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 of the star, which keeps it stable. What is the other force acting on the matter in the star?
Okay so, what we’re being told here is that we’re thinking about a star. So, let’s say this is our star. And, within the star there is a nuclear fusion process going on. Now, nuclear fusion as a process generates a lot of energy. And in fact, what we’re being told here is that the heat generated in this process exerts an outward force on the material around it. And since nuclear fusion occurs mainly in the core of the star, this means that the heat generated in the nuclear fusion process results in an outward force on the star from the core. In other words, the fusion process is trying to expand the star.
And what the question is telling us is that if this was the only process going on, then eventually the star would expand. It would get bigger. However, this is not necessarily the case. In fact, for large periods of their lifetime, the sizes of stars are relatively stable. So clearly, there must be a force counteracting this outward force exerted by the nuclear fusion process. And we’re trying to figure out what this balancing force is.
Now first things first, if the fusion process produces an outward force on the star, then in order to balance this out there must be some inward force acting on the star. In other words, the force that’s trying to actually shrink the star. And in order to work out what this force actually is, let’s think back to the beginning of this star’s lifetime. In other words, how this star formed in the first place.
Now often, stars begin their lives as clouds of gas and dust, very large clouds where the gas is distributed over large volumes. And, individual gas particles are actually quite far apart from each other. But then, because these gas particles are slowly drifting around inside the gas, eventually a couple of them will come close enough to each other that their gravitational attraction will become important.
So, let’s say that this particle here and this particle here are now close enough that their gravitational attraction now becomes fairly significant. And at this point, they start travelling around as a pair. Then eventually, that pair of particles will get closer to another particle, say. And the gravitational attraction of this pair to this particle will now also become important. And now, we’ve got a clump of three different particles. So eventually, what we can see happening is that these gas particles get closer to each other during their random motion. Until their gravitational forces become important enough that these gas particles start travelling around as a clump.
A result of this is that clumps like this get bigger and bigger. But also because the gas itself is made up of gas particles, the total volume occupied by the gas gets smaller and smaller. Now at this point, all of the particles are going to be strongly attracting all other particles due to the gravitational attraction. And eventually, this cloud of gas gets smaller and smaller and becomes more spherical in shape.
At this point, what we have is all of these gas particles in the gas very close to each other and strongly attracting each other. And because these particles are very close to each other, the volume occupied by this gas is much smaller than what it was before. But additionally because these particles are close to each other, the pressure in this gas cloud is massive. And it’s only getting bigger as the particles get closer and closer to each other.
All this has happened because of the gravitational attraction of these particles to each other. And eventually, the pressure inside this cloud of gas gets so large that nuclear fusion can start occurring in the center of this cloud. At this point, what we have now is a main sequence star. Now as we saw earlier, once fusion begins, it will start to exert an outward force on the star that’s trying to grow the star in size. However, that does not mean that the inward gravitational force on the star suddenly disappears. In fact, that force is still there. And so, we get to a point of equilibrium where the gravitational force is trying to shrink the star, whereas the force exerted by a nuclear fusion is trying to grow the star. Those two forces exactly counteract each other, and the star remains a stable size.
Now in this question, we’re trying to find out what this inward acting force is on the star. And as we’ve seen based on our description here, that force is gravity.