel runs out, stars start to collapse and the heat of the core goes up which allows for burning of heavier elements. The limit is iron. All fusion releases energy to prop up the star until iron is formed. Fusion past iron requires energy input so once iron forms, the star explodes within seconds.
During a supernova, a lot of heat is generated and a lot of fusion can happen at once. And there are a lot of free neutrons which can be captured by heavy nuclei.
It is thought that heavier elements can be created through the r-process which is rapid neutron capture. It is rapid in that a neutron is captured by an atom and then it captures more neutrons before a decay can occur. The rate is perhaps 100 neutron captures per second. This will pump up the number of neutrons in the nucleus of the atom. This only happens in very neutron rich environments. The heavy, neutron rich atoms will undergo decay to something stable but still remain heavier elements.
The r-process will also occur during neutron star collisions. These collisions might seed the interstellar gas with most of the heavier elements.
In a supernova the star has lost equilibrium so the pressure balance of fusion versus gravity is thrown out of balance. The star begins to expand, then as it exhausts its fuel it collapses down on itself hard. This sudden violent collapse forces the matter in the star’s core to be suddenly compressed so hard it forces the atoms together into a tiny space. Depending on how big the star was initially this either forms a black hole, or if it’s not as massive an explosion of the material as it rebounds releasing the pressure. If the atoms are compressed by enough pressure they can overcome the charge differences and be forced together or fused into heavier atoms which after the nova explodes are spread out into the universe.
There is also the S-process in stars, I think half of all elements heavier than iron are created by it. Basically this is there are a lot of particles flying around in a star. In the process of neutron capture which is where an atomic nucleus and one or more neutrons collide and form a new heavier element. This is a very slow process, which gives it its name the S process.
During a supernova, a whole lot of neutrons are created. Imagine an iron atom inside the exploding star. All these neutrons are flying by, and some have a chance of sticking to the iron atom. By the time the supernova is over, the iron atom has been covered in a whole lot of extra neutrons. That iron atom is now a very unstable isotope, and in the aftermath of the supernova it will radioactively decay. That means the extra neutrons turn into protons, and that’s exactly how you make heavier elements. For every neutron that turns into a proton, you go one element heavier.
As other comments already explained, nuclei heavier than iron are created by neutron absorption/conversion. Those between silicon and nickel are indeed [created by fusing helium into silicon](https://en.wikipedia.org/wiki/Supernova_nucleosynthesis#Silicon_burning) (and its daughter elements). This smashing of nuclei requires a lot of energy to overcome their repulsion, that is why a super energetic environment like that of supernova is required.
Latest Answers