載入中...
相關課程
登入觀看
⇐ Use this menu to view and help create subtitles for this video in many different languages.
You'll probably want to hide YouTube's captions if using these subtitles.
相關課程
0 / 750
- let's imagine we have a huge cloud of hydrogen atoms floating in space.
- when I say huge clouds-huge both in distance and in mass
- when we combine those hydrogen atoms it will just be this really really massive thing
- well we know that gravity will make the hydrogen atoms attracted to each other
- and you normally don't think about the gravity of atoms, but it will slowly affect these atoms
- as they slowing draw close to each other-as they slowly condense
- they slowly move towards the centre of mass of all of the atoms
- they slowly move in, and so if we fast forward,
- these clouds will get denser and denser and the hydrogen
- atoms are going to start to bump into each other, rubbing up with each other and interact with each other
- denser and denser and denser
- there is a huge mass of hydrogen atoms, so its temperature is going up
- and they will keep condensing, and condensing, until something really interesting happens
- let's imagine that they got really dense in the centre
- and there is a bunch of hydrogen atoms all over, really dense
- i can never really draw the actual amount of atoms here, just give you and idea
- -there is a huge amount of inward pressure from gravity
- everything wants to get to the centre of the mass of our entire cloud.
- the temperature here is approaching 10 million Kelvin
- and at that point something neat happen
- let's remember what a hydrogen atom look like
- i'm just gonna to focus on the hydrogen nucleus, which is a proton
- if you want to think about a hydrogen,
- it also have a electron orbiting around it
- and let's draw another hydrogen atom over here
- and this distance isn't to scale, and that isn't to scale either
- the nucleus is acutally much much smaller than the actual radius of the atom
- so we know from the columb forces, these electromagnetic forces
- these 2 positively charged nucleus would not want to get near each other
- but we do know that if they did get close to each other
- under huge temperature and huge pressure,
- and you were able to get the 2 close to each other
- then all of sudden the strong force will overtake, it is much stronger than the columb forces
- and these 2 hydrogens- their nucleus, will actually fuse together
- that is what actually happens, once this gets hot and dense enough
- and now we have enough temperature and enough pressure to overcome the columb forces
- and bring the 2 protons close to each other
- for fusion to occur- ignition
- the reason why fusion-i want to be very clear here
- the reason why it is not ignition and the reason why it is not combustion
- - not like your normal burning carbon with oxygen together.
- the reason why it is not combustion, and it is ignition
- and the reason why it is called ignition, is that when 2 of the protons fuse
- the resulting nucleus has a slightly smaller mass
- and so the first stage of this, you actually have 2 protons
- under enough pressure- obviously it will not happen
- there is enough pressure and then the strong interaction actually keeps them together
- one of these guys degrades into a neutron
- and the resulting mass of the combined proton
- is lower than the mass of each of the orginal proton
- by a little bit-and this little bit results in a lot of energy
- plus energy-and this energy
- is why we call it ignition
- and this energy provide with a bit of ourward pressure so that this thing dont keep collapsing
- so once we get pressure enough, fusion occurs, and this energy provides
- outward pressure to balance what is now a star
- so now we actually have ignition at the centre
- we have all the other molecules trying to get in
- providing the pressure for this fusion-ignition
- now, what is the hydrogen being fused into?
- well the first step of the reaction
- well I'm just doing the most basic kind of fusion that happens to stars
- the hydrogen gets fused into deuterium
- what is called heavy hydrogen
- this is still hydrogen.
- it has 1 neutron and 1 proton and is not a helium yet- it does not have 2 protons
- and the deuterium keeps fusing and we eventually end up with helium
- and you can see that on the periodic table
- okay i have lost my periodic table i will show you in the next video
- but we know that hydrogen in its atomic state has a atomic number of 1
- and it also has a mass of 1
- it has only 1 neucleon in its nucleus
- but it is being fused, it goes into hydrogen 2, which is deuterium
- which is 1 neutron 1 proton, which is 2 neucleons,
- and that eventually
- gets fused into helium - and im not going to go into that reaction
- and by definition, helium has 2 protons and 2 neutrons
- ok we are talking about helium 4 in particular- that isotope of helium- it has a atomic mass of 4
- and this process will release a ton of energy
- because the atomic mass of helium is slightly lower than
- 4 times the mass of each of the constitunent hydrogen
- so all of its energy from the fusion under super high pressure and super high temperature to happen
- keeps the star from collapsing
- and once the star is in this stage,
- once it is fusing hydrogen in its core into helium,
- it is now in its main sequence
- main sequence star
- and that is actually where the sun is right now
- and now there is question -
- oh what if there just wasnt enough mass to get to this level
- and there are actually things that cant quite reach the treshold to that level
- there are are few things that dont quite fuse to this level so they dont quite generate the heat
- or even smaller objects that just get to the point that there is just this huge temperature and pressure
- but fusion is not actually occuring inside of the core
- something like jupiter for example,
- several mass of the gases
- you will need to reach certain treshold of the mass
- with the pressure and temperature so large
- that you start with fusion
- but the smaller you are above the treshold, the slower fusion will occur
- but you are super massive, fusion will occur really really fast
- so that is just a general idea of how stars form and why they generally dont collapse on themselves
- and why they are this kind of balls of fusion reactions happening in the universe
- and in the next few videos we will talk more about what happens
- when the hydrogen fuel in the core starts to run out