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In 1967, Jocelyn Bell, a research student at Cambridge University, was studying distant radio sources with a special detector that had been designed and built by her advisor Antony Hewish to find rapid variations in radio signals. The project computers spewed out reams of paper, showing where the telescope had surveyed the sky, and it was the job of Hewish's graduate students to go through it all, searching for intersting phenomena. In September 1967 Bell discovered what she called "a bit of scruff" -- a strange radio signal unlike anything seen before.
What Bell had found, in the constellation of Vulpecula, was a source of rapid, sharp, intense, and extremely regular pulses of radio radiation. Like the regular ticking of a clock, the pulses arrive precisely every 1.33728 seconds. Such exactness led the scientists to speculate that perhaps they had found signals from an intelligent civilization. Radio astronomers even half-jokingly dubbed the source "LGM" for "little green men." Soon, however, three similar sources were discovered in widely separated directions in the sky.
When it became apparent that this type of source was fairly common, astronomers concluded that they were highly unlikely to bo signlas from "ET". By today hundreds of such sources have been discovered; they are now called pulsars.
The pulse periods of idfferent pulsars range from a little longer than 1/1000 s to nearly 10 s. At first, the pulsars seemed particularly mysterious because nothing could be seen at their location on visible-light photographs. But then a pulsar was discovered right in the center of one of the best-know supernova remnants--the Crab Nebula. In addition to pulses of radio energy, we can observe pulses of visible light and x rays from the Crab as well.
From observations of the several hundred pulsars discovered so far, astronomers have concluded that one new pulsar is born somewhere in the Galaxy every 25 to 100 years, the same rate at which supernovae are estimated to occur. Calculations suggest that the typical lifetime of a pulsar is about 10 million years; after that the neutron star no longer rotates fast enough to produce significant beams of particles and energy, and it is then no longer observable. We estimate that there are about 100 million neutron stars in our Galaxy.
According to present ideas, the Crab pulsar is rather young (only 900 years old) and has a short period, while the other, older pulsars have already slowed to longer periods. Pulsars thousands of years old have lost too much energy to emit appreciably in the visible and x-ray wavelengths, and are observed only as radio pulsars; their periods are a second or more.
There is one other reason that we can see only a fraction of the pulsars in the Galaxy. Consider our lighthouse model again. On Earth, all ships approach on the same plane--the surface of the ocean--so the lighthouse can be built to sweep its beam over that surface. But in space, ojbects can be anywhere in three dimensions. As a given pulsar's beam sweeps over a circle in space, there is absolutely no guarantee that this circle will include the direction of Earth. In fact, if you think about it, many more circles in space will not include the Earth. Thus we estimate that we are unable to observe a large number of neutron stars whose beams miss un entirely.
At the same time, it turns out that only 3 of more than 400 pulsars discovered so far are embedded in the visible clouds of gas that mark the remnant of a supernova. This might at first seem mysterious, since we know that supernovae give rise to neutron stars and that we should expect each pulsar to have begun its life in a supernove explosion. But the lifetime of a pulsar turns out to be about 100 times longer than the length of time required for the expanding gas of a supernova remnant to disperse into interstellar space. Thus most pulsars are found with no other trace left of the explosion that produced them. |
STARS