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let's imagine we have a huge cloud of hydrogen atoms floating in space.

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when I say huge clouds-huge both in distance and in mass

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when we combine those hydrogen atoms it will just be this really really massive thing

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well we know that gravity will make the hydrogen atoms attracted to each other

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and you normally don't think about the gravity of atoms, but it will slowly affect these atoms

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as they slowing draw close to each other-as they slowly condense

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they slowly move towards the centre of mass of all of the atoms

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they slowly move in, and so if we fast forward,

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these clouds will get denser and denser and the hydrogen

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atoms are going to start to bump into each other, rubbing up with each other and interact with each other

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denser and denser and denser

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there is a huge mass of hydrogen atoms, so its temperature is going up

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and they will keep condensing, and condensing, until something really interesting happens

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let's imagine that they got really dense in the centre

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and there is a bunch of hydrogen atoms all over, really dense

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i can never really draw the actual amount of atoms here, just give you and idea

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-there is a huge amount of inward pressure from gravity

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everything wants to get to the centre of the mass of our entire cloud.

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the temperature here is approaching 10 million Kelvin

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and at that point something neat happen

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let's remember what a hydrogen atom look like

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i'm just gonna to focus on the hydrogen nucleus, which is a proton

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if you want to think about a hydrogen,

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it also have a electron orbiting around it

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and let's draw another hydrogen atom over here

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and this distance isn't to scale, and that isn't to scale either

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the nucleus is acutally much much smaller than the actual radius of the atom

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so we know from the columb forces, these electromagnetic forces

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these 2 positively charged nucleus would not want to get near each other

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but we do know that if they did get close to each other

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under huge temperature and huge pressure,

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and you were able to get the 2 close to each other

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then all of sudden the strong force will overtake, it is much stronger than the columb forces

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and these 2 hydrogens- their nucleus, will actually fuse together

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that is what actually happens, once this gets hot and dense enough

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and now we have enough temperature and enough pressure to overcome the columb forces

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and bring the 2 protons close to each other

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for fusion to occur- ignition

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the reason why fusion-i want to be very clear here

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the reason why it is not ignition and the reason why it is not combustion

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- not like your normal burning carbon with oxygen together.

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the reason why it is not combustion, and it is ignition

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and the reason why it is called ignition, is that when 2 of the protons fuse

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the resulting nucleus has a slightly smaller mass

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and so the first stage of this, you actually have 2 protons

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under enough pressure- obviously it will not happen

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there is enough pressure and then the strong interaction actually keeps them together

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one of these guys degrades into a neutron

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and the resulting mass of the combined proton

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is lower than the mass of each of the orginal proton

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by a little bit-and this little bit results in a lot of energy

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plus energy-and this energy

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is why we call it ignition

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and this energy provide with a bit of ourward pressure so that this thing dont keep collapsing

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so once we get pressure enough, fusion occurs, and this energy provides

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outward pressure to balance what is now a star

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so now we actually have ignition at the centre

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we have all the other molecules trying to get in

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providing the pressure for this fusion-ignition

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now, what is the hydrogen being fused into?

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well the first step of the reaction

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well I'm just doing the most basic kind of fusion that happens to stars

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the hydrogen gets fused into deuterium

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what is called heavy hydrogen

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this is still hydrogen.

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it has 1 neutron and 1 proton and is not a helium yet- it does not have 2 protons

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and the deuterium keeps fusing and we eventually end up with helium

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and you can see that on the periodic table

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okay i have lost my periodic table i will show you in the next video

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but we know that hydrogen in its atomic state has a atomic number of 1

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and it also has a mass of 1

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it has only 1 neucleon in its nucleus

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but it is being fused, it goes into hydrogen 2, which is deuterium

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which is 1 neutron 1 proton, which is 2 neucleons,

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and that eventually

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gets fused into helium - and im not going to go into that reaction

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and by definition, helium has 2 protons and 2 neutrons

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ok we are talking about helium 4 in particular- that isotope of helium- it has a atomic mass of 4

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and this process will release a ton of energy

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because the atomic mass of helium is slightly lower than

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4 times the mass of each of the constitunent hydrogen

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so all of its energy from the fusion under super high pressure and super high temperature to happen

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keeps the star from collapsing

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and once the star is in this stage,

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once it is fusing hydrogen in its core into helium,

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it is now in its main sequence

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main sequence star

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and that is actually where the sun is right now

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and now there is question -

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oh what if there just wasnt enough mass to get to this level

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and there are actually things that cant quite reach the treshold to that level

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there are are few things that dont quite fuse to this level so they dont quite generate the heat

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or even smaller objects that just get to the point that there is just this huge temperature and pressure

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but fusion is not actually occuring inside of the core

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something like jupiter for example,

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several mass of the gases

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you will need to reach certain treshold of the mass

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with the pressure and temperature so large

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that you start with fusion

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but the smaller you are above the treshold, the slower fusion will occur

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but you are super massive, fusion will occur really really fast

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so that is just a general idea of how stars form and why they generally dont collapse on themselves

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and why they are this kind of balls of fusion reactions happening in the universe

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and in the next few videos we will talk more about what happens

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when the hydrogen fuel in the core starts to run out