In 1974 the first binary pulsar—two stars, at least one of which is a neutron star, that orbit each other—was discovered by Russell A. Hulse and Joseph H. Taylor, for which they shared the 1993 Nobel Prize in Physics. Using this binary system, they observed indirect evidence of gravitational waves and also tested the general theory of relativity. Several dozen binary pulsars are now known. In 1995 the orbiting Compton Gamma Ray Observatory detected the first object that bursts and pulses at the same time. This bursting pulsar, another class of pulsars, is currently the strongest source of X rays and gamma rays in the sky. Fewer than a dozen bursting pulsars are known to exist.
The intense magnetic field and plasma that are believed to surround a neutron star provide an effective source of radio waves. The high-energy electrons of the plasma spiral around the magnetic field and emit radio waves and other forms of electromagnetic radiation. This synchrotron radiation is highly directional, like a flashlight beam. If the neutron star is rotating, it will act like a revolving beacon and produce the observed pulses. The pulses recur at precise intervals, but successive pulses differ considerably in strength. Since 1968 more than 700 pulsars have been observed, with pulse rates from 4 seconds to 1.5 milliseconds; the very rapid ones are called millisecond pulsars. The interval between pulses decreases ever so slightly with the passage of time, and it is believed that the slower pulsers are the older stars while the rapid pulsers are the younger. Pulsars in the
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