Pulsars - Beeping Beacons in Space
By Chuck McPartlin
Monday, November 28, was the 50th anniversary of the discovery of pulsars. In 1967, a Ph.D. student at Cambridge University in England, Jocelyn Bell, was working under Dr. Antony Hewish on the subject of radio astronomy. Cambridge had built a dipole array receiver on flat ground covering an area of about 4 acres in July of 1967. The output from the receiver was in the form of long paper sheets with pen chart traces, like an old seismograph. Bell's job was to run the receiver and manually examine these traces, which were just under 100 feet long for each day's observations.
She saw what she called a small "scruff" in the pen traces. Dr. Hewish argued that it was just interference from a weak terrestrial source, but Bell found that the signal repeated at the siderial rate - the rate at which the stars seem to move across the sky, based on both the Earth's daily rotation and its motion around the Sun in its orbit. This was different from the man-made sources of radio noise that appeared on the charts.
By November 28, 1967, Bell and Hewish and Dr. Martin Ryle of the Cambridge radio astronomy group had managed to get a more detailed recording of the signal, which consisted of a sequence of radio pulses arriving every 1.3 seconds. Eventually, they found about a dozen of these precisely timed cosmic signals, with different pulse rates and directions in the sky. These were so different from previously known natural sources of radio noise that Bell and Hewish called them LGM signals, for "Little Green Men."
The signals they were seeing were all found to be associated with the positions of supernova explosion remnants in the sky. Theoretical astrophysicists studying supernovae had postulated that core-collapse supernovae could produce immensely dense compact remnants called neutron stars. These were essentially city-sized atomic nuclei made almost entirely of neutrons, and they could be rotating very rapidly. If they had a strong magnetic field, they could output beams of electromagnetic radiation along their magnetic poles, which didn't necessarily correspond to their rotational axes, and act as cosmic lighthouses, flashing their signals across the galaxy. Jocelyn Bell had discovered pulsars.
One of the most famous pulsars is at the center of the Crab Nebula, a supernova remnant that is visible in amateur telescopes. This is the result of a supernova that was seen on July 4, 1054 by Chinese astronomers, which was visible even in the daytime for about three weeks. Here's an image of the nebula, also known as Messier 1, from Hubble Space Telescope and Chandra X-Ray Telescope data.
It's called the Crab Nebula because in 1848 Lord Rosse in Ireland, with his big 72-inch reflecting telescope, was able to detect some of those filamentary tendrils, and thought they looked like crab legs.
And here is an image from a much smaller telescope in the wilds of Noleta on Wednesday evening, with a bright Moon washing out the sky. It is a composite of four 30-second DSLR exposures at ISO 6400 through a 130mm aperture refractor at f/6.3.
The website for the Chandra X-ray Telescope in the references below has some great time lapse videos of the Crab pulsar in action. The Crab pulsar is about 6,500 light years away, and emits 30.2 pulses per second. It is the corpse of a star that was initially about ten times the mass of our Sun. Its beams span the electromagnetic spectrum, with output in radio, infrared, optical, X-ray, and gamma-ray wavelengths.
Pulsars can be used as very precise clocks, and because each one has a slightly different rate, they have been proposed as accurate navigation beacons for spacecraft. Over 2,600 pulsars are currently known, and over half of them have been discovered by the star of the movie "The Dish," the Parkes radiotelescope in Australia.
Dr. Hewish and Dr. Ryle were awarded the Nobel Prize for Physics in 1974 for the discovery of pulsars. It was the first Nobel prize awarded for astronomical research. Jocelyn Bell's contribution was not recognized. Today she is Professor Dame Jocelyn Bell Burnell, DBE, FRS, FRSE, FRAS. That acronym salad after her name represents some of the many awards she has earned, and just about everybody thinks that the Nobel Prize should have included her, too.
References for a Cloudy Evening