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In the last video we started with a star in its main sequence like the sun. And inside the core of that

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star you had hydrogen fusion going on. So that is hydrogen fusion and then outside of the core

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you just had hydrogen. You just had hydrogen plasma.

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And we say plasma, its the electrons and protons of the idividual atoms

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have been disassociated because the temaptures and pressures are so high.

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So they really just, kind of, its like this soup of electrons and protons

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as opposed to proper atoms that we associate with at lower temperatures.

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So this is a main sequence star right over here.

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And we saw in the last video that this hydrogen is fusing into helium.

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So we start having more and more helium here.

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And as we have more and more helium the core becomes more and more dense

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because helium is a more massive atom. It is able to pack more mass into a smaller volume.

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So this gets more and more dense until, at some point, while the core is becoming

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more and more dense, that actually makes the fusion happen faster and faster

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because its more dense, more gravitational pressure, more mass wanting to get to it,

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more pressure on the hydrogen that's fusing, it starts to fuse hotter.

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So hydrogen fuses faster.

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In fact we even see this in our sun. Our sun today is brighter and hotter.

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Its fusing faster then it was when it was born 4.5 or 4.6 billion years ago.

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But eventually its going to get the point where in the core you only have helium.

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So at some point the entire core is all helium.

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Its going to be way denser than this core over here.

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All of that mass over there has been turned into helium.

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Not all of it, a lot of it has been turned into energy, but most of it is now in helium.

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Its going to be a much much smaller volume. The whole time the temperature