- What I wanna do in this video
- is talk a little bit about quasars.
- And that’s the short form for quasi-stellar radio sources.
- And this name is just a by-product
- of the first observations of quasars.
- Because all they’d look like were these kind of point-like
- sources of electromagnatic radiation, mainly in the radio part of the spectrum
- so that’s why we called them quasi-stellar radio sources.
- Now it turns out that they are neither stars
- or even quasi-stellar, and they’re actually not even—
- their main energy isn’t even being released in the radio frequency—
- in the radio band of the electromagnetic spectrum.
- They’re far more energetic than that.
- What they really are are the active nucleuses of galaxies.
- So let’s think about that a little bit.
- So if we have a supermassive black hole
- at the center of a galaxy.
- So let me draw that right over here.
- That’s our supermassive black hole.
- And maybe that’s the surface of the event horizon
- of the supermassive black hole
- The actual mass of the black hole is in the center of that event horizon.
- If there’s material that’s passing by this black hole
- it’s going to get attracted to it,
- and it’s going to form an accretion disk around it.
- This material is going to start rotating around this black hole
- and some of it, if it doesn’t have enough velocity,
- is going to actually fall into the black hole.
- So you have all of this material going around the black hole.
- And some of it,
- if it doesn’t have enough angular velocity,
- not enough to orbit around the black hole,
- it’s actually going to fall in.
- Now while things—
- let me label this; this is the accretion disk.
- So as things are getting faster and faster
- as they fall closer and closer to this black hole
- and bumping into each other more and more
- that gravitational potential energy from things falling into it
- is being turned into actual energy
- actual temperature
- so what you have is things start to get really, unbelievably
- unbelievably hot near the surface
- they get hotter and hotter as they fall closer and closer
- to that event horizon.
- And so near the event horizon itself,
- things are so intense that they’re actually releasing
- they’re actually releasing high-frequency
- electromagnetic radiation
- mainly in the x-ray part of the spectrum.
- Now I want to be very clear.
- So there’s two things here.
- One is, when you learn about quasars—
- or, when I was first exposed to quasars,
- in, like, a Nova special—
- they make you think that the radiation
- is somehow being released by the black hole itself
- and I would scratch my head.
- Because I was just told that nothing can escape
- the event horizon of a black hole,
- including electromagnetic radiation.
- So how could that be being emitted by the black hole?
- The answer is, it’s not being emitted by the black hole.
- It’s being emitted by the matter
- in the accretion disk
- that hasn’t quite gotten to the event horizon yet
- Once it’s inside of the event horizon,
- any electromagnetic radiation that it might emit
- will not be able to escape the black hole any more;
- will not be able to escape the actual event horizon.
- So all of this is from the accretion disk
- around the supermassive black hole.
- And the other question that used to
- pop in my mind
- is why does this kind of come out at these
- kind of perpendicular, kind of orthogonal to the plane
- of the actual accretion disk.
- At least my logic tells me, well things are not going to
- pop out along the direction of the accretion disk,
- because then they’re going to be absorbed by other things
- in fact that’s what’s going to cause other things
- to get heated up
- closer to the actual event horizon,
- so any energy that’s going out in that direction
- is just going to be absorbed
- and make other things hotter,
- and only when you go roughly perpendicular
- to the plane of the accretion disk
- is that energy aloud to kind of go transmit freely into space
- Now I want to be very clear:
- quasars are the most luminous things that we know of
- in the universe.
- The brightest, or actually, many quasars
- are on the order of a trillion suns in luminosity,
- so they can be brighter than an entire galaxy,
- and that’s just coming from material around a fairly small
- region of space,
- much, much, much smaller than an actual galaxy.
- It’s the very center, it’s kind of just the galactic core.
- Now another interesting thing about quasars
- and this kind of gives credence to this notion
- of a constantly changing universe,
- and even to some degree the Bing Bang itself,
- is you have these supermassive black holes
- that may be formed shortly after the Big Bang.
- Now you can imagine, at an early stage
- in the universe’s development,
- there would’ve been a lot of mass that would’ve been near
- these black holes that didn’t have quite the velocities
- to be able to escape them or be able to orbit around them,
- and so these would actually start falling into the black hole.
- And then, over time, all of the mass
- that had to fall into the black hole, the supermassive black hole
- will have fallen into the supermassive black hole,
- and if you imagine some future period of time,
- you should still have the supermassive black hole,
- but all you should see is mostly things orbiting around it.
- Anything that had to fall into it
- would’ve already fallen into it.
- So you’re just going to see things orbiting around it.
- And this is actually what we see.
- If we look around us. If we look at our Milky Way Galaxy.
- We don’t observe a lot of things falling in.
- For example, the Milky Way does not have an active nucleus.
- An active core.
- It is not currently a quasar,
- the center of the Milky Way Galaxy.
- The supermassive black hole there
- is not, I guess we could say, is not digesting
- or consuming material.
- But you could imagine that sometime in the Milky Way’s past
- there might’ve been a lot of material that didn’t have
- quite the velocity to be able to orbit, and so that was consumed,
- and when it was consumed, it would emit
- all of this x-ray radiation, and could be observed as a quasar.
- And that’s actually what we observe.
- The closest quasars, and we’ve observed more
- than 200,000 quasars.
- The closest quasars are on the order of
- 780 million—million!—light-years away.
- So what does that mean?
- We don’t observe quasars closer than 700 million light-years
- So what that tells us is, at least in our region of the universe
- the most recent quasars were 780 million years in the past.
- When we look at closer parts of the universe—
- so let me draw—
- let’s say this is the observable universe; this is us—
- so we only start to observe quasars
- at a certain distance away from us,
- and this distance is actually at a certain time in the past.
- because it took the light 780 million years to get to us.
- And most of the quasars are more than 3 billion light years away,
- which tells us that they only existed more than
- 3 billion years in the past, at a younger stage
- of the actual universe,
- when there was actual material for these
- supermassive black holes to consume
- at the center of galaxies.
- As you move closer in time to us, and most of that material
- has actually been consumed.
- And we just have material orbiting around
- these supermassive black holes,
- which we call galaxies,
- and so we don’t observe quasars anymore.
- And just to give an idea—
- I mean, you know, as with everything we learn in cosmology,
- there’s kind of these mind-bending concepts,
- unbelievable distances, unbelievable masses,
- unbelievable brightnesses, I guess you could think about it,
- but just to give a sense, the brightest known quasars
- devour on the order of 1000 solar masses per year.
- So that’s on the order of 10 earths per second,
- if I did my math right.
- Ten earths per second are being devoured by the brightest quasars.
- And it’s that energy of that mass
- that’s accreting around it that’s generating
- that’s generating all of that energy.
- And actually I should say,
- I shouldn’t even talk about it in the present tense.
- These brightest quasars, this happened in the past,
- we’re just observing it now.
- There are no—as—for all we know,
- the rest of the universe looks fairly similar
- to the way our universe does,
- so there really aren’t that many quasars around.
- Although you know, the other side of the coin might be
- even though most of the material has already been consumed,
- maybe even by our own supermassive black hole
- in the center of the Milky Way,
- at some point in the future, maybe it will be able to consume
- on some more stellar material, some more, well,
- any type of material in the future,
- and that might happen about 4, 5 billion years in the future
- when we actually collide with the Andromeda Galaxy.
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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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