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  • but they're the boring ones.

  • Let me tell you about the interesting ones.

  • The interesting ones are the ones that live in the middle of Galaxies, which are enormous.

  • They're very, very massive Are these rare exotic things or they quite commonplace that trees, they're fairly common because I work in Galaxies and we now believe that at the center of every galaxy is a supermassive black hole.

  • We're talking about millions of times the mass of the sun, So the ones that are made from stars, they're probably a few times the mass of the sun, maybe 10 times the mass of the sun.

  • Now we're talking ones that are maybe a 1,000,000 times 10 million times 100 million times the mass of the sun.

  • And as far as we know, pretty much every galaxy in the universe has one of these massive black holes look in its in its center.

  • The main thing it sort of connects to is how big the galaxy is.

  • And in probably in the sense that you would expect, which is you got a big galaxy attention, A big black hole in the middle of it.

  • If you got a wimpy little galaxy attention of a small black hole in the middle of it.

  • The short answer is nobody really knows.

  • Well, I think that's still a pretty open question that that really, um, takes us back to the very early universe in the formation of the first Galaxies.

  • So what is it ever is?

  • Seeding?

  • The growth of thes supermassive black holes is happening a very, very long time ago.

  • So this is about how to produces seed black hole.

  • We think we understand more or less how they grow by spike eating material by by accreting matter and growing forming in the early universe is still open.

  • Yeah, it could be the very first generation of stars we believe a very massive that could be several 100 solar masses on dhe you could you could form a black hole, maybe 100 several 100 solar masses from those first generation of these superstars on.

  • They might, for example, quickly gobble up the gas around them on grow very quickly.

  • Or it could be that you have lots of stellar mass, more ordinary 10 2050 solar mass black holes that quickly merged together on.

  • I don't think that that's very clear, whether it's lots of small things coming together or whether you create a fairly large seed all at once.

  • That then grows.

  • And it's particularly interesting when you realize that we can see now very powerful quasars very far back in time.

  • The most distant quasar now known is that red shift of 7.1 register 7.1 means we're seeing this quasar very powerful quasar about 708 100 million years after the Big Bang.

  • So looking back over 12 billion years back in time, it didn't have long to grow the black hole in middle.

  • It must be about a 1,000,000,000 solar masses to produce the power that we see.

  • A 1,000,000,000 solar masses of material has grown within a few 100 million years.

  • That's quite tight, so forming the black hole's growing and quickly just about feasible in a bottle.

  • But it said interesting questions actually worked.

  • I mean, you can kind of see how they grow in that.

  • One of the ways we know the Galaxies grow is by crashing into each other in merging to form big Galaxies, and you can see that in that process, what's gonna happen well originally you had two Galaxies, each with his own black hole.

  • They'll merge.

  • Now there's a process single dynamical friction which effectively, If you've got anything massive in the galaxy, it will over time, kind of spiral into the middle and end up in the middle.

  • So what's gonna happen is your two black holes in his murder system will end up spiraling down on merging in the center of the galaxy.

  • And actually astronomers are currently very excited about that because that merger between two black holes that lost a few seconds as they merge together produces a huge burst of these things called gravitational waves.

  • We know that Galaxies do merge.

  • They do come together under gravity and collide.

  • It's not impossible that in doing so, they're two supermassive black holes come together, but one of them gets kicked out through tidal forces.

  • It's a wondrous thing to think of a supermassive black hole, her fling through intergalactic space, but probably very unlikely that you know it would ever come our way.

  • But professor, surely the only way for a supermassive black hole to grow isn't just by eating other black, indeed, and so that process goes on as well.

  • And in fact, when we see these incredibly bright things called quasars and other types of active Galaxies where you got a galaxy with something incredibly bright going on in the middle, almost certainly the reason why they're something incredibly bright going on in the middle is because material is falling into the vicinity of that black hole on the game.

  • Once it falls into the black hole, you don't see it.

  • And so actually that wouldn't make it bright.

  • But he's very energetic.

  • Processes that occur closer, the black hole horizon are very likely creating some of these very energetic phenomenon we see, and that will make the grab black hole bro as well.

  • So it's kind of like the bustling crowd waiting to fall in gives off a whole of hate and energy.

  • Something like that, Yeah, and because it's falling in from such a long distance and because the black hole is so dense, it's got a long way to fall.

  • So if you think if I drop an apple onto the floor, it's only going that far and going bang.

  • But if I take the same amount of mass in the earth and squish it down into a really dense object dropped the apple from the same height.

  • It's got a lot further to fall.

  • It's gonna be going a lot faster by the time it reaches the black hole.

  • So it's converting that potential energy into thermal and kinetic energy.

  • So it's really transforming.

  • You know, something that's falling into something that's very energetic.

  • The trouble is that once it's snacked on everything that happens to come within reach of it, that's it because actually, things tend to stay on just the orbits that they're on to start with.

