1. Black Holes

2. When the Sun runs
out of hydrogen it
will start to burn
helium and expand
into a red giant.
When the helium runs
out, its gravity will
cause it to collapse
down to a very dense
white dwarf made of
carbon and oxygen.

3. The white dwarf will be about the size of the
Earth.
1 cm3 of it will have a mass of about 1 tonne.
Gravity on its surface will be about 1 million
times as much as on Earth.

4. A star 8 to 20 times the mass of the Sun will burn
the carbon and oxygen to heavier elements like
neon, silicon and eventually iron.
Once most of the core is iron,
it cannot burn any more and
will collapse to make a neutron
star and a supernova explosion.
The immense gravity will cause
the electrons to be pushed
into the nuclei to form a mass
of neutrons.

5. The diameter of a neutron star is about 15 km.
1 cm3 of a neutron star has a mass of about
500 million tonnes.

6. Gravity on the surface of a neutron star is
1 trillion times as much as on Earth.
A neutron star can spin at several hundred
revolutions per second.

7. The escape velocity from the surface of the Earth is
the speed at which you would have to throw an
object from the surface for it to escape the Earth’s
gravity and never fall back down again.
It is about 11 km/s.
The escape velocity from the surface of a neutron
star would be more like 100 000 km/s
– about 1/3 the speed of light.

8. If the star is more than 20 times the mass of the
Sun, when it collapses, the gravity will be so high
that even the neutrons get pushed into each other
and the whole star will collapse down to a single
point, called a singularity.
Gravity at the singularity is infinite.
Within a couple of kilometres of the singularity,
gravity is so strong that the escape velocity is
greater than the speed of light. As nothing can
travel faster than the speed of light, nothing, not
even light, can escape.

9. This bit of space is called a black hole.
The surface of the black hole is called the event
horizon. This is the distance at which the escape
velocity is equal to the speed of light, i.e. the
distance at which gravity is just strong enough to
hold photons down.

11. What does a black hole look like?

12. A black hole drifting through empty space will
not emit any light and therefore cannot be
seen directly.
However, its presence can be detected
because it bends star light coming past it.

13. There are probably millions of black holes like
this drifting among the stars in our galaxy.

14. The Earth might run into one one day.
Though, because the galaxy is so vast, the
chances of that happening in the next million
years are quite minute.

15. What would it be like to fall into a black hole?

16. It would look quite different depending on
your perspective.
If you were safely away from the black hole
watching someone else fall in, you would see
them speed up as they fell towards it, but
then slow down again as they approached
the event horizon.

17. Why would you see them slow down?
Einstein’s theory of general relativity says that in
very high gravitational fields, time slows down as
observed by someone outside the gravitational
field.
At the event horizon, time would stop. So you
would see them get closer and closer to the event
horizon, but never actually reach it. They would
seem to stay in suspended animation for ever.

18. But it wouldn’t seem like that from the perspective of
the person falling in.
The person falling in would see themselves falling faster
and faster towards the event horizon, then plunging
straight through it without any slowing down.
Like everything else that fell through the event horizon,
they would fall straight to the singularity at the centre
of the black hole.

19. Everything in the black hole would be
concentrated in the centre at the singularity.
It would take only a fraction of a second after
falling though the event horizon to reach the
singularity and be squashed to zero size.
And that would be it.

20. If the person were able to look back as they fell
through the event horizon, though, they would
see the whole future of the universe unfold
before their eyes, albeit rather dimly.
Unfortunately, it would all happen very quickly –
in a fraction of a second, then they would have
another fraction of a second to appreciate it
before being squashed into the singularity.

21. That’s hypothetical, though.
In actual fact the person would die before
they reached the event horizon.
They would be spaghettified.

22. As they approached the
event horizon, let’s say feet
first, because their feet were
closer to it than their head,
the gravitational pull on their
feet, would be a lot more
than on their head. This
would pull them out into a
long string like spaghetti.
This would be fatal.

23. So far we have talked about black holes left over
from when a star collapses.
These are called stellar-mass black holes because
they are about the same mass as a star.
But there are also super-massive black holes.

24. There is thought to be a super-massive black
hole at the centre of most galaxies – maybe
even all galaxies.
There is one at the centre of our galaxy, the
Milky Way. It has about 3 million times the
mass of the Sun.

25. Super-massive black holes probably formed
at the same time that the galaxy formed –
within the first billion years after the big
bang.

26. Some of the material of the galaxy would have
gone into orbit around the centre of the galaxy
forming stars and gas and dust clouds.
But some would have fallen into the centre. There
it would have formed massive stars which would
have burnt out very quickly and formed stellar-
mass black holes.
As there would have been a lot of these in a small
area, eventually, they would have swallowed each
other to form one large black hole.

27. Through the next 13 billion years, more stars
would have fallen into the central black holes
until their mass was millions or even billions
of times the mass of the Sun.

28. Unlike stellar-mass black holes, super-massive
black holes give out a lot of light and other
types of radiation from radio waves to X-rays.
As we know, no light can escape from inside
the event horizon.
The light is emitted by charged particles
falling into the black hole as they accelerate
towards the event horizon.

29. These black holes
tend to emit
radiation in all
directions but with
particularly strong
jets along the axis
of rotation.

30. Depending on how much matter is falling into
the black hole at the time, some super-
massive black holes produce little radiation,
while others produce a huge amount.
Quasars are distant galaxies which emit more
radiation from their super-massive black hole
than from all the stars in the galaxy
combined.

31. These quasars are seen as they were billions
of years ago, so it seems that very bright
central black holes were more common early
in the history of the universe than they are
now.

32. Eventually, more and more stars will fall into
these supermassive black holes, but it will
probably take hundreds of billions or trillions
of years before most of the galaxy has been
consumed.

33. By then all but the dimmest red dwarf stars
will have stopped shining. Looking out from a
surviving planet, we would see nothing but
blackness.

34. Evaporation of Black Holes
According to quantum theory, particles can
actually get out of a black hole over a very long
period of time.
This happens because there is a level of
uncertainty involved in the position of any
particle. This means that a particle inside the
black hole actually has a very small probability
of being outside the black hole at any instant in
time.

35. If it is outside at any time, then it can get away. This
process is called evaporation.
Once the universe is dead and everything that is
going to fall into a black hole already has, then it is
thought that black holes would slowly evaporate.
This would take much longer than trillions of years,
though.

36. That’s it for black holes.
I hope you don’t fall into one.