Little green men and Black holes

"Is a star completely blasted into dust in a supernova?"

No there is usually a core left behind and this is where the story gets interesting. Last time you asked a very intelligent question.

“Did I?”

You certainly did, you asked what stops gravity collapsing a star into nothing.

“..and you said it was the charged particles in atoms pushing away from each other like the poles of two magnets.”

Very good, you were listening. Well, that pushing effect which scientists call electron degeneracy is true up to a point.

“Electron degeneracy? What a great name for a rock band.”

Rock band?

“Rock, a new type of music, invented 50 years ago. You definitely need to get your entertainment circuits updated Albert, anyway you were telling me about gravity.”

Ah yes, gravity well if it gets too strong even the forces inside atoms aren’t strong enough and the electrons and protons in atoms get crushed together to end up as neutrons. Neutrons are not charged so can be squeezed together very tightly. A spoonful of pure neutrons would contain billions of tons. These dense neutron remnants are called neutron stars.

“So neutron stars must be tiny?”

They are. A neutron star with twice the mass of the sun might be only 15 miles across.

“So how can scientists see something that small thousands of light years away?”

When they first discovered them they thought they were alien radio stations. I told you earlier about radio telescopes. Well an astronomer Anthony Hewish from Cambridge University became interested in radio waves coming from stars. In 1967 one of his students, Jocelyn Bell, discovered a celestial radio source giving out regular radio pulses every 1.3373011 seconds. It was like radio-lighthouse in space.

“It wasn’t really a radio-lighthouse was it?”

No, but it was initially called LGM-1, short for Little Green Men because it seemed so fast and regular that an alien radio station seemed as likely as anything other explanation. Then they started finding more and more of them all over the galaxy, LGM-2, LGM-3, LGM-4. If you convert the radio signal into sound this is what a pulsar sounds like.

“So the universe does have a ticking clock then, like Newton said.”

No, it just has lots of spinning neutron stars. Like most things in space, the sun, the earth and the rest of the planets, neutron stars spin. And like ice skaters that spin faster as they pull their arms inwards, these solid lumps of matter spin faster and faster as they contract. You end up with a very small, very dense, star spinning around so fast that it can do a complete turn in a couple of seconds or even a fraction of a second. With each spin they give out a flash of radio-waves with every rotation which is why they are now called pulsars.

The Hubble telescope is so good that it has taken pictures that show the spinning of these stars.

“Are you serious?”

Oh yes take a look at this video. These are real pictures of the the supernova seen in the year 1054. The explosion left a cloud of gas and there is a pulsar in the middle of it. You can see the rotation and the ripples coming away from it. That’s the beauty of science, if you get it right everything starts to link together and make sense.

“I’m getting to see why you like science so much Albert.”

If you thought pulsars were interesting I have another surprise for you. Even stranger, and the only thing in the universe that we photons have to fear, is a black hole. They also can be created from exploding stars. If a collapsing star is a bit bigger than a neutron star, then the gravity will crush it to an even small size and create black hole. Like so many things in space, understanding black holes is all about gravity.

"Our friend Newton again?”

I don’t think Isaac would ever have imagined something like a black hole. In fact neither could I. A colleague of mine, Karl Schwarzschild, showed the my theories showed they could exist but I thought it was just a quirk of the equations. He even managed to do these calculations while he was serving in World War I calculating trajectories for artillery shells.

“Black holes and artillery shells?”

Well they both have to do with gravity but the gravity in a black hole is so strong even light can’t escape.

“Which is why they are black.”

Precisely. Here is how to make a black hole. Take the Earth and squeeze it into a ball less than an inch across and run as fast as you can. Most black holes are much bigger than that of course. There is probably a massive black hole in the centre of this galaxy, the milky way, which is about 10 million miles across. As long as the stuff inside is dense enough, gravity will do the rest. For all their mystery, the properties of black holes are still understandable in an apple falling way.

"Hmm, can't quite see how."

It's not as difficult to understand as you might think. You know that the gravitational force of the Earth stops you from throwing an apple into space.

"Of course, that's how Newton got the idea in the first place."

Exactly, if you throw an apple into the sky it falls back down again. Earth is therefore an apple-hole; apples cannot be thrown fast enough to escape the gravity of the Earth. Earth is also a bird-hole; birds cannot fly fast enough to fly into space. It is also an aeroplane-hole. To escape from the Earth you need a space rocket travelling at 25,500 miles an hour, any slower and the rocket will not be able to overcome gravity. Now imagine gravity getting stronger and stronger. The rocket will have to go faster and faster. The fastest thing in the universe is light and I’ll explain in a little bit why nothing can travel faster than us. So a black hole is a photon hole because photons cannot go fast enough to escape from the force of the gravity, in just the same way the Earth is an apple hole, bird hole and aeroplane hole.

“That makes sense.”

A long time before I invented relativity someone had already imagined that black holes could exist.

“Who?”

A Frenchman Pierre Simon de Laplace used this same argument in the year 1798, the year we've just reached, to predict that if a star had enough gravity it would stop light from escaping so become what he called a black star. It seemed such a crazy idea everyone ignored until I started coming up with just as crazy ideas.

"If you can't see black holes, how does anyone know they exist?"

Even though you can't see the holes themselves, you can see the effects of these strange things on neighbouring stars. As matter gets pulled in, it accelerates faster and faster and gives off x-rays in the process, the same sort of x-rays that are used in hospitals to take pictures of bones. These x-rays are a very high energy photon but are invisible and can go straight through your body and out the other side.

"So how do they escape?"

They escape because they are released just before the point of no return as matter is being pulled at huge speeds into the black hole. A special telescope that can detect x-rays from space can tell you where black holes might be. The constellation Cygnus, where we are coming from, contains the first detected object that might be a black hole.

"So what's it called? Vlad the absorber?"

You've been reading too many comics. No it ended up with a very boring name, X-1, because it was the first x-ray source detected in the constellation Cygnus.

“That’s the constellation we came from!

It certainly was, but luckily for us it was 5000 light years in opposite direction to the one we took towards Earth. So aren't you glad you ended up on this journey with me rather than being eaten up by a black hole.

"Oh, yes, but there is a lot space out there ahead of us and it all looks pretty black to me."