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Black holes

Black Holes. Created by Sal Khan.

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  • blobby green style avatar for user garcia.andrea482
    Could a blackhole ever be destroyed?
    (85 votes)
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    • duskpin ultimate style avatar for user Flying Emu
      When black holes meet, they do indeed form a bigger black hole.And yes black holes can get destroyed but not by another black hole.

      Black holes are not entirely giant 'sucking machines', they also give off energy. I'm not talking about quasars and spitting out of gamma ray bursts, I mean Hawking radiation. Because of this, black holes actually have a temperature.

      The temperature of a black hole is actually slightly above absolute zero, and so right before heat death, black holes will actually be giving off more energy than taking in, hence the decay of the black hole. It is predicted it would take 10^100 years for a super massive black hole to decay right before heat death (which is so far away I forgot how many zeros it had).
      (25 votes)
  • female robot grace style avatar for user Sabah
    So if light can't escape a black hole at a certain point, then how have people discovered them?
    (61 votes)
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  • orange juice squid orange style avatar for user Chand Vadalia
    Have any planets ever gotten sucked into a black hole that we know about?????
    (10 votes)
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    • leaf green style avatar for user Bill C.
      The "that we know about" parameter will make the answer an uneventful no. The problem with it is that the blackholes that we "know" about have either settled down and are just floating about waiting to hit something (or evaporate - hawking radiation) or are consuming stars or gas. The problem with viewing a planet being sucked into a black hole is that most planets will form near a star and when the blackhole starts sucking up the star the gasses will collide and the resulting friction will cause the event to become brighter and this effectively blinds us (this is like trying to look at a person standing between two headlights on a car at night and try to watch their lips move). The universe has been around long enough for just about anything to have happened, but I understand how cool it would be to witness it. :-)
      (15 votes)
  • male robot donald style avatar for user mark mulder
    do blackholes ever like run out of steam and disappear? and how do objects bigger than the black hole fit in it and how does it grow?
    (7 votes)
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    • piceratops ultimate style avatar for user AegonTargaryen
      Yes, black holes emit a little amount of radiation called Hawking Radiation (named after the discoverer). This radiation comes directly from the black holes mass and the black hole slowly gets smaller overtime. This takes an extremely long time though. The average stellar black hole will take an incredible 10^66 years to decay (10 with 66 zeroes). A supermassive black hole will take an unfathomable 10^92 years to decay completely. For reference, the universe is 1.37 x 10^10 years old. When the event horizon shrinks to size zero, everything that remains inside the black hole (much less than the original mass) bursts out in a titanic explosion.
      (13 votes)
  • starky tree style avatar for user OI'!!YA' THINK YOU CAN BEAT A SCOTSMAN!?
    could planets orbit around a black hole with enough speed?
    (14 votes)
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  • male robot hal style avatar for user rmishra72
    Say an object with the mass of the moon were to meet the event horizon of an average sized black hole, approximately how long would it take for the object to go from the event horizon to the singularity? Also, approximately how fast does a black hole travel?
    (10 votes)
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    • male robot hal style avatar for user Liam
      Since the force here involves the gravitational field strength, I would say that the rate of acceleration that an object gets sucked in is very close to speed of light. Since its mass is infinite, its gravitational strength would be infinite as well.
      However, it cannot go beyond speed of light due to Einstein's theory of relativity.
      Thus the actual time of an object sucked in would be indeed very fast. It is hard for us to observe the of black holes from the first place...

