In the 1960’s, Cambridge University built a radio telescope. After a few weeks operation, a young research student noticed something very weird. A source of radio waves that sent out a steady pulse, pulsing every one and a third seconds.
They had no idea what it was. It looked like a human made signal, but was definitely from very far away. It was dubbed LGM1 – the LGM standing for “little green men”.
There was serious thought that this might be initial evidence of extraterrestrial civilisation. Initially, nothing could be brought to mind which could naturally produce this type of frequent pulsing radio signal.
But then something even more amazing happened. They started to find even more of these signals, from all over the sky. Now, it’s not really possible that “little green men” are signalling us from loads of different locations all across the universe, in exactly the same way.
In the end, what they discovered was that the signals were from something even more bizarre and amazing than an extraterrestrial civilisation – They were coming from pulsars. Any science fiction writer in the last 150 years could come up with little green men – Pulsars just wouldn’t have been dreamt of.
So what’s a Pulsar? OK – the easiest way to explain it is by setting out the recipe for creating one.
1. Take a big star. It needs to be between about one and a half to five times the size of our own sun. Allow it to cook in its own nuclear reactions for billions of years.
2. Allow the star to expand as a giant, as the nuclear reactions change and expand outwards. In doing this, you will make many new elements in the core of the star.
3. let the star go Supernova – blowing away the outer regions of the star and spreading elements back into the interstellar regions. (We can use these later to make new stars and solar-systems, including all the great new elements made in 2 above.)
4. Keep the core. Make sure that the core left is no bigger in mass than about three times our own sun. (Be careful! if you keep too much mass in the core, you will spoil your neutron star and get a black hole instead. Whoops!)
5. Let the core collapse in on itself under its own gravity. There isn’t any nuclear reaction happening to support its own weight, so it will keep collapsing until something stops it. Something called neutron degeneracy pressure should eventually stop it.(degenreacy pressure is based on quantum theory, which is even weirder than Neutron stars.) It will now be REALLY small. A Neutron star with twice the mass of our sun would be squeezed into a ball about 10km from centre to edge. That means that the stuff making up the star is REALLY dense – a thimble full would weigh about 700,000,000 tonnes (seven hundred million tonnes)!! – heavy, man!
6. Remember to conserve angular momentum. This is a term for the way that, when you bring the weight of a spinning object towards the centre around which it is spinning, then the spinning speeds up. Think of a figure skater who starts spinning with their arms out wide but as they bring their arms in they spin faster and faster. (You can try this yourself spinning on an office chair with your arms and legs out . Bring them in and you will spin faster – fun!). You will find that speeding the original spin of the star as it collapses into this very small neutron star, causes some seriously fast spinning. A neutron star might rotate hundreds of times every second!
So there you have it – a neutron star. Super-tiny, super-old, super-dense and spinning super-fast. Weird!. To get from that to a pulsar, you just need one more bit. The Neutron star is likely to continuously emit radio waves from its magnetic poles. As the star rotates really fast, so the beam sweeps across space really fast. If we happen to be in a position to meet the sweeping beam, we will see it as a blinking pulse (to get the idea, think of a light-house beam sweeping outwards and how we would see it if we were on a small ship many miles away.)
Little Green Men would have been great. Pulsars, on the other hand, are just AWESOME!.