A Little Green Man

Astronomy has existed for as long as humans have existed; from Chinese Astronomers in 1054 observing a supernova brighter than the Sun (the now Crab Nebula exploding) to sailors charting the heaven’s constellations and using the stars to navigate (not forgetting the greats – Galileo, Copernicus, Newton). However, much of astronomy has developed thanks to the advancement of electromagnetic telescopes. Jodrell Bank – A radio telescope in Cheshire!

These telescopes have vastly shaped the universe we see for the last 50 years. Picture this: It’s 1967 and Jocelyn Bell Burnell is conducting research on high energy galaxies (or quasars). She picks up a regular repeating signal – a pulse – in the distant universe. With no real explanation, she names it LGM1, Little Green Man 1.

Funnily enough, she believed an alien civilisation were disrupting her PhD. (you can imagine how annoying it would be for an alien civilisation to interrupt you whilst you were doing homework… he he!) Unbeknown to her, Bell had just discovered a star, the size of a city, rotating up to 700 times a second. She had discovered…

…a pulsar!

It’s quite a nice theory to understand: the mechanisms behind a pulsar. After a star more massive than our Sun (between 10 and 30 times bigger) has fused everything it can*, it explodes in a supernova and leaves behind an extremely dense condensed ball of matter. Previously, protons, neutrons and electrons all existed within the star, happily in their own personal space (due to forces of repulsion and attraction). When forced into close proximity, the empty space that makes up 99.9999…% of an atom is squeezed out and electrons are forced into the nucleus. They then combine with protons to produce neutrons (sort of like beta decay, but in reverse!)

This is how a neutron star is formed. It’s just a sphere of pure compacted neutrons, but it’s 40 billion times denser than lead!

But what makes a neutron star a pulsar? First, the neutron star must have a companion. Before the supernova, pulsars exist as part of a binary star system (two stars ‘stuck’ together due to gravity holding them in orbit around each other). However, pulsars are very rare because it’s one thing having a companion, but another thing entirely keeping them (quite relatable to real life, wouldn’t you agree?). The supernova is so powerful an explosion that the force of gravity between the two stars must be strong enough so the companion doesn’t blow away.

The companion is essential because they provide fuel for the pulsar. Yes, you could say the pulsar eats its friend (Please can I note, DO NOT EAT YOUR FRIEND – it’s not a good move and they might not want to be your friend anymore 🙂 In a physicsy sense, the neutron star has such an intense magnetic field that it rips electrons from the companion’s surface.

*Here’s the tricky bit!*

The electrons travel away from the companion’s poles and towards the pulsar’s poles. As it moves away, the electron is accelerated to very high speeds; so fast, that the electrons emit an electromagnetic wave (often X-Rays). All that is left to add is that as the pulsar rotates, the poles where E.M waves are emitted move relative to us (Earth). Therefore,

the region of E.M wave emission pulses because it travels across our line of vision.

Let me just repeat this again (it’s crucial). We see a pulse as the area of E.M waves crosses our line of vision here on Earth.

And that’s it! When you understand the science of pulsars, you can see why Bell thought she’d stumbled across a beacon from alien civilisations. A pulsar acts just like a lighthouse!

*Coming Soon – Why Does the Sun Shine?