Every one of these stars will die.
But their fates will not all be the same.
This star, the same size as our own sun,
will fade to a hot but stable white dwarf star,
barely visible to our telescopes.
And this one will erupt into a fiery supernova
and then collapse into a hot, dense neutron star.
But a very few of these stars, the very massive ones,
will end their days as some of the strangest
objects in the universe.
Objects so massive, yet so compact that they trap light
itself behind a dark veil.
A star is really a violent competition
between nuclear physics
generating all the heat and power that makes them shine,
and gravitational physics, which wants to compress them
to a point.
Now nuclear physics holds gravity at bay,
in some cases for billions of years.
For the sun, fusing hydrogen into helium will keep it
supported for 10 billion years.
But for a star that's 30 times as massive as the sun,
after only a million years, the nuclear fuels run out.
And in fact it's kind of a funny thing.
There's this what people call onion layer process that
in the middle you have the highest temperatures so you
have the most energetic fusion happening in the centre,
but be making iron in the middle,
but then you are fusing helium farther out,
or fusing maybe oxygen, carbon farther out.
Once iron dominates the central core,
the star can no longer generate the energy needed
to counter gravity and the central core collapses,
radiating a burst of energy outward that signals
a much bigger cataclysm about to unfold.
As it compresses down, gets smaller and smaller,
gravities getting stronger and stronger,
and there's nothing left to stop it,
it's going to form either a proto-neutron start at
the centre, or in some cases, for very massive stars,
directly into a black hole.
Now the key is for our observational consequences,
the rest of the star doesn't know about this yet.
And the rest of the star starts collapsing down
on this missing core, this missing floor.
If it forms a proto-neutron star,
the rest of the star suddenly stops and bounces.
It goes flying out, and the difference in the size between
the original core and the final proto-neutron star is
so extreme, that bounce has a huge amount of energy.
Much more energy than is required to completely
blow the star apart.
And we have a name for that, as a supernova.
Most of the star is thrown off into space,
yet a remnant of the star remains at the core.
If what's left is less than about three solar masses,
it will form a neutron star.
But if it is more than three solar masses,
no force that we know will stop it from collapsing
into a dimensionless point called a singularity.
That is the massive compact heart of a black hole.
But what is a singularity?
Yeah, so, I guess in some level, maybe this collapsing
to singularity is not a logical conclusion at all,
because there is no logic to singularities.
Singularity is the point at which none of the
laws of physics, as far as we know, applies,
and I think most physicists think there is a theory that
we still don't know, the theory of quantum gravity,
in which you have a consistent description of
both quantum effects and gravitational effects.
Such a theory is missing right now, but if you have that,
then that's going to replace this idea of singularity,
which really doesn't make any sense.
Eventually it means that something like a space/time
point doesn't exist, because you cannot really
distinguish between arbitrarily near space/time points.
And that's what quantum gravity is mostly about is actually
explaining something, what is space/time and
what is quantum space/time?
And the problem is really to build up a model that
delivers an answer to what is quantum space/time,
but which also connects to all the other stuff we
know about the universe.
We don't know what happens there.
I can't tell you what that is.
But on the other hand, in some sense a black hole is
more than that singularity.
It's the set of interactions, the set of dynamics that's going
to happen to everything around it as a consequence
of that singularity.
It's the gravitational field, or the space/time that's
left behind by this collapse.
To see how a black hole reveals itself through its gravity,
we'll travel to an established black hole that orbits
in a binary system with a blue giant star.
The first thing we notice is how star stuff is being pulled,
or accreted, from the surface of the star by the
tidal forces generated by the nearby black hole.
If we follow the accretion trail,
it will lead directly to the black hole.
To a physicist, this is all the black hole is.
If I'm an astronomer, I never see that.
And we see a lot of things that we think are black holes.
And the reason why we see them is we don't see black
holes all by themselves.
We see black holes in the middle of stuff.
Lots of stuff.
And it's the interaction of the black hole with that stuff that
produces all of the observable signatures that we can go after.
Stuff orbits around black holes very similarly to stuff
orbiting around the sun, and if I'm far enough away,
I really couldn't tell the difference between a black hole
that had the mass of a sun, and a star that had
the mass of a sun.
But that black hole is much, much more compact.
And so as stuff falls down, it accelerates and it gains
energy as it falls.
And this stuff that's falling in towards the black hole is
accelerating and gaining energy, turning that energy
into random motions, which then ends up shining.
Heats of the accretion flow and it shines.
All this gas rushing headlong in this traffic jam
to get in towards the black hole as it spirals in,
rubbing up against each other, liberating all of
this gravitational potential energy as it falls in,
ends up shining brighter than galaxies in some cases.
At a certain distance from the centre,
there is a zone of complete darkness.
An area where the effect of gravity is curving space/time
so much that nothing, not even light, can escape.
Like trying to escape against a current that is carrying
you faster and faster towards a waterfall,
there is a distinct point where the current is so strong
the swimmer can no longer make any progress upstream.
In the same way, light that travels too close to a black
hole will become trapped, unable to escape because
space is falling inward faster than the speed of light.
This edge of darkness is called the "event horizon, "
and it gives the black hole its name.
For the black hole, once you've crossed the horizon,
there's no going out; everything goes in.
That's why we call them black holes.
The other way we see black holes is through outflows.
That sounds a little counter-intuitive.
How does a black hole push anything going out?
After all, it's a horizon that's characterized by stuff
going in and not coming back.
Well it's that spin.
It's the fact that the black hole goes around.
And if I accrete a bunch of gas, and that gas has magnetic fields
in it, those magnetic fields feel the influence of the
rotation of the black hole and they're dragging those
field lines around, and they spiral them up, and matter
accelerates out along this cylindrical or nearly
conical in some cases field line structure.
And we call those jets.
So jets are basically elongated plumes of gas or
plasma that seem to be moving at relativistic, very,
very fast speeds, speeds close to the speed of light,
and basically being ejected from everything
a black hole has sucked in.
So far we have been describing stellar black holes.
Black holes formed by the death of the most massive stars.
In fact, you are looking at one of the most famous,
named Cygnus X-1.
Yet, stellar black holes are so compact that they are very
difficult to observe from Earth.
Luckily, astrophysicists have discovered
another type of black hole.
These are called super massive black holes and they are
millions of times larger than their stellar cousins.
They are found in the centre of most galaxies,
revealing themselves by way of their immense gravity.
These balls of light show the movement of stars near the very
centre of our own galaxy.
Each is travelling in orbit around the same invisible mass.
These orbits betray the presence of a super massive black
hole over four million times the mass of our sun.
We don't know precisely how they are formed,
yet they are our best chance of observing black holes directly,
and solving the mysteries about one of the universe's
strangest objects.