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White and black dwarfs | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy


In the last video, we
started with a star in its main sequence,
like the sun. And inside the
core of that star, you have hydrogen
fusion going on. So that is hydrogen fusion,
and then outside of the core, you just had hydrogen. You just hydrogen plasma. And when we say plasma, it’s
the electrons and protons of the individual atoms
have been disassociated because the temperatures
and pressures are so high. So they’re really just
kind of like this soup of electrons and protons,
as opposed to proper atoms that we associate with
at lower temperatures. So this is a main sequence
star right over here. And we saw in the last
video that this hydrogen is fusing into helium. So we start having more
and more helium here. And as we have more
and more helium, the core becomes
more and more dense, because helium is a
more massive atom. It is able to pack more
mass in a smaller volume. So this gets more
and more dense. So core becomes more dense. And so while the core is
becoming more and more dense, that actually makes the fusion
happen faster and faster. Because it’s more dense,
more gravitational pressure, more mass wanting
to get to it, more pressure on the
hydrogen that’s fusing, so it starts to fuse hotter. So let me write this, so
the fusion, so hydrogen fuses faster. And actually, we even
see this in our sun. Our sun today is
brighter and hotter. It’s fusing faster
than it was when it was born 4.5 or
4.6 billion years ago. But eventually you’re
going to get to the point so that the core,
you only have helium. So there’s going to
be some point where the entire core is all helium. And it’s going to be way denser
than this core over here. All of that mass over there has
now been turned into helium. Not all of it. A lot of it has been
turned into energy. But most of it is now
in helium, and it’s going to be at a much,
much smaller volume. And the whole time, the
temperature is increasing, the fusion is getting
faster and faster. And now there’s this
dense volume of helium that’s not fusing. You do have, and we saw this in
this video, a shell around it of hydrogen that is fusing. So this right here is
hydrogen fusion going on. And then this over here
is just hydrogen plasma. Now the unintuitive
thing, or at least this was unintuitive
to me at first, is what’s going on the
core is that the core is getting more and more dense. It’s fusing at a faster rate. And so it’s getting
hotter and hotter. So the core is
hotter, fusing faster, getting more and more dense. I kind of imagine it’s
starting to collapse. Every time it collapses, it’s
getting hotter and more dense. But at the same time
that’s happening, the star itself
is getting bigger. And this is actually
not drawn to scale. Red giants are much, much
larger than main sequence stars. But the whole time
that this is getting more dense, the
rest of the star is, you could kind of view
it as getting less dense. And that’s because this is
generating so much energy that it’s able to more
than offset, or better offset the gravitational
pull into it. So even though this
is hotter, it’s able to disperse the
rest of the material in the sun over a larger volume. And so that volume is so big
that the surface, and we saw this in the last video, the
surface of the red giant is actually cooler– let me
write that a little neater– is actually cooler than the
surface of a main sequence star. This right here is hotter. And just to put
things in perspective, when the sun
becomes a red giant, and it will become a
red giant, its diameter will be 100 times the
diameter that it is today. Or another way to be put it,
it will have the same diameter as the Earth’s orbit
around the current sun. Or another way to view it
is, where we are right now will be on the surface or
near the surface or maybe even inside of that future sun. Or another way to put it, when
the sun becomes a red giant, the Earth’s going to be
not even a speck out here. And it will be
liquefied and vaporized at that point in time. So this is super, super huge. And we’ve even thought about it. Just for light to
reach the current sun to our point in orbit,
it takes eight minutes. So that’s how big one
of these stars are. To get from one side of the star
to another side of the star, it’ll take 16 minutes
for light to travel, if it was traveling
that diameter, and even slightly
longer if it was to travel it in a circumference. So these are huge,
huge, huge stars. And we’ll talk about
other stars in the future. They’re even bigger than this
when they become supergiants. But anyway, we have the
hydrogen in the center– sorry. We have the helium
in the center. Let me write this down. We have a helium
core in the center. We’re fusing faster
and faster and faster. We’re now a red giant. The core is getting hotter
and hotter and hotter until it gets to the temperature
for ignition of helium. So until it gets to
100 million Kelvin– remember the ignition
temperature for hydrogen was 10 million Kelvin. So now we’re at 100 million
Kelvin, factor of 10. And now, all of a
sudden in the core, you actually start to
have helium fusion. And we touched on this
in the last video, but the helium is fusing
into heavier elements. And some of those
heavier elements, and predominately, it
will be carbon and oxygen. And you may suspect this is how
heavier and heavier elements form in the universe. They form, literally, due to
fusion in the core of stars. Especially when we’re talking
about elements up to iron. But anyway, the core is now
experiencing helium fusion. It has a shell around it of
helium that is not quite there, does not quite
have the pressures and temperatures to fuse yet. So just regular helium. But then outside
of that, we do have the pressures and
