Quasars

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Quasars

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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
electromagnetic
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.
So, anyway, hopefully that gave you some food for thought.