[MUSIC]. Fusion reactions occur as energy producing naturally or spontaneously occurring reactions that are possible under the great pressures and temperatures of stars. So under those conditions you can begin to put protons and neutrons together to form helium atoms. The helium atoms can be pushed together to form carbon atoms. And the carbon atoms can combine with other helium atoms to give you heavier, like, oxygen and so on. So all of the elements that we have in the solar system, from hydrogen and helium which were formed in the Big Bang, up to iron 56, are produced by these fusion reactions. And all of these fusion reactions release huge amounts of energy, and it's that energy that we see within stars and our own sun which gives us the heat and the light that is emanating from those. So all of these are exothermic reactions and they release energy, and they are what illuminate the stars. And this is how we build the periodic table from helium up to the iron group elements. And here's just another series of possible reactions in which you can build even heavier. The top one, for example, you take 2 carbon. They go together to form either oxygen 16 or neon 20, sodium 23 and so on. So these are naturally occurring. They occur in stars. They are the fuel which is basically driving the energy that's omitted from the stars. And so the question now is, does a single star just produce one type of thing at a time? And the answer is no. Depending on the size of the star, and how much pressure and temperature is built up in the center of those stars, you have the possibility of simultaneous reactions occurring in an onion like structure within different stars. The bigger the star, the more layers you might have. And the more simultaneous reactions that you might have in these different layers. The end of the line for this process, is the production of iron 56. You cannot take iron 56 and combine it with anything else and release energy. And so once you begin to build up an iron, nickel or iron group core to a star, you're basically out of gas and the whole system is going to collapse. And a star condenses and explodes. In some kind of a super nova, or it could just dissipate in some form. If we return back to this diagram, which we talked about earlier and we divided up into fusion reactions on the left-hand side, and neutron capture reactions on the right-hand side. Then we've got the elements now from the Big Bang, hydrogen and helium. We have all the elements up to iron 56 built by these fusion reactions within stars. And now we have to go beyond that iron 56, and explain the existence of all these heavier elements, which have lower binding energy which are therefore less stable than iron 56. How do we populate the heavier elements that we have within our solar system? This occurs within stars as well, hence the term stellar nuclear synthesis. And all stars have got high neutron fluxes, that's to say free neutrons, which are flying around with very high amounts of energy, and especially high neutron fluxes occur within supernovas, when these stars run out of fuel and explode in spectacular fashions as we can actually observe with astronomy. So with all these neutrons flying around with these high energies both in modest amounts in normal star processes and also in very high amounts in supernovae. Let's explore the possibility of using these neutrons to build the heavier elements. This is a small section of the chart of the nuclides that I presented earlier. In which you have neutrons along the X axis and protons along the Y axis. This is a segment that presents the element silver, cadmium, indium, tin and antimony. And we are going to use these elements to explore what happens to nuclei in stellar environments where they are bombarded by free neutrons. If you take and add a neutron to silver, with a mass of 109. And when I say mass, that means the number of neutrons and protons combined together, then you, you hit that nuclei with a neutron. That becomes mass-110, because you added another neutron, but silver-110 is not stable. And so it decays to cadmium-110. You've just built cadmium-110, you hit cadmium-110 with a neutron and you build 111. You hit it with another neutron, 112, and so on. Once you get over to the far end or the far right side of cadmium to 114, and you build 115, 115 is not stable, and 115 will very rapidly decay to indium 115, and so you can see by this process of just simply adding more and more neutrons to existing nuclei. You go up the up the periodic table, or up the chart of the nuclides in this case, to heavier and heavier elements, and you get all the way up to uranium by this means. But this does not explain the existence of isotopes like cadmium 116, for example. Because you can see there's a gap there. And when you form 115 it goes to indium 115. So, to explain the existence of cadmium 116, you have to come up with another process. The previous process was called s-process nucleosynthesis, that is the slow delivery of neutrons to existing nuclei, so that you can populate the chart of the nuclides by this fashion. In order to get past the barrier where you have an unstable nuclei which will naturally decay to another element, you have to have such a rapid number of neutrons in the environment bombarding these nuclei that you even though cadmium 115 is unstable, you hit it before it decays, and with another neutron, and you can form cadmium 116 by that process, and cadmium 116 is stable. So, once you have it, it doesn't decay away, and so you can therefore build these, what are referred to as neutron-rich nuclides by this R process. Where R refers to as the rapid process of nucleosynthesis, and this is probably occurring mainly or only in supernova events, where you have these massive fluxes of neutrons as a result of the explosions of stars. So we have s process nuclear synthesis and r process nuclear synthesis, and what we haven't explained is elements that lie on the left side of the diagram or isotopes like tin. Tin 112 here in this diagram, in which you have, they're proton rich or neutron poor nuclei. And those are formed by a process of so-called p-process nucleosynthesis, in which if you have free protons in stellar environments, you build up an inventory of proton rich nuclei, just in the same fashion as you build up neutron rich nuclei. And as a last point here, I'll say that the very most proton rich and the very most neutron rich isotopes are those which are the rarest in nature, because of the difficulty of getting past these barriers. Okay, so, what we have discussed so far to review back to what we had seen earlier in terms of this B2FH model. We have, first of all, the Big Bang products of hydrogen, helium, minor amounts of lithium, beryllium, and boron. And then you have, the second, that occurs right at the very beginning with the Big Bang. The second event or second type of production of elements are from, lithium, all the way up to the iron group elements. Those are stellar fusion products. That's the process of burning material in stellar environments to give you the light and heat that emanates from stars. And the third thing is, beyond iron 56 predominately you have this P, S, and R process nucleosynthesis, some of them related to supernovae, some of them recurring in natural stars which give you the heavier elements. And then finally let's put this into some kind of a context. The Big Bang is thought to occur approximately 13.7 billion years ago from today. Our solar system is 4.56 billion years old. So, there's 9 billion years of time from the Big Bang through to the formation of our solar system. So, the composition of our solar system isn't unique. No, it's not unique. If our solar system had formed sometime significantly earlier than it did, we would have less of the heavy elements than we have today, because there wouldn't have been enough time gone by in order to be able to build up this inventory of the heavy elements. If our solar system was younger, or another way of saying it is, if we look at a star that exists that formed after our solar system, would it have the same composition as our solar system? No it would have an increased or enhanced inventory of these heavier elements, because there would be more time passed, and more stars would have produced heavier elements and so we would have an enriched heavy element composition in that stellar system. [MUSIC]