temperatures for hydrogen to continue to fuse. So out here, you do
have hydrogen fusion. And then outside
over here, you just have the regular
hydrogen plasma. So what just happened here? When you have helium fusion
all of a sudden– now this is, once again,
providing some type of energetic outward
support for the core. So it’s going to counteract
the ever-increasing contraction of the core as it gets more
and more dense, because now we have energy going outward,
energy pushing things outward. But at the same time
that that is happening, more and more hydrogen in this
layer is turning into helium, is fusing into helium. So it’s making this inert
part of the helium core even larger and larger and
denser, even larger and larger, and putting even more
pressure on this inside part. And so what’s actually going
to happen within a few moments, I guess, especially from a
cosmological point of view, this helium fusion is
going to be burning super– I shouldn’t use– igniting or
fusing at a super-hot level. But it’s contained due
to all of this pressure. But at some point, the pressure
won’t be able to contain it, and the core is
going to explode. But it’s not going to be one of
these catastrophic explosions where the star is
going to be destroyed. It’s just going to release a
lot of energy all of a sudden into the star. And that’s called
a helium flash. But once that happens, all of
a sudden, then now the star is going to be more stable. And I’ll use that in quotes
without writing it down because red giants, in
general, are already getting to be less stable
than a main sequence star. But once that
happens, you now will have a slightly larger volume. So it’s not being contained
in as small of a tight volume. That helium flash kind
of took care of that. So now you have helium fusing
into carbon and oxygen. And there’s all sorts of
other combinations of things. Obviously, there’s many elements
in between helium and carbon and oxygen. But these are the
ones that dominate. And then outside of that,
you have helium forming. You have helium
that is not fusing. And then outside of that, you
have your fusing hydrogen. Over here, you have
hydrogen fusing into helium. And then out here in
the rest of the radius of our super-huge
red giant, you just have your hydrogen
plasma out here. Now what’s going to
happen as this star ages? Well, if we fast forward
this a bunch– and remember, as a star gets denser
and denser in the core, and the reactions happen
faster and faster, and this core is expelling
more and more energy outward, the star keeps growing. And the surface gets
cooler and cooler. So if we fast forward
a bunch, and this is what’s going to happen to
something the mass of our sun, if it’s more massive,
then at some point, the core of carbon and
oxygen that’s forming can start to fuse into
even heavier elements. But in the case of
the sun, it will never get to that 600 million
Kelvin to actually fuse the carbon and the oxygen. And so eventually you will have
a core of carbon and oxygen, or mainly carbon and
oxygen surrounded by fusing helium surrounded by
non-fusing helium surrounded by fusing hydrogen,
which is surrounded by non-fusing hydrogen, or just
the hydrogen plasma of the sun. But eventually all of
this fuel will run out. All of the hydrogen will
run out in the stars. All of this hydrogen, all
of this fusing hydrogen will run out. All of this fusion
helium will run out. This is the fusing hydrogen. This is the inert helium,
which will run out. It’ll be used in
kind of this core, being fused into the carbon
and oxygen, until you get to a point where
you literally just have a really hot core of
carbon and oxygen. And it’s super-dense. This whole time,
it will be getting more and more dense as heavier
and heavier elements show up in the course. So it gets denser and
denser and denser. But the super dense thing will
not, in the case of the sun– and if it was a more massive
star, it would get there– but in the case of
the sun, it will not get hot enough for the carbon
and the oxygen to form. So it really will just be this
super-dense ball of carbon and oxygen and all of the
other material in the sun. Remember, it was superenergetic. It was releasing tons
and tons of energy. The more that we
progressed down this, the more energy was
releasing outward, and the larger the radius
of the star became, and the cooler the
outside of the star became, until the outside just
becomes this kind of cloud, this huge cloud of gas around
what once was the star. And in the center– so I could
just draw it as this huge– this is now way far
away from the star, much even bigger than the radius
or the diameter of a red giant. And all we’ll have left is
a mass, a superdense mass of, I would call it,
inert carbon or oxygen. This is in the case of the sun. And at first, when it’s hot, and
it will be releasing radiation because it’s so hot. We’ll call this a white dwarf. This right here is
called a white dwarf. And it’ll cool down over many,
many, many, many, many, many, many, years, until it
becomes, when it’s completely cooled down, lost
all of its energy– it’ll just be this superdense
ball of carbon and oxygen, at which point, we would
call it a black dwarf. And these are obviously
very hard to observe because they’re
not emitting light. And they don’t have quite
the mass of something like a black hole that
isn’t even emitting light, but you can see how it’s
affecting things around it. So that’s what’s going
to happen to the sun. In the next few
videos, we’re going to talk about what would
happen to things less massive than the sun and what would
happen to things more massive than the sun,
although I think you can imagine the more massive. There would be so much
pressure on these things, because you have so
much mass around it, that these would begin
to fuse into heavier and heavier elements
until we get to iron.

