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The Discovery of Pulsars

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What would you do if you found a radio source from outer space? Well, no astrophysicist or astronomer would be surprised. Nearly every natural outer space object emits or reflects some sort of radio signal, including the Sun. What if the radio source was pulsing on and off? Still, it would not arouse much interest, to be quite frank. But - what if the pulses were constant? Now, that might turn some heads. It might seem more akin to something from science fiction, but constant pulses have been found.

But what could these radio pulses be? Alien life attempting to communicate with us? Or some sort of navigational beacon for alien space ships? Or what, exactly? This entry is about the discovery of a constant radio pulse from outer space, and all that transpired.

Little Green Men

In 1967, at Cambridge University, Professor Antony Hewish and his team of students were completing the creation of a radio telescope specifically designed to look for the scintillation, or twinkling, of radio signals coming from quasars1. He assigned graduate student Jocelyn Bell to analyse the printed readout charts that came from the telescope's computer. Her task was to wade through miles of printouts, and analyse the results.

One day, Bell noticed something odd. On a printout, there was a type of pulse she had never seen before. She first thought these were perfectly ordinary, and probably from an Earth-bound source - a clock ticking, perhaps. These unexciting sources had been observed before, whereas this pattern seemed new. She dubbed it 'Scruff', to signify it as meaningless interference.

Bell later mentioned her findings to Hewish, and they decided to set up a high-speed recorder. However, they failed to pick up the signal again for several months. When Bell returned from a Christmas holiday - during which time Hewish had decided to keep the recorders running - she finally found what she had been looking for. A clear series of rapid pulses, each about 1/3 of a second long, all equal in strength, and with equal spacing of around a second. This behaviour had never been seen in nature before.

Bell and Hewish speculated that the source could be extraterrestrial intelligence. The 'scruff business' was detracting from her assigned duties, and she began to gripe about 'little green men getting in the way'. Bell decided to name the first radio source LGM-1, for 'Little Green Men 1'. She decided to keep LGM-1 secret until she had more evidence to work with, as she was aware of the interest and speculation that would be generated by the find. Inevitably, however, the news did leak out.

Hewish's team decided to publish an article in Nature Magazine. Quickly, Jocelyn Bell became something of a celebrity, and many articles about the discovery were being printed, along with masses of conjecture. Astronomers and astrophysicists around the world were called upon to translate the findings in 'ordinary' language. Astronomy circles were also thrown into uproar - well, for astronomy circles, anyway. New LGMs were being discovered everywhere, although inevitably some were hoaxes. Was it extraterrestrial intelligence after all?

Hewish's team took several things into account. One very important fact was the Doppler Effect. The Doppler Effect is when, for example, a train blows its whistle as it is roaring past, and the pitch of the whistle changes to the stationary listeners. This effect is more precisely called Doppler-shift. If this signal really was coming from an inhabited planet, it would be orbiting a sun. This would cause Doppler-shifting of the radio signal, and the pulses would change frequency. This was not the case: the pulses came at a constant frequency, ruling out an inhabited planet. Therefore, it was clear that the LGMs were not little green men, after all, but something entirely different.

As it was known that nearly all natural celestial objects emit radio waves, the LGMs might be some sort of natural celestial object. It became apparent a new name must be given to this whole new class of objects, so they were called 'pulsars'. This was shorthand for 'Pulsating Star'. At the time, many astronomers around the world were pondering how these pulsars managed to keep regular time. Eventually, three main hypotheses were put forward, with each under constant fire from supporters of the others. They were later dubbed 'The Three Clock Hypotheses' because they all explained, in different ways, the accuracy of the timekeeping method.

The Three Clock Hypotheses

The first of these clock hypotheses was the 'Binary Spark' hypothesis. The idea was that in a binary system - which consists of two stars orbiting each other - they would brush by each other at high speeds, and 'spark', or transfer energy between themselves. This would produce the requisite radio emission. Also, the mass of the stars would conceal the 'sparks' for some time, and they would only be visible during two phases of the entire orbital period. Even so, the stars would have to be moving incredibly fast in order to produce such quick pulses. For the stars to move at such a speed, it was speculated that it would have to be a binary system of white dwarfs, or possibly neutron stars. Neutron stars were purely conjecture at the time, and their use called into question the validity of this hypothesis, which had already been on shaky ground - no orbital periods had ever been observed at such high speeds.

