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Supermassive black holes

Supermassive Black Holes. Created by Sal Khan.

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  • male robot johnny style avatar for user Jonny Redekopp
    Do black holes ever end? Or do they just keep growing?
    (132 votes)
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    • duskpin seed style avatar for user RichDaRach
      Yes, even a black hole has a finite life. This discovery came about when Stephen Hawking discovered that black holes should radiate energy due to quantum mechanical processes. This radiation is called Hawking radiation. As a black hole radiates energy, it shrinks and the more it shrinks, the more it radiates (this is the nature of the radiative process) and so finally it will completely evaporate. However, the timescale for this is extremely long: a black hole of the mass of the Sun will take more than a billion times a billion times a billion times a billion times a billion times a billion times the age of the universe to evaporate completely! So it is not a process which has any significant effect for the black holes we find in astrophysical situations.
      (28 votes)
  • marcimus pink style avatar for user Sophia
    At , what is the name of the black hole in the middle of the Milky Way?
    (18 votes)
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  • purple pi purple style avatar for user eddieqiao
    is it possible to see a black hole on earth when you are standing and what does it look like
    (14 votes)
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    • primosaur ultimate style avatar for user Derek M.
      Black holes can't be seen. No light can escape them. However, according to Hawking, radiation is being emitted from them all the time due to virtual particles coming to existence by the event horizons of these black holes. When this happens, one of the virtual particles gets sucked in, making the other one become a "real" particle, so in order for thermodynamics and conservation of energy to not be violated, the black hole must release gravitational potential energy. This apparently happens all the time according to quantum mechanics, so eventually, black holes will evaporate.
      (21 votes)
  • marcimus pink style avatar for user ¤¤LAGEEZ¤¤
    If Super Massive Black Holes were at the centers of galaxys,why wouldn't the Black Holes eat up the galaxys?
    (7 votes)
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    • male robot hal style avatar for user Charles LaCour
      Black holes are not some incredible force of destruction in the universe that they are all too often portrayed as.

      If you were to take all of the mass of the earth except for a thin shell for us to stand on and compress it into a black hole we would not feel any difference.

      The force of gravity from a mass at a specific distance doesn't change based on the density of the matter. The equation F = (G * M1 * M2)/(r^2) has no term for density. Only G (a constant) M1 and M2 the masses and r the distance between the centers of mass of M1 and M2.
      (23 votes)
  • male robot johnny style avatar for user awesome.inc.5000
    Is it actually possible to go through a black hole, or even send a space probe into a black hole?
    (10 votes)
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  • purple pi purple style avatar for user Jonathan Nativ
    Does the massive gravitational pull of supermassive black holes have any effect on other black holes that are smaller?
    (3 votes)
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  • hopper happy style avatar for user rick ram
    at about solar mass is mentioned? what is it actually? what's its amount
    (6 votes)
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  • duskpin ultimate style avatar for user Bert Depoorter
    Black holes are cosmological objects that are so dense that they even absorb light. But, seeing light has no mass, how can it be attracted? Newton's law states F = (M1*M2*G)/(d^2). The mass of the black hole is very high, but the mass of light equals 0. Therefore, the equation comes like this: F = (M1*0*G)/(d^2) = 0. In my opinion, this means that light travels straigth forward and isn't attracted by black holes. What's wrong whit my reasoning? Can anyone explain this to me?
    (4 votes)
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    • male robot hal style avatar for user Andrew M
      What Mark said is correct, and also:
      1) The REST mass of light is zero, but light is never, ever at rest, so that's not relevant.
      2) Newton's law was for two point masses (or spherical masses). Is light a point mass? No. So applying Newton's law to light that way does not make sense.
      2) Still, if you insist on applying Newton's law, recall that F = ma and F = mg. Now do algebra:
      ma = mg
      divide by m
      a = g
      Newton's law therefore predicts that a will be equal to g and it doesn't say anything about "only if something has mass". Newton himself did not realize that his own law would predict that light would be affected by gravity, but it does. The problem turns out to be that the effect predicted by Newton's law is only 1/2 the observed effect. Einstein's relativity gets the prediction correct.
      (5 votes)
  • duskpin seed style avatar for user RichDaRach
    What makes a black hole BLACK?
    (0 votes)
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  • ohnoes default style avatar for user Evan Moser
    Wait wait wait. How does Sal's theory at make sense? Well not his theory but the one he supports. How would galaxy's form around black holes, when the event horizon keeps pulling things in?
    (2 votes)
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    • male robot hal style avatar for user Charles LaCour
      In the early universe the same density fluctuation that produced the clumping of matter to produce galaxies produced the black holes in the center. Black holes are not a some special super sucking object, they obey the same laws of gravitation that other objects do. If we were to replace the sun with a black hole of the same mass the orbits of the planets would not change.
      (7 votes)