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Comments
  • Could you stand on a black dwarf? That would be AMAZING. Say the Sun went through all of these phases in the next few hours, nom noming earth, cooling down, et cetera. What would happen to the outer planets such as Jupiter? Could you chill out on the black dwarf and watch Jupiter and Saturn go round and round?

  • what do you do for a career?
    you know an awful lot, and I'd like to think that if I knew as much as you that I'd be able to find a decent career?

  • @bevon17 He does this for a living, but he used to be a hedge fund analyst.

    On his website he says, "My background is in math, computer science, and investment management".

  • @vpletap Also the website says" Sal received his MBA from Harvard Business School. He also holds a Masters in electrical engineering and computer science, a BS in electrical engineering and computer science, and a BS in mathematics from the Massachusetts Institute of Technology."

  • @YuureiInu Well, it is capable of burning but our sun isn't big enough to get to the 600M K that it needs to get to start fusing the O and C. Much larger stars are capable of this

  • Another problem with black dwarves is, that it takes really long for the white dwarf to cool.
    In fact it would take at least 100,000 times longer than the current age of the universe, so there won't be any black dwarves for a very long time.

  • I think what he said was "more mass wanting to get to it" As in the context the core was getting more dense, and when things get dense its gravitational pull gets stronger. That's why black holes do what they do 🙂

  • I think its because it isn't hot enough. Oxygen and carbon are heavier then hydrogen and helium, so if it isn't hot enough they won't fuse. I know that bigger hotter stars can go all the way to fuse iron before its life cycle ends, and might become a black hole.

  • The nuclei within the star are part of a plasma, and therefor have no orbiting electrons.

    "Burning " in the traditional non-nuclear sense is a chemical reaction in which the nuclei of elements do not chang they just share/donate/steal electrons and become bonded (ie. Oxygen and Hydrogen to Water).

    "Burning" in the nuclear sense, as is found in stars, can burn the carbon and oxygen and in fact can go as far as iron but no further providing the star has enough mass to allow the fusion to begin.

  • The diameter of the earth's orbit is nearly 200,000,000 miles. The diameter of the sun at it's equator is near 1,000,000 miles. Even if as a red giant the sun's volume were to increase a million times in three dimensional space, the diameter would only increase a hundred times. Therefore the sun's diameter would be only nearly 100,000,000 miles. Making it's radius only nearly 50,000,000 miles Leaving the then cooler surface of the sun nearly 50,000,000 miles short of the earth. 

  • ples somebody help me! In the video he said the white dwarf only contain Carbon and Oxygen! So where did the elements in between go? For example, Nitrogen, Lithium..

    It says same in my book! This is so damn annoying!

  • I find it unintuitive that a relatively small white dwarf object without input of new energy, immersed in cold space, takes such a long time to cool down.

  • I don’t think sal has the right idea about the fusion processes in the core of stars. He said that helium begins to fuse because it is denser. I’m pretty certain that’s not why helium begins to fuse. The reason helium begins to fuse is because it takes more extreme temperature and pressure to fuse helium than hydrogen. When hydrogen becomes depleted, there is not enough temperature and pressure to fuse the now helium core. Thus the core contracts because there is no longer energy being released from fusion to counter the inward gravitational pull. Since the core contracts, it heats up and this is why helium begins to fuse.

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