Lesson Video: Fusion in Stars Physics

Video Transcript

In this video, we will be discussing the process that occurs throughout the main sequence phase of a star’s life. This process is known as nuclear fusion.

Let’s begin, then, by looking at the formation of a star. A star begins to form when a very large cloud of gas and dust, mainly formed of hydrogen gas since hydrogen is the most common element in the universe, starts to collapse in on itself due to the force of gravity. Every bit of gas in the cloud attracts every other bit of gas. And eventually, the cloud of gas shrinks and becomes more and more dense as all of the material in it is packed into a smaller and smaller volume.

This also results in the cloud of gas getting hotter. And the pressure in the gas increases as it gets smaller. Eventually, the central part of the gas cloud, the core, gets hot enough and the pressure in the core gets high enough that a process known as nuclear fusion can begin to occur. Once this process begins in the cloud of gas, it’s now officially known as a star. It’s in its main sequence phase.

Now, this process, nuclear fusion, also releases energy. And all of this energy that’s released ends up exerting an outward force from the core of the star. This means that there is now something to combat the inward force due to gravity, which is what caused the star to form in the first place. Except, now, there is a direct competition between the inward gravitational force and the outward force caused by the energy released by the fusion process. And these two forces strike a balance. That is, they cancel each other out. And the star remains at a stable size; it doesn’t get any bigger or smaller for a very large part of its life.

Now, as we said earlier, this star is mainly made up of hydrogen. And hydrogen happens to be the fuel, so to speak, for this nuclear fusion process that occurs in the core of the star. And in fact, what’s actually happening in the core of the star is that hydrogen fuses to form helium. And this process releases lots of energy. Now, this process releases lots of energy because the fuel, hydrogen, is actually a very light element. Other elements can indeed undergo nuclear fusion, but, as a general rule, the heavier the element undergoing nuclear fusion, the less energies released.

So, during the main sequence phase of a star’s life, when there’s lots of hydrogen in the core, it’s the hydrogen that is fusing to form helium. However, at some point, the hydrogen in the star’s core all runs out. At this point, what we find in the star’s core is a lots of helium formed from the nuclear fusion process. And all of the hydrogen has been depleted. Now, at this point, helium starts to fuse. And the reason it does so is because helium is the lightest available element when all the hydrogen has run out.

Now, as we’ve mentioned already, the nuclear fusion process will generally release more energy if the elements undergoing fusion are lighter. And so, in this case, when all the hydrogen has run out, the lightest available element is helium. So, the star’s core fuses helium. But this fusion process releases less energy than when hydrogen was being fused. And this continues until all the helium runs out. At this point, in stars that have enough mass, the next lightest available element, which is lithium, begins to fuse.

It’s important to note here that lighter stars can fuse only lighter elements, whereas heavier stars can keep fusion going to produce heavier elements. These heavy stars start by fusing hydrogen until it runs out. Then, they fuse helium until that runs out. And this step-by-step process continues, where all the lightest available elements are used up in the fusion process occurring in the core. And then, the next lightest available element starts fusing. But every time a slightly heavier element starts fusing in the core, the energy produced by the fusion process gets smaller and smaller.

In fact, the heaviest element that can be produced by the fusion process is iron. Fusing elements heavier than iron is actually extremely difficult because doing this absorbs more energy than it gives out. And where would this energy come from, when the source of the energy within the star was the fusion process itself. In other words, in the core of a star, where fusion can occur because the temperatures and pressures are high enough, only elements as heavy as iron can be produced.

But on the periodic table, we can see much heavier elements than iron. We can see elements that have larger atomic numbers than iron. In the universe, elements as heavy as uranium are produced naturally. But the ones heavier than iron all the way up to uranium are produced by another process entirely. When a very large star, a star with much more mass than our sun for example, reaches the end of its life, it undergoes a massive explosion. This massive explosion is known as a supernova. It’s in this massive explosion, where large amounts of energy are produced, that heavier elements could be formed.