The second proposed clock mechanism was the 'Binary Lens' hypothesis. This also involved a binary system. This hypothesis, however, varied as to the cause of the radio emission. The two stars, instead of brushing past each other, acted as lenses, their gravity focusing the natural radio emission of the other into a beam. Thus, the stars would have rotated together and acted like a lighthouse. When a lighthouse is viewed from a certain angle, it appears to pulse. This is because the beam of light sweeps past the viewer, illuminating the area for a short while. This clock mechanism, like the 'Binary Spark', would also have required a very fast orbital period (four seconds at maximum), and would also have required a very small star, such as a white dwarf as mentioned earlier.

The third proposed clock mechanism differs somewhat from the others. It was known as the 'Rotation' hypothesis, and would have produced much the same 'lighthouse' effect as in the 'Lens' hypothesis. It consisted of a star that was spinning with some sort of spot that emitted radio waves. This clock system would have required the star to rotate quickly, instead of orbiting. This was much more plausible. For this to occur, however, the star would need to be very small, because a single spot would take a longer time to rotate around a large object than a spot of the same size on a smaller object. Therefore, the object must have been rotating incredibly fast, but also be very small to allow the spot to rotate so quickly.

These three clock hypotheses were the subject to intense debate in scientific circles. However, two major discoveries proved one theory alone to be valid.

Process of Elimination

At the height of the clock hypothesis debate, in mid-1968, two more pulsars were found. One in the constellation Vela, and one in the Crab Nebula. These were aptly named the Vela pulsar and the Crab pulsar. The Crab pulsar was spinning extremely fast, at 30 rotations per second. This strained the first and second hypotheses, as orbital periods had never been observed at that speed. It simply was not possible for this to occur with a pair of white dwarfs. The Crab pulsar was also embedded in nova remnants. This was not the case with LGMs 1-4. A startling fact suddenly became apparent: the Crab pulsar was not spinning as fast as it was when it was first recorded.

This was the final demise of the first and second hypotheses, because orbital periods do not slow down over time - they speed up. However, rotation does slow down over time. Therefore, by process of elimination, as well as support from data, it was decided that pulsars were spinning stars.

The discovery of the Crab pulsar also led to greater understanding of the origins of these objects. At the time, it was the fastest pulsar known, and was the only one within nova remnants. This implied that pulsars could have been 'born' in the midst of a nova, and that pulsars were actually stars that had imploded.


But what of their discoverer, in all this commotion? Where did Jocelyn Bell go, and what did she think of these happenings, and did she get any recognition for her discovery? Bell had married, and was now Jocelyn Bell-Burnell. But after all this excitement had subsided, Antony Hewish won a Nobel Prize for 'his' discovery. This upset many people, as Bell-Burnell had been a major factor2 in the observations, and the discoverer, if not the ultimate identifier. Bell-Burnell adopted something of a rebellious stance. She wrote up her thesis paper (which did not mention pulsars at all), and left Cambridge. She later became a professor at the Open University and, in 2007, was made a Dame.

So what can be learned from pulsars? What uses do they have to us?

Pulsars can be used as navigational lighthouses. The Pioneer 10 spacecraft, launched in 1972, carried a plaque showing the distances, in binary, from our solar system in relation to 14 different pulsars, and the time between pulses3. As pulsars slow down over time, these pulsing distances would appear slower than represented on the plaque. If smart enough - and sufficiently interested - beings on other planets would be able to figure out where the craft came from, and what the creators looked like. Also, the well known project, SETI@home (Search for Extra Terrestrial Intelligence), is now searching for pulsars, to add to the database of over 1,500 known examples.

The discovery of pulsars has shown what the first contact with extraterrestrial intelligence may be like. It has given humanity a glimpse into the death of stars. It will, perhaps, someday show extraterrestrial intelligence how to find the Earth. Before that happens, humankind may even find extraterrestrial intelligence - we just have to keep looking.


  • Greenstien, George. Frozen Star, New York, New York: Freundlich Books, 1983. 13-31.

  • Kraus, John D. Radio Astronomy, 1966. 2nd ed. Powell, Ohio: Cygnus-Quasar Books, 1986.

  • McDonough, Thomas R. The Search For Extraterrestrial Intelligence, Stephen Kippur, 1987.

  • Reference Book Article: Columbia University Press. 'Pulsar'.

  • Magazine Article: Zimmerman, Robert. 'A Square Dance in Space'.

  • Book: Thorne, Kip S. Black Holes & Time Warps: Einstein's Outrageous Legacy. Walbaum: Haddon Craftsmen, Inc., 1994.

1Quasars are believed to be young galaxies. Quasar stands for Quasi-stellar object.2Although it should be noted that she voluntarily handed over chart analysis of the pulsars to Hewish's team before the prize was won.3This same 'Map of the Galaxy' was also used on the Voyager spacecraft.

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