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The discovery of pulsars, half a century ago
The Crab Nebula, Messier 1, is seen here in a photo by Paul Ricketts of the University of Utah Physics and Astronomy Department and members of the Salt Lake Astronomical Society. It was taken in late 2017 by remote control from the university's South Physics Building in Salt Lake City, using the 32-inch telescope in the Willard L. Eccles Observatory near Milford, Beaver County, Utah. The image was assembled from numerous sub-exposures by Don J. Colton. - photo by Paul Ricketts and the Salt Lake Astronomical Society; processing by Don J. Colton

I remember the discovery of pulsars; or maybe I don’t recall the actual announcement, but discussions about them soon afterward. With the passage of half a century, it’s hard to sort out. Word came in February 1968 that scientists had detected radio beacons in the cosmos, of the strangest type ever recorded, signals that repeated rapidly and at precise intervals. Nobody could resist wondering if the signals were the product of a spacefaring civilization.

The findings came through the operation of a new type of radio telescope designed by Antony Hewish of Cambridge University and operated by graduate student Jocelyn Bell, who first noticed the odd pulses; Hewish was her faculty adviser. The project was intended to open a new way to find quasars, extremely powerful sources of radio waves from far beyond the Milky Way.

But these signals originated within our galaxy. The first source to be discovered sent "sharp pulses" exactly every 1.33 seconds. The astronomers called the source LGM-1 for Little Green Men.


To quote NASA's description of this important discovery:

"Bell's first two years at Cambridge were spent assisting in the construction of an 81.5-megahertz radio telescope that was to be used to track quasars. The telescope went into operation in 1967. It was Jocelyn Bell's job to operate the telescope and to analyze over 120 meters (a little over 393 feet) of chart paper produced by the telescope every four days. After several weeks of analysis, Bell noticed some unusual markings on the chart paper. These markings were made by a radio source too fast and regular to be a quasar. Although the source's signal took up only about 2.5 centimeters (just under one inch) of the 121.8 meters (almost 397 feet) of chart paper, Jocelyn Bell recognized its importance. She had detected the first evidence of a pulsar."

Bell's next source came in at 1.2 second intervals. As the American Physical Society wrote, the likelihood was low that two extraterrestrial societies would be communicating in the same way from far-separated sections of the galaxy. By the time Hewish was awarded the first Nobel Prize for Astronomy in 1974, more than 130 pulsars had been discovered.

Although Bell and student assistants built the telescope over two years; although she ran the instrument and analyzed the data; although she discovered pulsars, Hewish received the Nobel Prize for the discovery and she didn't. To be fair, in 1965 Hewish and a Nigerian astrophysicist named Samuel Ejikeme Okoye, his student at the time, detected radio emissions in the Crab Nebula that they attributed to a remnant star's flaring. Later this source also turned out to be a pulsar. Not to take anything away from Hewish's fine accomplishments — and he did acknowledge Bell in his Nobel lecture — the Nobel Prize Committee's neglecting her is another example in the shameful history of the scientific establishment slighting women.


As radio astronomers turned up more pulsars, scientists quickly realized they were too widespread to represent intelligent signals. Probably no civilization could construct transmitters so many light-years apart in space. Today more than 2,000 have been charted, and throughout the Milky Way Galaxy others must exist whose beams don't happen to fan out in our direction.

But what could generate these swiftly repeating signals? Not flares from an ordinary star, as such a massive object couldn't rotate fast enough that the source would reappear every 1.2 seconds. Centrifugal force would rip it to shreds before it reached a speed that was a minute fraction of that.

An obvious answer presented itself. Decades earlier, theorists had predicted the existence of objects that could spin quickly enough, the strange objects dubbed neutron stars.

In 1934, a year after the discovery of the neutron — a subatomic particle with no electrical charge — two European astrophysicists posited the existence of neutron stars. They were Fritz Zwicky, born in Bulgaria and educated in Switzerland, and the German Walter Baade. Both worked at the California Institute of Technology, Pasadena, when they made the proposal.

Zwicky and Baade calculated that a "super-nova" (a term they invented for an exploding star) could collapse to the point that its electrons and protons were squeezed together into an unbelievably dense object made up mainly of neutrons. Princeton astronomer professor Adam S. Burrows, in a February 2015 article published by the Proceedings of the National Academy of Sciences of the United States of America, writes that before the Zwicky-Baade papers, "the concept of a dense 'neutron star' the size of a city but with the mass of a star like the sun, did not exist."

"When the core of a massive star undergoes gravitational collapse at the end of its life, protons and electrons are literally scrunched together, leaving behind one of nature's most wondrous creations: a neutron star," wrote Robert Naeye on a NASA site. "Neutron stars cram roughly 1.3 to 2.5 solar masses into a city-sized sphere perhaps 20 kilometers (12 miles) across. Matter is packed so tightly that a sugar-cube-sized amount of material would weigh more than 1 billion tons, about the same as Mount Everest!"

Neutron stars are created by the supernova explosion of massive — but not too massive — stars. The remaining core implodes but an effect called "neutron degeneracy," a state where the neutrons are packed as tightly as possible, prevents further collapse. (When even more massive stars explode, the collapse is unstoppable and a black hole results. If the original star isn't quite massive enough to form a neutron star, the supernova creates a white dwarf star. Smaller stars apparently don't go supernova.)

Given enough energy, something the size of a city, like 12 miles across, could rotate every 1.2 seconds. The Smithsonian Astrophysical Observatory explains that neutron stars "spin rapidly, and when they have associated magnetic fields, charged particles caught in them emit electromagnetic radiation in a lighthouse-like beam that can sweep past the Earth with great regularity every few seconds or less."

In 1054, astronomers in China and Japan and a physician living either in Constantinople or Cairo reported that a brilliant star had appeared in the sky. It was bright enough to be visible "like Venus" during the daytime for 23 days. The first observation was on July 4, 1054, and the "guest star" could be seen in the evenings until April 6, 1056, wrote F. Richard Stephenson of the University of Durham, England, and David A. Green of the Mullard Radio Astronomy Observatory, Cambridge, England.

The light was from a supernova explosion, and records of its celestial coordinates prove it was the eruption that created the Crab Nebula, also known as Messier 1. "If the blast had occurred 50 light-years from Earth, astronomers believe that all living things could have been destroyed by radiation," says "A History of the Crab Nebula," posted by Kopernik Observatory and Science Center, Vestal, N.Y. Lucky for us, the supernova was 6,500 light-years away.

Astronomers found that the heart of the Crab Nebula is a spinning neutron star, beaming radio and X-ray pulses 30 times a second.

Joe Bauman writes an astronomy blog at and is an avid amateur astronomer. His email is
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