Supermassive black holes Supermassive Black Holes
Supermassive black holes
- In the videos on massive stars and on black holes, we learned that if the remnant 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 is an event horizon around these black holes.
- And if anything gets closer or goes within the boundary of that event horizon,
- there is no way that it can ever 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 -- I'll do it in purple, although it's supposed to be black --
- This entire surface 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 is because they are 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, lets 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 (the plural of supernova).
- Now, there is another class of black holes here, and they are somewhat mysterious.
- They are called supermassive black holes.
- To some degree the word super isn't big enough.
- They are not just a little bit more massive than stellar black holes --
- they are a lot more massive.
- They are on the order of hundreds of thousands to billions
- of solar masses.
- A hundred thousand to billions of times the mass of our sun.
- And what's interesting about these, other than the fact that they're 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 thirty-three 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 than can accrete around it,
- (So let's imagine you have a regular . . .
- So I'll draw the event horizon around it.
- The actual black hole's going to be in the center of that,
- or the mass of the black hole will be in the center of it.)
- And then over time you just have
- 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 would expect to see more black holes in between --
- maybe black holes with a hundred solar masses, or a thousand solar masses, or ten thousand 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 inclination is leaning 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.
- 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 the gravitational pull will pull them 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 single point;
- we've never gone into the center of a black hole.
- Just the mathmetics of the black hole --
- 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 infitely 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 the matter in the universe was in a much denser space
- because the universe was smaller.
- So let's say 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 radiation
- is at that point, the universe was relatively uniform.
- It was super, super dense, but it was relatively uniform.
- So in 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 close to it, but it's very close to it in every direction.
- So it would be pulled . . .
- 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 fluctuaion effects --
- it becomes slightly non-uniform.
- So let's say it becomes slightly non-uniform.
- 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 non-uniform.
- 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
- is going to be less than the gravitational pull towads those areas.
- And the more things get pulled towards it, the less uniform it's going to get.
- So you can imagine, in that primordial universe,
- very shortly after the Big Bang,
- when things were very dense and closely packed together,
- we may -- we may -- have had the conditions
- where these supermassive black holes could have formed,
- where you had so much mass in such a small volume,
- and it was just not-uniform enough so that you could 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 the even 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, it might go into the black hole.
- But if it's going past it fast enough,
- it'll just start going in orbit around the black hole.
- And so you can 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?"
- We think there is one.
- We think there is one because we've observed stars orbiting very quickly
- around something at the center of our Milky Way.
- And the only plausible explanation
- for things orbitting 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 four million times the mass
- of the sun.
- So hopefully that give you a little food for thought.
- There aren't just only stellar collapsed black holes.
- Or maybe there are,
- and maybe they somehow grow into supermassive black holes,
- and 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 un-uniform,
- things condensed into each other.
- And what we're going to talk about in the next video is
- how these supermassive black holes could help generate
- unbelievable sources of radiation,
- even though the black holes themselves aren't emitting them.
- And those are going to be quasars.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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