How Pulsars Were First Detected
There are quite literally countless numbers of stars in the sky. Some are quite obvious, such as our own sun, others we can see with the naked eye and are nearby such as the constellations of the Milky Way Galaxy while others are much further away and much harder to detect. It has taken many centuries since the Mayan astronomers, Galileo and others who have put forth their discoveries in the heavens before we reach the point in time on how pulsars were first detected.
Pulsars are a certain type of star that sometimes can be seen at night, most often with the help of a telescope. Pulsars are actually neutron stars that quickly spin on their axes all the while emitting light that seems to pulsate on and off as if they were lighthouses. A few hundred pulsars have been discovered since the first and their pulses range from seconds to sub-milliseconds.
The Mullard Radio Astronomy Observatory at the University of Cambridge in July 1967 placed online a large radio telescope capable of a frequency of 81.5MHz with the expectations of “investigating the angular structure of compact radio sources by observing the scintillation caused by the irregular structure of the interplanetary medium.” To start the observations were made with weekly scans of the entire sky at a declination range of -08° <d <44°. Signals were detected that were weak and sporadic as if they were an interference of sorts. After a time it was realized that it was not interference, but actually coming from the heavens. It was at that point in November 1967 that observations began to be recorded. It was originally thought that although the signals came from space they certainly must have been manmade from perhaps probes that had been sent out in orbit or maybe signals from the moon the Earth or just planetary radar. These ideas were turned down “because the absence of any parallax shows that the source lies far outside the solar system”.
Continuing observations were made and recorded. Astronomers Jocelyn Bell and her advisor Doctor Anthony Hewish, as well as their team, were those who discovered pulsars. Following is an account in Bell’s own words of the discovery. “At the end of November '67 I got it on the fast recording. As the chart flowed under the pen I could see that the signal was a series of pulses, and my suspicion that they were equally spaced was confirmed as soon as I got the chart off the recorder. They were 11/3 seconds apart…. Then Scott and Collins observed the pulsations with another telescope with its own receivers, which eliminated instrumental effects. John Pilkington measured the dispersion of the signal which established that the source was well outside the solar system but inside the galaxy…. [Near Christmas time and] shortly before the lab closed for the night I was analyzing a recording of a completely different part of the sky, and in amongst a strong, heavily modulated signal from Cassiopea A at lower culmination (at 1133) 1 thought I saw some scruff. I rapidly checked through previous recordings of that part of the sky, and on occasions there was scruff there [that] showed itself to be a series of pulses, this time 1.2 seconds apart.” This was the discovery of the first pulsar.
Within Taurus you will find the Crab Nebula, also known as M1, which is the home of the Crab Pulsar, the a new type of pulsar that actually seems to be blinking in gamma rays. “The gamma-ray-only pulsar lies within a supernova remnant known as CTA 1 located about 4,600 light-years away in the constellation Cepheus. Its lighthouse-like beam sweeps Earth's way every 316.86 milliseconds. The pulsar, which formed in a supernova explosion about 10,000 years ago, emits 1,000 times the energy of our sun.”
Science hasn’t stopped it patrolling of the universe and all it has to offer. More and more facts are being discovered and investigated every day thanks to people like Drs. Bell and Hewish.