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Pulsars Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 17, 2011

So What Are They?

Theories concerning the nature of pulsars began to arise after an astronomer named Thomas Gold hypothesized that they are fast-spinning neutron stars generating intense EM fields. The key to the shortness of pulsar periods, which had baffled astronomers and physicists, was the angular-momentum effect described earlier in this chapter and illustrated in Fig. 14-3.

The radio signals we observe coming from pulsars are apparently the result of the rapid spin combined with relativistic effects on the magnetic fields surrounding the object. According to Gold, the intensity of the magnetic field near a collapsing neutron star can grow to several trillion times the intensity of Earth’s magnetic field—that is, trillions of gauss. Because the gravitation near the neutron star is so intense, it “warps” space and causes the magnetic lines of flux to “pile up” close to each other. This effect is exaggerated by the rapid spin of the object. These bunches of magnetic flux lines sweep around and around as the neutron star spins. From a distance, to observers located in positions such that the bunches of flux lines sweep across their instruments, the magnetic bursts appear as EM energy. This energy can occur, theoretically, at any wavelength, from low-band radio through microwave, infrared (IR), and even into the visible spectrum.

If Thomas Gold’s theory is correct, it is reasonable to suppose that the rotational periods of pulsars should increase gradually. All spinning objects rotate more slowly as time passes because they lose momentum. Careful observations of pulsar periods, made over long stretches of time and with accurate time-measurement devices, have confirmed that this does happen. Pulsar periods invariably get longer as time goes by. This lends support to Gold’s theory.

The Mystery Goes On

The energy from some pulsars arrives in bursts so intense that they can be seen with optical telescopes, and their images can be captured on photographic film. A pulsar in the Crab Nebula , which is believed to be the remnant of a supernova explosion approximately 1,000 years ago, has a period of only 0.03 second and emits visible “flashes.” The flashing went unnoticed for many years because most optical observation is done using time-exposure photography. In such a photograph, a visible pulsar looks like an ordinary star.

Pulsars remain mysterious, and the mechanisms by which they operate stretch the limits of credibility. Although variations of Thomas Gold’s hypothesis have been accepted, no one is completely certain what makes these things tick. It is impossible to manufacture a pulsar in a laboratory. Computer modeling is helpful but not conclusive because the output of a computer can only be as good as the input data. Some pulsars have been observed to suddenly cease their “transmissions” for several seconds or even minutes and then start up again. How can we explain that?

Practice problems of this concept can be found at: Extreme Objects in Our Galaxy Practice Problems

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