I am a bit confused regarding the nuclear fusion that occurs during star formation. For example, suppose there is a huge hydrogen cloud. It gets more mass and therefore pulls in more and more hydrogen, but it could just do this endlessly and get endlessly big. What causes it to stop growing? Is it because of the heat generated by the huge mass?

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Maeve Holloway · Accepted Answer

This is not meant as a detailed description of how fusion starts in stars: I just want to convince you that it can start, and where the energy comes from to start it.Let&#039;s start with a big ball of hydrogen (and let&#039;s assume it&#039;s not spinning very fast). There are two things which determine what happens to it:- it has a lot of mass, and so gravity wants it to get smaller;- pressure wants it to get bigger.If we were very careful and built this ball very slowly and carefully we could get to a state where it just sat in equilibrium (so long as it was not too big when bad things famously happen) with pressure just counteracting gravity.But in fact what happens is that it starts off with quite a low pressure, so gravity wins, and starts making it smaller. What this means is that all the hydrogen atoms start moving down the gravitational potential gradient: they are losing gravitational potential energy. But energy is conserved, so they must be gaining some other kind of energy. And that&#039;s kinetic energy: they start moving faster, and bouncing around off each other as you&#039;d expect.Well, a thing where the atoms have a lot of kinetic energy like this is hot: that&#039;s what being hot is. So the ball of gas bargains gravitational potential energy into heat, and starts getting really hot.Some of this heat -- in fact half of it, thanks to the virial theorem -- gets radiated away into space, but it&#039;s a big ball of gas, so its surface area is relatively small compared to its volume, so the rate at which it can radiate stuff away is relatively low. So as it continues to shrink it gets really hot, especially in the centre.Really hot means that the hydrogen atoms in the centre are moving really fast (they also get ionized so they end up as a plasma). The centre of the ball of the gas also gets really dense as the thing falls in, so the atoms (really, the protons) start being more likely to crash into other protons, and to do so with lots of energy.If the ball is too small, then eventually the pressure becomes high enough to halt the collapse and then the thing sits there radiating away heat and slowly cooling.But if the ball is big enough then the centre gets hot and dense enough that some of these collisions have high enough energy that fusion starts. This releases more energy, so the temperature in the centre climbs further, and the protons have more kinetic energy and more fusion happens. It&#039;s kind of astonishing that this process settles down at some point to being a star which is stable for millions or billions of yeas rather than causing what you&#039;d naively expect, which is a huge explosion as fusion runs away and blows the thing to bits.

I am a bit confused regarding the nuclear fusion that occurs during star formation. For example, sup

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