In the summer of 1967, Anthony Hewish and his collaborators at the University of Cambridge detected, by accident, radio broadcasts in the skies that in no way resembled those that had been detected until then. They arrived on very regular impulses at intervals of only 1 1/3 seconds. To be exact, at intervals of 1, 33730109 seconds. The emitting source was called "pulsating star" or "pulsar" in abbreviation (pulsating star in English).
A fairly large number of such pulsars were discovered during the next two years, and the reader will surely wonder why they were not discovered before. The case is that a pulse radiates a lot of energy in each pulse, but these impulses are so brief that on average the intensity of radio waves is very low, going unnoticed. Moreover, astronomers assumed that radio sources emitted energy at a constant level and paid no attention to intermittent impulses.
One of the fastest pulsars was the one found in the Crab Nebula, proving that it radiated in the visible area of the electromagnetic spectrum. It turned off and on in perfect synchronization with the radio pulses. Although he had been observed many times, he had hitherto passed through an ordinary star. No one ever thought of watching him with a detection device delicate enough to show that he winked thirty times a second. With such rapid pulsations, the light seemed constant, both for the human eye and for ordinary instruments.
But what is a pulsar? If an object emits energy at periodic intervals, it is experiencing some physical phenomenon at those intervals. It can be, for example, a body that is expanding and contracting and that emits an impulse of energy in each contraction. Or it could revolve around its axis or around another body and emit an impulse of energy in each rotation or revolution.
The difficulty was that the pulse rate was very rapid, from one pulse every four seconds, to one every 1/30 of a second. The pulsar had to be a very hot body, otherwise it could not emit so much energy; and, besides, it had to be a very small body, because otherwise, I couldn't do anything with that incredible speed.
The smallest hot bodies scientists had observed were white dwarf stars. These may have the mass of our sun, they are as hot or hotter than him, and yet they are not larger than the Earth. Could it be that these white dwarfs produced impulses when expanding and contracting or rotating? Or were they two white dwarfs spinning around each other? But for many laps that astronomers gave to the problem, they could not understand that white dwarfs moved quickly enough.
As for even smaller objects, astronomers had theoretically foreseen the possibility of a star contracting brutally under the attraction of gravity, squeezing the atomic nuclei against each other. The electrons and protons would interact and form neutrons, and the star would become a kind of neutron jelly. A "neutron star" like this could have the same mass as the Sun and measure only ten miles in diameter.
Now, a neutron star had never been observed, and being so small it was feared that, even if they existed, they were not detectable.
However, such a small body could rotate fast enough to produce the impulses. Under certain conditions electrons could only escape at certain points on the surface. By turning the neutron star, electrons would be fired like water from a sprinkler; at each turn there would be a time when the jet pointed in the direction of the Earth, sending us radio waves and visible light.
Thomas Gold, of Cornell University, thought that, in that case, the neutron star would lose energy and the pulsations would be increasingly spaced, which proved to be true. Today it seems very likely that pulsars are those neutron stars that astronomers believed undetectable.
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