      I don't know if this has helped, but I thought that it wouldn't be bad to share my mind.
      (1 vote)
  • purple pi purple style avatar for user Jousboxx
    I've seen A few computer-generated videos of black holes "eating" stuff, and I noticed that it always spirals around instead of going straight. Why does it do this instead of going straight into the black hole?
    (4 votes)
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  • male robot johnny style avatar for user Denis
    If the photon has a mass of 0 e.v, why is it under influence of gravity? In other terms, why the photon is not able to escape beyond event horizon?
    (8 votes)
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    • mr pink red style avatar for user KrootonTheBudderMan
      Well, I don't think anything could be more infinite than the other. It's not like infinity could go on for infinity. That just doesn't make sense. Regular or supermassive black hole, neither is more infinite than the other. The only thing we can actually make a sense of with black holes is density. A lot of us know that supermassives are really dense. Maybe even all of us. I mean it's not like we can see them or anything, it's just that...well...we know stuff.
      (1 vote)
  • starky ultimate style avatar for user 1 Liu, Joseph
    I always thought that a black hole is a 4-d dimension, is that true?
    (3 votes)
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  • piceratops seedling style avatar for user amblau34
    What can possibly happen if we just keep adding mass to the black hole ? Will its infinite density will just keep increasing or will it stop at some point
    (6 votes)
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Video transcript

