2. When the Sun runsout of hydrogen itwill start to burnhelium
and expandinto a red giant.When the helium runsout, its gravity
willcause it to collapsedown to a very densewhite dwarf made
ofcarbon and oxygen. 3. The white dwarf will be about the size of
theEarth.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 burnthe carbon and
oxygen to heavier elements likeneon, silicon and eventually
iron.Once most of the core is iron,it cannot burn any more andwill
collapse to make a neutronstar and a supernova explosion.The
immense gravity will causethe electrons to be pushedinto the nuclei
to form a massof 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 is1
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 anobject from the surface for it to escape the Earthsgravity
and never fall back down again.It is about 11 km/s.The escape
velocity from the surface of a neutronstar 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 highthat 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
cantravel 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 eventhorizon. This is the
distance at which the escapevelocity is equal to the speed of
light, i.e. thedistance at which gravity is just strong enough
tohold photons down. 10. What does a black hole look like? 11. A
black hole drifting through empty space will not emit any light and
therefore cannot be seen directly. However, its presence can be
detectedbecause it bends star light coming past it. 12. There are
probably millions of black holes likethis drifting among the stars
in our galaxy. 13. The Earth might run into one one day.Though,
because the galaxy is so vast, thechances of that happening in the
next million years are quite minute. 14. What would it be like to
fall into a black hole? 15. It would look quite different depending
onyour perspective. If you were safely away from the black
holewatching someone else fall in, you would seethem speed up as
they fell towards it, butthen slow down again as they approachedthe
event horizon. 16. Why would you see them slow down?Einsteins
theory of general relativity says that invery high gravitational
fields, time slows down as observed by someone outside the
gravitational field.At the event horizon, time would stop. So
youwould see them get closer and closer to the event horizon, but
never actually reach it. They wouldseem to stay in suspended
animation for ever. 17. But it wouldnt seem like that from the
perspective ofthe person falling in.The person falling in would see
themselves falling fasterand 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 centreof the black hole.
18. Everything in the black hole would beconcentrated in the centre
at the singularity.It would take only a fraction of a second
afterfalling though the event horizon to reach the singularity and
be squashed to zero size.And that would be it. 19. If the person
were able to look back as they fellthrough the event horizon,
though, they would see the whole future of the universe
unfoldbefore their eyes, albeit rather dimly.Unfortunately, it
would all happen very quickly in a fraction of a second, then they
would haveanother fraction of a second to appreciate it before
being squashed into the singularity. 20. Thats hypothetical,
though.In actual fact the person would die before they reached the
event horizon.They would be spaghettified. 21. As they approached
the event horizon, lets say feetfirst, because their feet were
closer to it than their head,the gravitational pull on theirfeet,
would be a lot more than on their head. Thiswould pull them out
into a long string like spaghetti. This would be fatal. 22. So far
we have talked about black holes left over from when a star
collapses.These are called stellar-mass black holes becausethey are
about the same mass as a star.But there are also super-massive
black holes. 23. There is thought to be a super-massive blackhole
at the centre of most galaxies maybeeven all galaxies.There is one
at the centre of our galaxy, theMilky Way. It has about 3 million
times themass of the Sun. 24. Super-massive black holes probably
formed at the same time that the galaxy formed within the first
billion years after the big bang. 25. Some of the material of the
galaxy would have gone into orbit around the centre of the
galaxyforming stars and gas and dust clouds.But some would have
fallen into the centre. There it would have formed massive stars
which wouldhave burnt out very quickly and formed stellar- mass
black holes.As there would have been a lot of these in a smallarea,
eventually, they would have swallowed eachother to form one large
black hole. 26. Through the next 13 billion years, more starswould
have fallen into the central black holesuntil their mass was
millions or even billionsof times the mass of the Sun. 27. Unlike
stellar-mass black holes, super-massive black holes give out a lot
of light and othertypes of radiation from radio waves to X-rays. As
we know, no light can escape from insidethe event horizon. The
light is emitted by charged particles falling into the black hole
as they accelerate towards the event horizon. 28. These black
holestend to emit radiation in alldirections but withparticularly
strongjets along the axis of rotation. 29. Depending on how much
matter is falling intothe 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 moreradiation
from their super-massive black hole than from all the stars in the
galaxy combined. 30. These quasars are seen as they were billionsof
years ago, so it seems that very brightcentral black holes were
more common early in the history of the universe than they arenow.
31. Eventually, more and more stars will fall intothese
supermassive black holes, but it willprobably take hundreds of
billions or trillions of years before most of the galaxy has been
consumed. 32. By then all but the dimmest red dwarf starswill have
stopped shining. Looking out from a surviving planet, we would see
nothing butblackness. 33. Evaporation of Black HolesAccording to
quantum theory, particles canactually 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 theblack hole actually has a very small
probabilityof being outside the black hole at any instant in time.
34. If it is outside at any time, then it can get away. Thisprocess
is called evaporation.Once the universe is dead and everything that
isgoing 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. 35. Thats it for black holes.I hope
you dont fall into one.
