The Discovery of Pulsars

Pulsars were discovered because of remarkable persistence on the part of Jocelyn Bell, at the time a graduate student at Cambridge University.

An economy-size radio telescope for studying radio source scintillation had been constructed by eager student labor and it consisted of over a thousand wooden posts each about 10 ft high with miles of wire strung between them. This "telescope" was built before computers were pervasive. Pen recorders were used to display the data by drawing a line on a paper chart that automatically unrolled as the machine created its recording. Analysis of the radio observations required inspection of these charts and the measurement of deflections from the so-called baseline, the normal path of the line drawn on the paper in the absence of a radio source in the beam of the antenna. In modern radio observatories such data are directly fed into computers, where the information is lost from sight until the final numbers are printed out. However, the scintillation experiment produced data on 400 ft of chart paper every day, all of which had to be examined by eye.

The antenna was located near Cambridge and plenty of locally produced radio interference (such as automobile ignition) contributed to the deflections of the pen. These deflections appeared similar to those expected from scintillating radio sources, and Bell, studying her ratio of400 ft of paper a day, soon became an expert in recognizing which was which. In the course of her work she experienced what other graduate students assigned the tiresome task of studying endless amounts of data sometimes discover—the alert brain is capable of the most extraordinary feats of memory regarding apparent trivia. She noticed that the recordings showed a faint signal that could not be explained by interference or scintillation or any other natural causes then known to astronomers.

Over the months Bell looked at miles of paper charts and found that this "little bit of scruff," as she affectionately named the signal, persisted and that it occurred at night when the sun was below the horizon when no scintillating sources were to be expected. Furthermore, the "scruff" appeared 4 min earlier each day.

The stars move across the skies at a different rate from that of the sun. Another way of stating this is that the length of the solar day, 24 h, is different from the length of the day measured with respect to the stars, which is 23 h and 56 min. All the stars thus appeal- in slightly different directions, as seen with respect to the horizon, from night to night, an effect that is noticeable to even the casual observer over periods of a month or so. A given star appears directly south, say, 4 min earlier every day.

The "bits of scruff" were unlikely to be human-made interference, which would tend to occur either randomly or at the same time each night Since the time of arrival of the "scruff" of which she recognized four distinct sources, shifted by about 4 min per day, the signals had to be coming from something associated with the starry heavens. When the research group at Cambridge finally confronted the reality of the discover}', they studied the new radio sources more carefully and were astonished to find that the signals were pulsating with impressive regularity, so regular that it required the best available clocks to measure the arrival time of these "pulse trains." One of the original sources, named CP 1133 (for Cambridge Pulsar at right ascension 11 h 33 min), was found to emit a radio pulse once ever>' 1.33730110168 s. The radio signals were so regular that at first the radio astronomers considered the possibility that they had detected messages from extraterrestrial intelligence (ET), or LGMs, "little green men," as they jokingly called to the first four mystery sources.

The pulsating radio sources turned out to be anything but ET. They are related to an extraordinary object called a neutron star that spins incredibly rapidly and emits radio signals in a beam that sweeps the heavens just as a lighthouse sends its light over the ocean. Every time a neutron star beam swept past the earth a pulse of radio waves was detected.

Pulsars run with clock-like regularity and there is only way to do that in an astronomical context. Some object must be spinning rapidly. Also, each pulsar has its characteristic pulse shape, which refers to the way the radio intensity varies during the pulse. These pulse shapes are the "signature" for each pulsar. From the duration of the pulse, compared to the time between pulses, it is found that the typical pulsar beam is between 10 and 20° wide. All pulsars are also slowing down, some almost imperceptibly yet measurably. This is a natural consequence of aging through the loss of energy by radiation. Some of the really old pulsars even skip beats for minutes at a time.

Because of the very short periods and their relative faintness, pulsars are very difficult to detect. The first few were found because they were fairly strong radio sources and their periods of a few seconds allowed them to be discovered by the radio telescope system designed to observe radio source scintillation.

Two enduring memories from the beginning of the pulsar saga have stayed with me ever since. The first relates to a story told me by a colleague from the NRAO who visited us at Jodrell Bank, and then went on to the Cambridge University radio astronomy group, There, while being given a tour of a laboratory by a group of their scientists, he noticed a couple of boxes of computer cards (used in the "old" days to input data to a computer) on a shelf that were labeled LGM 1 and LGM 2. He then returned his attention to his host who was describing some aspect of their research, and when the visitor turned back to take a quick look at the mystery boxes they had been reversed. He was polite enough not to question the work they were doing related to little green men! It turned out they had found the pulsating signals and were not about to talk about it until they had a better idea about what they had found,

My other memory highlighted for me a conviction that theoreticians in any branch of science can probably dream up explanations for any mystery if you only give them an hour or so to think. It doesn't mean they are right, though. In this case in Charlottesville, VA, right after the pulsar discover)'1 had been announced and before anyone had yet figured out what the mystery source of radio pulses might be, a theoretician who shall remain nameless gave a talk about the pulsating radio sources to an attentive audience and listed something like a dozen alternative theories that might account for the weird objects. None of them turned out to be correct! Had one of them been on the mark he could have claimed credit for solving the mystery. Such is life.

Was this article helpful?

0 0

Post a comment