Supermassive Black Holes Supermassive Black Holes
Supermassive Black Holes
⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.
- 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.
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site