Attack of the Little Green Men: the discovery of pulsars

It’s 1967. A radio signal is picked up by astrophysicist Jocelyn Bell-Burnell and radio astronomer Antony Hewish at a UK observatory. Originating from the Vulpecula constellation- a faint pattern of stars known as ‘the fox’- the signal is similar to other examples of celestial noise in every way except one; its near perfect timing. It is almost as if it’s a distress beacon, a SOS from the depths of space.

Radio signals , are a common occurrence in space, with nearly every object either emitting or reflecting a signal of some sort, even the Sun. Around the time the mysterious message was picked up, Hewish was in the process of constructing a radio telescope with the purpose of picking up radio emissions from quasars- the luminous centres of galaxies. Graduate student Bell-Burnell was given the task of analysing the charts of information produced by the telescope, when she came across the Vulpecula signal.

It was a clear series of steady pulses, each with equal strength, length and spacing around the second. Such a phenomenon had never been documented before and its implications were not lost on Hewish and Bell-Burnell, who jokingly named the signal Little Green Men 1 (LGM-1).

As it turned out, the undoing of LGM-1′s alien explanation lay in the very uniformity that had prompted it. One important factor in ruling out the theory was a phenomenon known as the Doppler shift- the reason the pitch of a police siren appears to change as it whizzes past you. As the siren moves away, the sound waves produced are more spread out, affecting the sound of the siren. This principle can also be applied to light waves.

If the origin of the signal was an inhabited planet in the Vulpecula constellation, it would be orbiting a sun and the frequency of the pulses would vary, due to the Doppler effect. As this was not the case, the origin of the pulses must have been from a relatively stationary object, such as a star. This lead to the birth of the term ‘pulsar’ – short for ‘pulsating star’.

However, the regularity of the pulses continued to baffle scientists. Two possible explanations were offered. The first was that two orbiting stars brushing past one another could produce the signal, causing a spark of energy that was picked up by the telescope. The second was that as the two stars passed each other, their combined gravity focused the radio emissions of the star into a beam-in the same way the lens of a lighthouse focuses its light source.

It was the discovery of a pulsar in the Crab Nebula that led scientists to finally agree on a third theory. It was because the Crab pulsar was slowing down, a phenomenon which happens to spinning objects, not orbiting ones. Pulsars, it was decided, were spinning stars, emitting a beam of radiation that was detectable when pointing at the Earth, again like a lighthouse.

So what began as an alien message ended in the discovery of pulsars. Ironically, these navigational lighthouses are now being put on spacecraft in case an intelligent civilisation comes across them. In 1972, the Pioneer 10 spacecraft was launched carrying a plaque that listed, in binary the relation of our solar system to 14 different pulsars.

Whether the little green men are intelligent enough or sufficiently interested to get in touch is a question only the future can answer.

Other Elements articles in which you might be interested:

1. Blast from the past: the discovery of cosmic background microwave radiation
2. Eye in the sky
3. Spying on the neighbours