Supermassive Black Holes - Sixty Symbols

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 are they quite commonplace like 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 million 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 it 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 if you've got a big galaxy it tends to have a big black hole in the middle of it if you've got a wimpy little galaxy it tends to have 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 and the formation of the first galaxies so what is it ever exceeding the growth of these supermassive black holes is happening a very very long time ago so this is about how to produce a seed black hole we think we understand more or less how they grow by 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 are very massive there could be several hundred solar masses and you could you could form uh you know a black hole maybe a hundred several hundred solar masses from those first generation of these superstars and they might for example quickly gobble up the gas around them and grow very quickly or it could be that you have lots of stellar mass more ordinary 10 20 50 solar mass black holes that quickly merge together and i don't think that that's very clear whether it's lots of small things coming together or whether you 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 a redshift of 7.1 redshift 7.1 means we're seeing this quasar very powerful quasar about 700 800 million years after the big bang so we're looking back you know over 12 billion years back in time it didn't have long to grow and the black hole in the middle it must be about a billion solar masses to produce a power that we see so a billion solar masses of material has grown within a few hundred million years and that's quite tight so forming the black holes growing them quickly enough just about feasible in the model but it's an interesting question as to how it actually worked i mean you can kind of see how they grow in that one of the ways we know that galaxies grow is by crashing into each other emerging to form big galaxies and you can see that in that process what's going to happen well originally you had two galaxies each with its own black hole they'll merge now there's a process they called dynamical friction which effectively if you've got anything massive in a galaxy it will over time kind of spiral into the middle and end up in the middle so what's going to happen is your two black holes in this merge system will end up spiraling down and merging in the center of the galaxy and actually astronomers are currently very excited about that because that merger between two black holes that last 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 there are two super massive 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 hurtling through intergalactic space um 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 holes it could just eat all the stars around the place 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've got a galaxy with something incredibly bright going on in the middle almost certainly the reason why there's something incredibly bright going on in the middle is because material is falling into the vicinity of that black hole and again once it falls into the black hole you don't see it and so actually that wouldn't make it bright but these very energetic processes that occur close to the black hole horizon are very likely creating some of these very energetic phenomena we see and that will make the black hole grow as well so it's kind of like the bustling crowd waiting to fall in gives off a whole lot of heat 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 drop the apple from the same height it's got a lot further to fall it's going to 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 right because actually things tend to stay on just the orbits that they're on to start with and so the time scale 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'll eat 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 are in this state where we see a very bright nucleus where they're clearly in these actively eating phases 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 is not eating anything very much at the moment does the supermassive black hole at the center of the milky way have a name sagittarius a star it tends to get called um because it sort of coincides with a radio source which was what was originally given that name and 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 it's 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 are whizzing around at quite high speed which means there's a pretty strong gravitational field that's holding them there but there's nothing much to see and so there's you know several million 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 not actually giving out much light and so that's probably the strongest evidence for one of these supermassive black holes there is 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 it's a lot of mass and not much else so probably although every galaxy has this supermassive black hole they're not always in an active phase they may go through phases or 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 that 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 disc that disc gets really hot because the material in it is is you know 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 going to 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 say a disc galaxy you got black hole in the middle and you blow it out this way it means that that galax that gas is not available to produce more stars for example one of the things that we the astronomers are very interested in is is galaxies tend to form stars but some galaxies stop forming stars and so we need a mechanism a sort of feedback mechanism that somehow stops a 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 then actually shut the process down and can act as a kind of a a regulator a feedback process we're we're forming stars it's a normal staffed spiral galaxy and at the moment the black hole in the middle isn't doing anything very much so in a few billion years the andromeda galaxy is going to come crashing into the milky way it could be that's that would stir things up to a create a sudden burst of style formation but actually b 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 can go through a number of generations of these things so it could well be in the past that the milky way went through these active phases maybe that slowed 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 wind the clock back we would have seen the black hole in the middle of the milky way doing more dramatic things than it does now but then we wouldn't have our gas well there's more gas there's always more gas arriving so eventually it will kind of you may deplete it through one of these events but then maybe more of it will get accreted more material will fall in so actually even galaxies which have gone through the shutdown process they can be rejuvenated and 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 it may snack on a little gas cloud or something and it does light up in that in that event i can generally describe them in terms of classical theory so i can talk about you know electromagnetic fields that you know you're normally familiar with the classical force of gravity that pulls pulls me

Channel: Sixty Symbols

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Keywords: sixtysymbols, supermassive black hole, black hole, galaxies, quasars

Id: kWSYuskD-zM

Length: 10min 9sec (609 seconds)

Published: Sun Apr 10 2016