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.
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.
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.
The diameter of a neutron star is about 15 km. 1 cm3 of a neutron star has a mass of about 500 million tonnes.
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.
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 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.
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.
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 escape velocity is equal to the speed of light, i.e. thedistance at which gravity is just strong enough to hold photons down.
What does a black hole look like?
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.
There are probably millions of black holes like this drifting among the stars in our galaxy.
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.
What would it be like to fall into a black hole?
It would look quite different depending on your perspective. If you were safely away from the black holewatching 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.
Why would you see them slow down?Einstein’s 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 would seem to stay in suspended animation for ever.
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.
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.
If the person were able to look back as they fellthrough 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.
That’s hypothetical, though.In actual fact the person would die before they reached the event horizon. They would be spaghettified.
As they approached the event horizon, let’s say feetfirst, 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.
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.
There is thought to be a super-massive blackhole at the centre of most galaxies – maybe even all galaxies.There is one at the centre of our galaxy, theMilky Way. It has about 3 million times the mass of the Sun.
Super-massive black holes probably formed at the same time that the galaxy formed – within the first billion years after the big bang.
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 smallarea, eventually, they would have swallowed each other to form one large black hole.
Through the next 13 billion years, more starswould have fallen into the central black holesuntil their mass was millions or even billions of times the mass of the Sun.
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 inside the event horizon. The light is emitted by charged particles falling into the black hole as they accelerate towards the event horizon.
These black holes tend to emit radiation in alldirections but withparticularly strongjets along the axis of rotation.
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 moreradiation from their super-massive black hole than from all the stars in the galaxy combined.
These quasars are seen as they were billions of years ago, so it seems that very brightcentral black holes were more common early in the history of the universe than they are now.
Eventually, more and more stars will fall into these supermassive black holes, but it willprobably take hundreds of billions or trillions of years before most of the galaxy has been consumed.
By then all but the dimmest red dwarf starswill have stopped shining. Looking out from a surviving planet, we would see nothing but blackness.
Evaporation of Black Holes According 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.
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 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.
That’s it for black holes.I hope you don’t fall into one.