What I want to do in this video is talk a little bit
about quasars.
And that's a short form for quasi-stellar radio sources.
And this name is just a byproduct
of the first observations of quasars.
Because all they looked like were
these kind of point-like sources of electromagnetic 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 actually, their main energy isn't even
being released 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.
So 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 is 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
and not enough to orbit around the black hole,
it's actually going to fall in.
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.
And so what you have is things start
to get really, 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 high-frequency electromagnetic radiation,
mainly in the X-ray part of the spectrum.
Now, I want to been very clear.
So there's two things here.
One is when you learn about quasars,
or when I first was exposed to quasars in like a Nova special,
they make you think that the quasar, 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?
And 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 anymore,
will not be able to escape the actual event horizon.
So all of this is from the accretion disc
around the supermassive black hole.
And the other question that used to pop in my mind
is why does it come out at these kind of perpendicular,
orthogonal to the plane of the actual accretion disk?
And at least my logic tells me, well, things
aren't going to pop out-- they're 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
allowed to kind of go and transmit freely into space.
Now, I want to be very clear.
Quasars, these 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 Big 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 have been a lot of mass that
would have 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, into the supermassive black
hole, will have fallen into the supermassive black hole.
And if you imagine at 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 have 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 Galaxy
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, digesting, is not digesting or consuming
material.
But you could imagine at some point in the Milky Way's past,
there might have been a lot of material
that didn't have quite the velocity to be able to orbit.
And so that was consumed.
And as 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 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-- 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 that distance is actually also
a certain time in the past.
Because it took the light 780 million years to get to us.
And actually, 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.
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 these are, as everything we learn in cosmology,
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 quasars,
the brightest known quasars, devour
on the order of 1,000 solar masses per year.
So that's on the order of 10 Earths, 10 Earths per second,
if I did my math right.
10 Earths per second are being devoured
by the brightest quasars.
And it's that energy of that mass that's acreting around it
that's generating all of that energy.
And actually, I should say-- I shouldn't even
talk about it in the present tense.
This happened in the past.
We're just observing it now.
For all we know, the rest of the universe
looks fairly similar to the way our universe does.
And so there really aren't that many quasars around.
Although 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 or 5 billion years
in the future when we actually collide with the Andromeda
Galaxy.
So anyway, hopefully that gave you some food for thought.