In the last video, we saw that if we started with a massive star about nine to 20 times the mass of the Sun, and when it finally matures the remnant of the star is roughly, or that remnant core of the star is roughly 1 and 1/2 to 3 times the solar mass or the mass of the Sun, then this remnant right here-- and let me just be clear, this nine to 20 times is the mass of that star when it's in its main sequence. This 1 and 1/2 to three times is the mass once it's shed off a lot of the, I guess, outer material of the star. And this is really the mass of the remnant of the star, kind of the core of the star. But that remnant, once it stops fusing, once it stops having outward pressure, and once it has enough density, this, we saw in the last video, will cause a supernova. It will cause a shock wave to move out through the rest of the material and essentially cause it to blow up. And this will condense into a neutron star. Now, in this video what I want to talk about is what if we're starting with a star that has a mass more than-- and this is give or take-- but we don't know the actual firm boundaries here. But what if we have a star that is more than 20 times the mass of the Sun? And this is kind of the original mass before the star burns itself out. Or when that star is kind of reached this old age, once it has an iron core, it has more than-- so I could say the remnant-- the dense remnant has more than three to four times the mass of the Sun. And remember, it's going to have three to four times the mass of the Sun. But it's going to be far denser. It's just going to be a core. It's going to be in iron-nickel core that's no longer fusing. So what happens to these stars? So it turns out that these are so massive that even the neutron degeneracy pressure will not be enough to keep the mass from imploding. And these stars, all of the mass in these stars will just keep imploding. So we imagine, in the first, in kind of Sun-like stars, things would collapse into white dwarfs. So maybe I should draw that in white. So they would collapse into white dwarfs. Now, that's not white either. There you go. They would collapse into a white dwarfs eventually. So this is a white dwarf. And here, the pressure that's keeping this from collapsing further is electron degeneracy pressure. The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore. But if the pressure gets large enough, then you have the neutron star. So you have even more mass and even a smaller-- and I'm not drawing this to scale-- neutron stars are tiny. White dwarf stars are on the scale of an Earth-like like planet. Neutron stars, we learned in the last video, are on the scale of a city. So these are superdense, super tiny. And this has more mass than this over here. In fact, maybe I should just draw it as a dot just so you have a sense of how dense it is. It's really just like one big atomic nucleus or, well, it's still small. But it's size of a city. It's like a nucleus the size of the city. But this right here is a neutron star. And what's unintuitive about what I'm drawing is each of these smaller things have more mass. This overcame the electron degeneracy pressure to collapse even further. But if the mass is large enough, and this is what we're talking about in this video, even the neutron degeneracy pressure will not be able to keep that mass from collapsing. And there's even theoretical quark stars where the quark degeneracy pressure-- but if you get even beyond that, then it all collapses into a single point-- and I'm simplifying here-- but it collapses into a single point of infinite density, infinite mass density. And this is really the mass of a black hole. And I'm calling it the mass of a black hole, because there's different ways how you could view where a black hole starts and ends or what exactly is the black hole. So this is all the mass of the black hole or we could say of the original star. So when we're talking about that remnant being times three to four solar masses, all of that mass is now being contained. Well, not all of it. Some of it is released as energy during the supernova. And that was also true of the neutron star. But most of that mass is now being contained in this infinitely small point. And you'll hear physicists and mathematicians talk about singularities. And singularities are really points, even in mathematics, where everything breaks down, where nothing starts to make sense anymore, where the mathematical equations don't give you a defined answer. And this is a singularity because you have a ton of mass in an infinitely small space. You essentially have an infinite density right here. And this is hard to visualize. But you have kind of an infinite curvature in space/time right here. And I can't visualize that. So maybe we'll think about that in more videos. But the reason why I said that there's different ways to think about where a black hole is, or where it starts and ends, is that this is where the mass is. And if there was any other mass that was over here, it would obviously be attracted to this mass and then become part of that singularity. It would add to that mass, that already huge mass, that's in an infinitely small point in space. But the reason why the boundary is hard to define is because there's some point in space around that singularity at which no matter what that thing is, no matter how much energy that thing has, it will not be able to escape the gravitational influence of the black hole, of that ultradense mass. So even if it was electromagnetic radiation, even if it was light, and even if it's a light that's shone away from the mass, it will eventually have to go back. It will not be able to escape the gravitational influence. And so the boundary where if you're within that boundary-- that's really a sphere-- so that boundary around the singularity where if you're within the boundary, no matter what you do, no matter if you're electromagnetic radiation, you're never going to be able to escape the black hole. If you are beyond that boundary, you might be able to escape the black hole. So this guy could escape. This guy over here, no matter what he does, is going to have to go back into the black hole. This boundary right here is called the event horizon. This right here is the event horizon, another word used in a lot of science fiction movies. And for good reason, because it's fascinating. And we'll actually learn in future videos-- hopefully, about Hawking radiation-- we'll see that that is not radiation from the black hole itself. It's the byproduct of quantum effects that are occurring at the event horizon. But the event horizon, it's this kind of point in space, or this sphere in space, or this boundary in space. Anything closer or within the event horizon has to eventually end up in the singularity, contributing to that mass. Anything on the outside has a chance of escaping. So what would a black hole look like? Well, not even light can escape from it. So it will be black. It will be black in the purest sense. It will not emit any type of radiation from the black hole itself, from that mass. And so here are some depictions I got from NASA of black holes. And so just to be clear what's happening here. What you're seeing here is black. You can view that as the black hole. When people talk about the black hole, that's often what they're talking about. But there's a point of infinite density at the center of this black sphere right here. And what you see as that black sphere, that really is the boundary of the event horizon. So this right here is the boundary of the event horizon. And what we're seeing right here is the accretion disk around the black hole. As all of this matter gets closer and closer to it, it's being squeezed more and more. It's moving faster and faster and getting hotter and hotter. And that's why the way this artist depicted it, it looks like the stuff over here is redder and hotter than the stuff further out. It's just accelerating as it approaches that event horizon. Once it's in the event horizon, we cannot even see the light it is emitting, even though it would be starting to become unbelievably energetic. Here's some other pictures. This is a picture of a star being ripped apart-- not a picture. This is actually an artist's depictions. All of these are artist depictions. We never were able to get such a good pictures of actual action occurring near black holes. These are artist depictions. But this is a star being ripped apart by a black hole. So this star is getting pretty close to this black hole. Already out here, where the star is, it's very strong gravitational attraction. So any mass that's being emitted from the star in that direction is slowly being pulled into the black hole. So the star is kind of being ripped apart by the black hole. This is maybe a better depiction of it. This is the star at first. And once it becomes under the influence of the black hole's gravitation, it starts to kind of elongate and gets ripped apart. And its matter starts spiraling in closer and closer to that black hole. And then once it's in the event horizon, we won't even see it anymore. Because even the light from that matter, that intensely hot matter that's entering into the black hole, cannot even escape the black hole itself. Anyway, hopefully you found that interesting. And I want to be clear, we still don't understand a lot about black holes. In fact, this whole notion of a singularity, the fact that all the math and all the theory breaks down at the singularity, is a pretty good sign that our theory isn't complete. Because if our theory is complete, we would maybe get something a little bit more sensical than just all of our equations not making sense at that infinitely dense point. Anyway, hopefully you found that interesting.