Birth of stars | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy


Let’s imagine we
have a huge cloud of hydrogen atoms
floating in space. Huge, and when I say huge
cloud, huge both in distance and in mass. If you were to combine
all of the hydrogen atoms, it would just be this
really, really massive thing. So you have this huge cloud. Well, we know that gravity
would make the atoms actually attracted to each other. It’s– we normally don’t think
about the gravity of atoms. But it would slowly
affect these atoms. And they would slowly
draw close to each other. It would slowly condense. They’d slowly move
towards the center of mass of all of the atoms. They’d slowly move in. So if we fast
forward, this cloud’s going to get denser and denser. And the hydrogen atoms
are going to start bumping into each other and
rubbing up against each other and interacting with each other. And so it’s going to get
denser and denser and denser. Now remember, it’s a huge
mass of hydrogen atoms. So the temperature is going up. And it’ll– they’ll
keep condensing. They’ll just keep
condensing and condensing until something really
interesting happens. So let’s imagine that they’ve
gotten really dense here in the center. And there’s a bunch of
hydrogen atoms all over. It’s really dense. I could never draw the
actual number of atoms here. This is really to
give you an idea. There’s a huge amount of
inward pressure from gravity, from everything
that wants to get to that center of mass
of our entire cloud. The temperature here is
approaching 10 million Kelvin. And at that point,
something neat happens. And to kind of realize the
neat thing that’s happening, let’s remember what a
hydrogen atom looks like. A hydrogen– and
even more, I’m just going to focus on
the hydrogen nucleus. So the hydrogen
nucleus is a proton. If you want to think
about a hydrogen atom, it also has an electron orbiting
around or floating around. And let’s draw another
hydrogen atom over here. And obviously this
distance isn’t to scale. This distance is
also not to scale. Atoms are actually–
the nucleus of atoms are actually much,
much, much, much smaller than the actual
radius of an atom. And so is the electron. But anyway, this just
gives you an idea. So we know from
the Coulomb forces, from electromagnetic forces,
that these two positively charged nucleuses will
not want to get anywhere near each other. But we do know from our– from
what we learned about the four forces– that if they did get
close enough to each other, that if they did get– if
somehow under huge temperatures and huge pressures you were able
to get these two protons close enough to each other,
then all of a sudden, the strong force will overtake. It’s much stronger
than the Coulomb force. And then these two
hydrogens will actually– these nucleuses would actually
fuse– or is it nuclei? Well, anyway, they would
actually fuse together. And so that is what
actually happens once this gets hot
and dense enough. You now have enough pressure
and enough temperature to overcome the
Coulomb force and bring these protons close enough
to each other for fusion to occur, for fusion ignition. And the reason why– and
I want to be very careful. It’s not ignition. It’s not combustion in
the traditional sense. It’s not like you’re
burning a carbon molecule in the presence of oxygen. It’s not combustion. It’s ignition. And the reason why
it’s called ignition is because when two
of these protons, or two of the nucleuses
fuse, the resulting nucleus has a slightly smaller mass. And so in the first
stage of this, you actually have two protons
under enough pressure– obviously, this would not happen
with just the Coulomb forces– with enough pressure
they get close enough. And then the strong interaction
actually keeps them together. One of these guys
degrades into a neutron. And the resulting mass
of the combined protons is lower than the mass
of each of the original. By a little bit, but
that little bit of mass results in a lot of
energy– plus energy. And this energy is why
we call it ignition. And so what this energy
does is it provides a little bit of
outward pressure, so that this thing
doesn’t keep collapsing. So once you get pressure
enough, the fusion occurs. And then that energy
provides outward pressure to balance what is now a star. So now we are at
where we actually have the ignition at the center. We have– and we still have all
of the other molecules trying to get in providing the pressure
for this fusion ignition. Now, what is the hydrogen
being fused into? Well, in the first step of
the reaction– and I’m just kind of doing the most basic
type of fusion that happens in stars– the hydrogen
gets fused into deuterium. I have trouble spelling. Which is another way of
calling heavy hydrogen. This is still
hydrogen because it has one proton and
one neutron now. It is not helium yet. This does not have two– it
does not have two protons. But then the deuterium
keeps fusing. And then we eventually
end up with helium. And we can even see that
on the periodic table. Oh, I lost my periodic table. Well, I’ll show
you the next video. But we know hydrogen
in its atomic state has an atomic number of 1. And it also has a mass of 1. It only has one
nucleon in its nucleus. But it’s being fused. It goes to hydrogen-2,
which is deuterium, which is one neutron, one proton
in its nucleus, two nucleons. And then that
eventually gets fused– and I’m not going into the
detail of the reaction– into helium. And by definition, helium has
two protons and two neutrons. So it has– or we’re
talking about helium-4, in particular, that
isotope of helium– it has an atomic mass of 4. And this process
releases a ton of energy. Because the atomic mass of
the helium that gets produced is slightly lower than
four times the atomic mass of each of the
constituent hydrogens. So all of this energy, all this
energy from the fusion– but it needs super high pressure, super
high temperatures to happen– keeps the star from collapsing. And once a star
is in this stage, once it is using hydrogen–
it is fusing hydrogen in its core, where the
pressure and the temperature is the most, to form helium–
it is now in its main sequence. This is now a main
sequence star. And that’s actually where
the sun is right now. Now there’s questions of,
well, what if there just wasn’t enough mass to get
to this level over here? And there actually
are things that never get to quite
that threshold to fuse all the way into helium. There are a few things
that don’t quite make the threshold of stars
that only fuse to this level. So they are generating
some of their heat. Or there are even
smaller objects that just get to
the point there’s a huge temperature and pressure,
but fusion is not actually occurring inside of the core. And something like Jupiter
would be an example. And you could go several
masses above Jupiter where you get
something like that. So you have to reach
a certain threshold where the mass, where the
pressure and the temperature due to the heavy mass,
get so large that you start this fusion. And– but the smaller you
are above that threshold, the slower the
fusion will occur. But if you’re super
massive, the fusion will occur really, really fast. So that’s the general idea
of just how stars get formed and why they don’t
collapse on themselves and why they are
these kind of balls of fusion reactions
existing in the universe. In the next few
videos, we’ll talk about what happens once that
hydrogen fuel in the core starts to run out.

