... They may be fuel-depleted stars that are pulsating or rotating
By Antony Hewish
It is ironical that astronomys latest discovery, the pulsars, should have been stumbled on unexpectedly during an investigation of quasars, those starlike radio sources whose origin is still one of the outstanding problems of astrophysics. ... Whereas the true nature of the pulsars is still far from clear, it may be an opportune moment to take stock of the situation.
One obvious question to ask is why pulsars remained undiscovered for so long although powerful radio telescopes have been in operation for at least 10 years. ... To find the pulsars it is necessary to use a sensitive radio telescope operating at meter wavelengths with the additional requirement that observations of the same areas of sky must be repeated with a recording system that has a sufficiently rapid response to show up the radio flashes. ... The radio telescope was trained on these places, and after some weeks three more pulsars had been added to the list. By this time we felt reasonably confident that the pulsars were a natural phenomenon. ... Nine pulsars are now known. ... One of the southern pulsars, PSR 2045, has the longest period yet observed: 1. ... It is now known that pulsars radiate over a very wide band extending from 40 megahertz to 3,000 megahertz. ... One feature common to all the pulsars is that the amplitude variations are more rapid at lower frequencies. ... Referring back to our earlier consideration of the physical size of the pulsars, this shows that in all cases their diameter cannot exceed about 15,000 kilometers. ...
Radio astronomers will clearly be busy for some time searching for more pulsars and unraveling the properties of those already found. Another line of investigation is the possibility of observing pulsars with optical telescopes. ... The Cambridge dipole array that originally detected the pulsars was not designed for accurately pinpointing sources, but at the same observatory Sir Martin Ryle operates a large radio telescope system that employs the principle known as aperture synthesis. ... The pulsars CP 0950 and CP 1133 radiate no visible light down to the limit of the photographic plates. ... In view of the absence of light from at least two of the pulsars, and their close similarity in all other respects, the optical evidence obtained so far must be treated with caution.
No satisfactory explanation for the pulsars has yet been suggested. ... We are currently aware of only two types of star, white dwarfs and neutron stars, that are as small as the pulsars must be. ... As we groped for some explanation for the pulsars we were impressed by the fact that the pulsation periods calculated for white dwarfs and neutron stars came fairly close to the observed periods of the pulsars. ... 1 second are reported for this model, which would certainly account for the known pulsars. ...
Many other ideas have been suggested to explain pulsars, some of which can be classified as "lighthouse" theories. ... Difficulties arise with most of them, and clearly theoreticians have much work to do before the truth about pulsars is known. ... Quite apart from what pulsars actually are, however, they are likely to become important in widely different realms of astronomy. ... It may be that pulsars are sufficiently accurate astronomical clocks to supply the answer. ... Pulsars are exciting objects and we can expect a great deal from them.
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