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Pulsar In astronomy, short for pulsating radiosources, indicating radio sources that have the characteristic of emitting periodic short pulses of radio waves. The first pulsar was discovered in 1967 by two radio astronomers at the University of Cambridge, England: A. Hewish and his student J. Bell. Today there are about 500 pulsars, all (except one located in the Small Magellanic Cloud) belonging to the Milky Way. Probably, however, they are far more numerous: it is estimated that, in our galaxy, there are around 100,000. Pulsars are indicated by the abbreviation PSR followed by two numbers that describe the position in the sky: eg., PSR 0531 + 21 is the pulsar that has a right ascension of 5 hours and 31 minutes and a declination of 21 ° north. It seems now certain that they should be identified with neutron stars, whose existence had been hypothesized since the 1930s. It is objects having masses comparable with that of the sun rays and the order of 10 km, in which the material is in a state of enormous compression. Their radioemission is attributed to the presence of intense magnetic fields, combined with a rapid rotation. The waves would be emitted from a pulsar only around its magnetic axis: this, on the other hand, is inclined to the axis of rotation, so that the beam of radio waves, such as that of light emitted from a headlight, sweeps the sky in every around. A distant observer, located on Earth, intercepts the beam at regular time intervals, and then sees an emission ‘pulsed’, with a equal to the period of rotation of the star period of occurrence of the signal. In 2003 it was discovered the first double pulsar, ie a pulsar system formed in orbit around each other. The pulsar are observable in a broad spectrum of radio frequencies, which can extend from a few tens of MHz to over 10 GHz. The most intense emission falls, in general, in the band between 100 and 1000 MHz.