39 Replies to “Birth of stars | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy”

  1. Seriously you have such a great way of explaining things, and I just love the way you draw it. Even as a layman you really make a lot of sense.

  2. what playlist is this under? Physics, or is Sal starting a new astronomy playlist? I dl Khan Academy, so I want to know what to sort it under.

  3. Question: at 3:59

    Two protons are still lighter than a proton / neutron nucleus. Does one of the protons absorb an electron before turning into a neutron? Or maybe there is some sort of other processes that creates the neutrons in the deuterium atoms?

  4. This is awesome. I need to bring whatever you're using into my classroom and quit the board and markers once and for all… Thanks for doing these!

  5. I thought a neutron is heavier (or have more mass) then a proton… So when they fuse do they both lose a bit of mass ( The degraded proton to a neutron keeping a little bit more bass) to form energy??

  6. ok, so in the last video you said gravity is weaker than EM force, then how come they are able to come close enough? i mean shouldnt the stronger (EM) force repel them before the strong force kicks in?

  7. @crorens picture a a pileup in football. The more players are on top, the more the guy on the bottom is being pushed into the ground. Similarly, the way hydrogen condenses in a cloud, the particles on the outside shells exert additional pressure to the ones at the very center and eventually a reaction occurs.

  8. Neutrons weigh SLIGHTLY more than protons so how come when the proton turns into a neutron, the two weight a little less than before? :S

  9. I got told today i had to do a 10minute presentation on Stellar evolution. I may just link your videos! I've been using them that much to write the presentation!

  10. Hydrogen accretes. Reaches critical mass/temperature/pressure. Star switches on. Why do stars vary so greatly in size? Shouldn't they all be roughly as big as each other? Can anyone please explain, in simple terms.

  11. 3.56: something is wrong there. 1 proton + 1 neutron, in fact, have greater mass than 2 protons. Neutron has greater mass than the proton. Must be another explanation for the energy release in fusion.

  12. If it takes certain amount of pressure and heat for nuclear fusion to happen then why stars are not the same size and some stars are smaller and some are much bigger ?

  13. I was under the impression that the extreme pressure and temperature at the core still wasn't enough to overcome the repulsive force between protons, and that the protons actually had to quantum tunnel through the energy barrier in order to get close enough to interact strongly

  14. this video made me understand life and everything that makes up life. I needed to confirm all of this that I heard with out any BS calculations or formulas or any of that bs. thank you

  15. How come when a proton turns into a neutron that the mass that is left creates energy when the weight of proton is less than the mass of the neutron? Would you not need mass to turn proton to a neutron?

  16. The Khan doesn't know what dark matter and energy is! If he did he could see the electromagnetic spectrum (like a pebble in still water) and see the quantum entanglement in light fields. Gravity is nothing more than vibrating molecules surrounding a mass in space, gravity is the electromagnetic spectrum or dark energy. Dark matter producing dark energy or electromagnetic radiation or aether is space time. Dark matter is Antihydrogen fusion converting antihelium to light!

  17. What about Boyles Gas Law? Wouldn't the heat drive the gases apart again? Don't gases seek equality in pressure?

    Why doesn't our own atmosphere collapse into a small star?

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