So, now, that we’ve looked at fusion and stars as well as understood a bit about how elements heavier than iron are formed, let’s take a look at an example question.

Which element the main sequence stars primarily use for nuclear fusion?

Okay, so, to answer this question, let’s first recall that the main sequence is literally the main phase of a star’s life. If this is our star, slightly egg-shaped for some reason, then the main sequence is set to begin when in the core of the star, the temperatures and pressures are high enough for nuclear fusion to begin. But the fact is that this star was formed due to the gravitational collapse of clouds made up of mainly hydrogen gas.

Now, hydrogen is the most abundant element in the universe. It is also the lightest possible element in the universe because it literally just needs one proton in its nucleus. And it may or may not have some neutrons. But even if it was just one proton that would classify as hydrogen. So, it seems highly likely, then, that what’s fusing in the core of the star is hydrogen because the star is mainly made of hydrogen.

But the other important thing is that nuclear fusion is a process that releases energy. In other words, nuclear fusion is the energy source of our star. And when nuclear fusion is happening, as a general rule, the lighter the element being fused, the more energy is released by this process. And since hydrogen is the lightest possible element in our universe, as well as what these stars are made up of, that points us very strongly in the direction of hydrogen being the element that main sequence stars primarily use for nuclear fusion.

Okay, let’s move on to another example question.

The heat generated through nuclear fusion in a star’s core exerts an outward force on the material around it. This would cause the star to expand, but it is balanced by another force acting upon the material in the star, which keeps it stable. What is the other force acting on the matter in the star?

Okay, so, in this question, what we’re being told is that we’ve got a star. And in the core of this star, nuclear fusion is occurring. Now, this nuclear fusion process is exerting an outward force due to all of the energy released from this fusion process. And this theoretically would cause the star to expand. But the star remains a stable size because of another force that balances the force generated due to this nuclear fusion process. We want to try and work out what this other force is.

So, firstly, we can realize that in order to balance the outward-acting force from the nuclear fusion process, we need to think about an inward-acting force, one that would potentially cause the star to collapse in on itself if the force from the nuclear fusion process wasn’t there to counteract it. And this force is actually the force that causes our star to be a star in the first place. Because let’s recall that stars are formed from what were initially large clouds of hydrogen gas.

Now, each little bit of the gas, each lower hydrogen molecule, has some small amount of mass. But then, anything with mass is attracted to anything else with mass due to the force of gravity. And it’s this force of gravity that causes this cloud of hydrogen gas to collapse in on itself. The cloud shrinks with all of the hydrogen molecules being packed into smaller and smaller volumes.

And the fact that the cloud shrinks means that the temperature and the pressures in the gas cloud increase massively. Until eventually the temperature and the pressure in specifically the core of this gas cloud is large enough for nuclear fusion to occur. At which point, the fusion process starts exerting the outward-acting force that balances the gravitational force that caused the inward collapse of the hydrogen cloud in the first place. And so, we can say that the inward-acting force is gravity. This is what balances the force exerted by the nuclear fusion process and keeps the size of the star relatively stable.

So, now, that we’ve had a look at a couple of examples, let’s summarise what we’ve talked about in this lesson. We firstly saw that hydrogen is the most abundant element in the universe. And it is hydrogen that fuses in the core of a star during its main sequence phase. Secondly, we saw that it’s energetically favorable to fuse lighter elements, which is why hydrogen is initially fused forming helium. Once all the hydrogen in the star’s core runs out, helium begins to fuse. Because helium is the next lightest available element in the star’s core.

It’s important to note that as heavier and heavier elements begin to fuse, the energy produced by these fusion processes decreases. And in fact, there’s a limit to what elements the fusion process can produce. In fact, the heaviest element that can be formed by a fusion process is iron. And elements heavier than iron are not formed in a fusion process, but rather in a supernova explosion undergone by large stars towards the end of their life cycles.