  • And so the timescale for any more food to arrive is actually very long.

  • So indeed, what you say is true that a black hole will grow kind of within its sphere of influence.

  • It leave everything up.

  • But once it's done that, it's kind of run out of things to eat.

  • And so actually, at that point it will probably no longer be accreting.

  • And that's probably why some Galaxies air in this state where we see a very bright nucleus where they're clearly in these actively eating faces, where others, others like the Milky Way.

  • There's really not much that much going on in the center of our galaxy just because probably the black hole's not eating anything very much at the moment.

  • Just a supermassive black hole at the center of the Milky Way have a name.

  • Secretary is a star.

  • It tends to get called because it sort of coincides with a radio source, which was what was usually given that name.

  • It's only subsequently we realized that's probably where the black hole is.

  • That's actually it is a source of radio waves.

  • It's pretty quiet.

  • I mean, you can see it blink from time to time.

  • Maybe it has a little snack on something, and suddenly there's a little bit of brightness from there.

  • But basically the main reason we know there's a black hole in the middle of the Milky Way isn't by seeing the black hole is actually by seeing the stars around it.

  • We've actually over over years, a number of astronomers have been following the motions of stars, and it looks like there's a whole bunch of stars near the middle of the galaxy, which is whizzing around at quite high speed, which means it's a pretty strong gravitational field that's holding them there.

  • But there's nothing much to see.

  • And so this, you know, several 1,000,000 times the mass of the sun you can figure out has to be the mass in order to generate these kind of orbits.

  • But it's no actually giving out much light.

  • And so that's probably the strongest evidence for one of these supermassive black holes that is, that there's clearly something very massive there.

  • Otherwise, you wouldn't have these stars following these orbits, but actually it's not doing anything other than being very massive.

  • Which is kind of the definition of a black hole is a lot of mass and not my chills.

  • So probably, although every galaxy has the supermassive black hole, they're not always in an active phase.

  • They may go through phases where they may only switch on once in the early part of the Galaxy.

  • Physically, even though it's a very, very small fraction of the size and mass of the galaxy, we think that the black hole itself plays an important role in governing The evolution of back galaxy Black holes at the center of Galaxies can actually drive material out, which is a really counterintuitive thing, and that's because material comes down towards the black hole, it has to lose angular momentum.

  • It usually forms a disk that this gets really hot because the material in it is is you rubbing up against each other and produces radiation that radiation can drive a wind that then blows material out of the black hole.

  • So this before it's actually got in.

  • So it's not gonna push the stars around.

  • Nothing but gravity can really do that.

  • But there's a lot more to a galaxy than just stars.

  • There's gas and dust, all the raw material for making more stars.

  • And if you blow the gas out of the galaxy, so if you've got it, say, a disk galaxy, you've got black hole in the middle and you blow it out.

  • This way, it means that that galaxy that gas is not available to produce more stars, For example, one of the things that the astronomers are very interested in is is Galaxies tend to form stars, but some Galaxies start forming stars, so we need a mechanism, a sort of feedback mechanism that somehow stop the galaxy from forming stars.

  • And one of the things it could be is if a whole bunch of material gets secreted onto a galaxy that will tend to make it make lots of stars.

  • But if that then makes the nucleus very active, so it blows material out that can actually shut the process down on Connect as a kind of, ah, regulator of feedback process, we're forming stars.

  • It's just normal staffed spiral galaxy.

  • And at the moment, the black hole in the middle isn't doing anything very much.

  • So in a few 1,000,000,000 years, the Andromeda Galaxy is going to come crashing into the Milky Way.

  • It could be that that would stir things up to a create a sudden burst of star formation, but actually be maybe even fire off the black hole in the middle of one or the other, or both of these systems to actually shut down the star formation process.

  • Galaxies could go through a number of generations of these things, so it could well be in the past that the Milky Way we went through these active phase is maybe that slow down or shut down star formation.

  • Or maybe it never quite got to that point.

  • So it could well be that if you if we could kind of want wind the clock back.

  • We wouldn't see the black hole in the middle, the Milky Way doing more dramatic things than it does now.

  • But then we wouldn't have our guests.

  • There's more gases with more gas arriving, so eventually it will kind of you may deplete it through one of these events, but then maybe more of it, we'll get a created, more material will fall in.

  • So actually, even Galaxies which have gone through the shutdown process they could be rejuvenated didn't start forming stars again.

  • Interestingly, though, we have seen because we've spent so long now looking at this black hole in our neighborhood in the center of our galaxy, we've actually seen not just the stars orbiting around it, but we've occasionally seen things fall in.

but they're the boring ones.

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超巨大ブラックホール - 60のシンボル (Supermassive Black Holes - Sixty Symbols)

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    林宜悉 に公開 2021 年 01 月 14 日
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