Video transcript

In the videos on massive stars and on black holes, we learned that if the remnant of a star, of a massive star, is massive enough, the gravitational contraction, the gravitational force, will be stronger than even the electron degeneracy pressure, even stronger than the neutron degeneracy pressure, even stronger than the quark degeneracy pressure. And everything would collapse into a point. And we called these points black holes. And we learned there's an event horizon around these black holes. And if anything gets closer or goes within the boundary of that event horizon, there's no way that it can never escape from the black hole. All it can do is get closer and closer to the black hole. And that includes light. And that's why it's called a black hole. So even though all of the mass is at the central point, this entire area, or the entire surface of the event horizon, this entire surface of the event horizon-- I'll do it in purple because it's supposed to be black-- this entire thing will appear black. It will emit no light. Now these type of black holes that we described, we call those stellar black holes. And that's because they're formed from collapsing massive stars. And the largest stellar black holes that we have observed are on the order of 33 solar masses, give or take. So very massive to begin with, let's just be clear. And this is what the remnant of the star has to be. So a lot more of the original star's mass might have been pushed off in supernovae. That's plural of supernova. Now there's another class of black holes here and these are somewhat mysterious. And they're called supermassive black holes. And to some degree, the word "super" isn't big enough, supermassive black holes, because they're not just a little bit more massive than stellar black holes. They're are a lot more massive. They're on the order of hundreds of thousands to billions of solar masses, hundred thousands to billions times the mass of our Sun, solar masses. And what's interesting about these, other than the fact that there are super huge, is that there doesn't seem to be black holes in between or at least we haven't observed black holes in between. The largest stellar black hole is 33 solar masses. And then there are these supermassive black holes that we think exist. And we think they mainly exist in the centers of galaxies. And we think most, if not all, centers of galaxies actually have one of these supermassive black holes. But it's kind of an interesting question, if all black holes were formed from collapsing stars, wouldn't we see things in between? So one theory of how these really massive black holes form is that you have a regular stellar black hole in an area that has a lot of matter that it can accrete around it. So I'll draw the-- this is the event horizon around it. The actual black hole is going to be in the center of it, or rather the mass of the black hole will be in the center of it. And then over time, you have just more and more mass just falling into this black hole. Just more and more stuff just keeps falling into this black hole. And then it just keeps growing. And so this could be a plausible reason, or at least the mass in the center keeps growing and so the event horizon will also keep growing in radius. Now this is a plausible explanation based on our current understanding. But the reason why this one doesn't gel that well is if this was the explanation for supermassive black holes, you expect to see more black holes in between, maybe black holes with 100 solar masses, or a 1,000 solar masses, or 10,000 solar masses. But we're not seeing those right now. We just see the stellar black holes, and we see the supermassive black holes. So another possible explanation-- my inclinations lean towards this one because it kind of explains the gap-- is that these supermassive black holes actually formed shortly after the Big Bang, that these are primordial black holes. These started near the beginning of our universe, primordial black holes. Now remember, what do you need to have a black hole? You need to have an amazingly dense amount of matter or a dense amount of mass. If you have a lot of mass in a very small volume, then their gravitational pull will pull them closer, and closer, and closer together. And they'll be able to overcome all of the electron degeneracy pressures, and the neutron degeneracy pressures, and the quark degeneracy pressures, to really collapse into what we think is a single point. I want to be clear here, too. We don't know it's a single point. We've never gone into the center of a black hole. Just the mathematics of the black holes, or at least as we understand it right now, have everything colliding into a single point where the math starts to break down. So we're really not sure what happens at that very small center point. But needless to say, it will be an unbelievably, maybe infinite, maybe almost infinitely, dense point in space, or dense amount of matter. And the reason why I kind of favor this primordial black hole and why this would make sense is right after the formation of the universe, all of the matter in the universe was in a much denser space because the universe was smaller. So let's say that this is right after the Big Bang, some period of time after the Big Bang. Now what we've talked about before when we talked about cosmic background is that at that point, the universe was relatively uniform. It was super, super dense but it was relatively uniform. So a universe like this, there's no reason why anything would collapse into black holes. Because if you look at a point here, sure, there's a ton of mass very close to it. But it's very close to it in every direction. So the gravitational force would be the same in every direction if it was completely uniform. But if you go shortly after the Big Bang, maybe because of slight quantum fluctuation effects, it becomes slightly nonuniform. So let's say it becomes slightly nonuniform, but it still is unbelievably dense. So let's say it looks something like this, where you have areas that are denser, but it's slightly nonuniform, but extremely dense. So here, all of a sudden, you have the type of densities necessary for a black hole. And where you have higher densities, where it's less uniform, here, all of a sudden, you will have inward force. The gravitational pull from things outside of this area are going to be less than the gravitational pull towards those areas. And the more things get pulled towards it, the less uniform it's going to get. So you could imagine in that primordial universe, that very shortly after the Big Bang when things were very dense and closely packed together, we may have had the conditions where these supermassive black holes could have formed. Where we had so much mass in such a small volume, and it was just not uniform enough, so that you could kind of have this snowballing effect, so that more and more mass would collect into these supermassive black holes that are hundreds of thousands to billions of times the mass of the Sun. And, this is maybe even the more interesting part, those black holes would become the centers of future galaxies. So you have these black holes forming, these supermassive black holes forming. And not everything would go into a black hole. Only if it didn't have a lot of angular velocity, then it might go into the black hole. But if it's going pass it fast enough, it'll just start going in orbit around the black hole. And so you could imagine that this is how the early galaxies or even our galaxy formed. And so you might be wondering, well, what about the black hole at the center of the Milky Way? And we think there is one. We think there is one because we've observed stars orbiting very quickly around something at the center of the universe-- sorry, at the center of our Milky Way. I want to be very clear, not at the center of the universe. And the only plausible explanation for it orbiting so quickly around something is that it has to have a density of either a black hole or something that will eventually turn into a black hole. And when you do the math for the middle of our galaxy, the center of the Milky Way, our supermassive black hole is on the order of 4 million times the mass of the Sun. So hopefully that gives you a little bit of food for thought. There aren't just only stellar collapsed black holes. Or maybe there are and somehow they grow into supermassive black holes and that everything in between we just can't observe. Or that they really are a different class of black holes. They're actually formed different ways. Maybe they formed near the beginning of the actual universe. When the density of things was a little uniform, things condensed into each other. And what we're going to talk about in the next video is how these supermassive black holes can help generate unbelievable sources of radiation, even though the black holes themselves aren't emitting them. And those are going to be quasars.