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.
