Hi, and welcome back to my videos for General Chemistry 2. Today we'll finish up our discussion of nuclear reactions by talking in more depth about the fusion reactions that happen in stars. That'll give us a chance to talk about the most energetic nuclear reactions of all, and we'll also find out where all the elements in the periodic table came from in the first place. To get there, let's start by remembering that in a fusion reaction, two small nuclei combine to form a single larger nucleus. We learned more about that in video 38, so you might want to go back and watch that one if you've forgotten what we talked about. For example, here's a fusion reaction in which carbon 12 and oxygen 16 react to form silicon 28. As we saw in video 38, reactions like this one produce a tremendous amount of energy, but also require huge amounts of energy in order to initiate them. In fact, the technology necessary to produce commercial power using fusion reactions is still too expensive to be practical, but reactors are now being built that might eventually make fusion energy available to consumers. One such reactor is the International Thermonuclear Experimental Reactor, or ITER, which is under construction in France, it might begin experiments as soon as 2025. However, fusion reactions are actually fairly common in space because every star we see in the sky is undergoing fusion reactions and most have been doing so for billions of years. So, how do fusion reactions begin in stars? Well, when a star forms it begins its life as a simple cloud of hydrogen gas, which no fusion reactions occurring inside. Gravity starts to pull the gas into its spherical shape, and as gravity pulls the gas together, it becomes denser and denser in the center. The amount of gas we're talking about is almost impossible to comprehend. For example, the sun has a mass of about 2 times 10 to the 30th kilograms. That's well over 300,000 times the mass of the earth. Because the mass is so large, gravity pushes to hydrogen atoms close together and this generates enormous amounts of heat. That heat is high enough to start fusion reactions. The gas that makes up the sun is mostly hydrogen, and that was even more true with the start of the sun's life. The main fusion reaction taking place in the sun is this one, in which two hydrogen-1 nuclei deform a hydrogen-2 nucleus and a positron. You might recall from video 37 that a positron is an antimatter particle. It has the same mass as an electron, but has a positive charge instead of a negative one. The hydrogen-2 nucleus produced in this reaction can then undergo another fusion reaction, this one, in which it fuses with another hydrogen-1 to form a helium-3 nucleus. Finally, two helium-3 nuclei can react to form a helium-4 nucleus and two hydrogen-1 nuclei. All three of those reactions are occurring in the sun right now, and you can see the result. Almost all the light in heat at the sun results from trillions of these reactions happening every second. Like most fusion reactions, each of these produces a large amount of energy, especially the third one, which produces about 1.24 billion kilojoules per mole. That's an immense amount of energy. Stars like our sun will continue to perform these reactions for between 1 million and 200 billion years. The heavier the star, the shorter its life will be. Our sun will probably continue these reactions for about another 5 and a half billion years. That's when things get really interesting. At about that time, the amount of hydrogen in the sun will finally start to run out. But that doesn't mean all the fusion reactions will stop. Instead, as the hydrogen runs out, gravity pulls the gas in the star closer together, and the star shrinks slightly. But that means the nuclei are closer together, so more heat is generated. Eventually, the heat is high enough to start some new, even more intense fusion reactions. The main one is this, in which three helium-4 nuclei fuse to form a carbon-12 nucleus. The carbon can also react with another helium to form an oxygen-16 nucleus. These reactions are particularly interesting to think about because it turns out that almost all the carbon and oxygen in the universe was formed in these fusion reactions. Before the first stars began to shine and perform these fusion reactions, there was almost no carbon or oxygen in the universe. That means all the carbon in your body, and all the oxygen in the atmosphere, and all the carbon and oxygen in the proteins and DNA in the living things on Earth, was created in stars that existed before the sun formed. And that means that all the other elements on the periodic table that are heavier than lithium were also created in stars, because fusion reactions are what's needed to build up heavy nuclei from lighter ones. In fact, the elements from Beryllium to Nickel are able to be formed by fusion reactions that happen in stars like our sun near the end of their lives. The fusion reaction needed to form an element is increasingly difficult as the elements get heavier, and so they require a higher and higher temperature. The intense heat makes the star expand, and so our sun will become larger, becoming a type of star called a red giant. The fusion of helium and other elements takes place much faster than the fusion of hydrogen in, so this part of the star's life only lasts about 130 million years, much less than the billions of years that the hydrogen fusion lasted. But eventually all the helium and other elements in a star will run out. Once they form iron or nickel, not even the intense heat generated inside the star is enough to cause the iron or nickel to undergo fusion. Instead, the outer layers of the star begin to cool off, and they get blown away by the hotter layers underneath. The result can be a very beautiful gas cloud called a stellar nebula. You've probably seen beautiful pictures of these if you were interested in astronomy. This is probably the eventual future of our sun. There's still a dim star underneath all the gas, but it's a much cooler type of star called a white dwarf, in which all the fusion reactions have stopped. But wait, I mentioned that elements as heavy as nickel can be formed by fusion reactions in a star like our sun. What about the heavier elements? Those are actually difficult to make using fusion. The temperatures needed for the fusion reactions are very rare anywhere in the universe, but actually they do happen. In order to perform those fusion reactions, we need a star that's at least 12 times more massive than the sun. Stars like those perform the same fusion reactions that the sun will perform right up to the production of iron and nickel. Just like in a smaller star, the fuel needed for those reactions will eventually run out. But unlike a smaller star like our sun, the star doesn't form a nebula and a white dwarf. Instead, once the fusion reactions stop, the gravity caused by the star's great mass is so large that it causes the star to collapse, all at once, in just a fraction of a second. The energy caused by all that gas collapsing so quickly generates a huge amount of heat all at the same time. It's so intense that the temperature of the resulting explosion is high enough to cause fusion reactions that produce all the rest of the elements on the periodic table heavier than iron. So everything all the way up to uranium and plutonium is produced in just a fraction of an instant at the end of a star's life. As we probably know, that kind of explosion is called a supernova. And that means that all the heavy elements on Earth, including the ones in your body like iodine, zinc, and copper, were formed in a supernova before the sun existed. That's pretty neat to think about, and that's also a good note to end the course on. I hope you enjoyed watching these videos for general chemistry. If you're hungry for more, please take more science courses, especially chemistry. I'm also making a series of videos like these for the course physical chemistry, so please feel free to watch those if you can. In the meantime, good luck on your final exams, and don't stop learning new and fun things whenever you can. Have fun!