The key that unlocked the door to our understanding of collapsed stars was the discovery of pulsars. In 1968, several sources of radio waves were found to be pulsing with great rapidity and astonishing regularity. They blinked on and off with the precision of a clock that loses only one second in a million years! The time between pulses varies from one object to the next, but it is typically one second or less. The most energetic pulsar is associated with the Crab Nebula, a remnant of a supernova explosion. The Crab pulsar has a pulsation period of about 1/30 of a second.
The regularity of the pulses can be understood only in terms of the rotation of a very large body, that is, a star. The rapidity of the pulsation, especially in the case of the Crab pulsar, can be explained by only one type of star-a neutron star. Any other star, even a white dwarf, would break apart if it were to rotate as fast as the pulsars must be rotating.
The rapid rotation of a pulsar is a natural consequence of the way in which they are formed. When an object collapses, any rotational motion it has is greatly amplified. A familiar example of this is the figure skater, who greatly speeds up rotation by pulling in his arms. In the formation of the neutron star, the core of a star has collapsed under the action of its own gravity to a diameter only one ten-thousandth or less than its original diameter. Since the rotational frequency increases as the square of the collapse factor, by the time the core reaches the neutron star stage, it could be rotating 100 million times faster than it originally was.
Another consequence of the gravitational collapse is that the magnetic fields tied into the core of the star are greatly amplified. A rotating magnet can produce strong electrical effects. This is the principle underlying electrical generators in automobiles, power plants, and pulsars.
Electrical potential differences of trillions of volts are produced by the rapidly rotating magnetic field of neutron stars. These voltages act to pull particles off the surface of the neutron star and accelerate them to high energies. This happens mainly near the magnetic poles of the star, producing two diametrically opposite beams of high energy particles. These particles produce beams of radiation, which, because of the rotation of the star, show up as a sequence of regular pulses. The exact mechanism for producing the beams of radiation is still not understood.
The neutron star in the Crab Nebula pulses at all wavelengths from radio to gamma rays, but it is the exception. A survey of radio pulsars with the HEAO I experiment detected only one pulsing X-ray source-the Crab...
...pulsar. With HEAO 2, two pulsars have been discovered in supernova remnants. One has a pulse period of 0.15 seconds, and appears to be a system similar to the Crab Nebula. The other has a period of 3.5 seconds, more characteristic of an X-ray binary star (see Chapter 15). Recently two radio pulsars with extremely short periods, less than a hundredth of a second, have been discovered. These objects do not show pulsed X-ray emission nor are they associated with a radio or X-ray remnant. Apparently they are very old neutron stars with very weak magnetic fields. Evidently very rapid rotation and a strong magnetic field are required to produce X-ray pulses.
The discovery of pulsars proved the existence of a new type of object, the neutron star. It also showed the importance of a source of energy that had been overlooked, the energy contained in the rotational motion of the neutron star.
One of the most striking manifestations of the power of this energy source is the Crab Nebula. The Crab Nebula was produced by a supernova explosion that was observed to occur in 1054 A.D. by Chinese, Japanese, Arabian, and possibly North American Indian "astronomers." Today it is observed as a cloud that stretches across several light years and produces intense radiation from radio through gamma ray wavelengths. A careful study of the characteristics of the radiation from this cloud shows that it comes from very high energy electrons interacting with the magnetic field in the cloud.
 The existence of this cloud of high energy particles posed a major puzzle for astrophysicists. The particles producing the X- and gamma radiation should radiate away most of their energy in a few years or less, yet the cloud has been around for a thousand years.
The discovery of a pulsar in the center of the Crab Nebula solved the problem almost overnight. The neutron star there was spinning around 30 times a second, generating electromagnetic fields that in turn produced the high energy particles in the cloud as a whole.
The energy for the particles is coming ultimately from the rotational energy of the neutron star. If this is true, then the neutron star should be slowing down by an amount that can be estimated from the amount of energy lost in the form of radiation. In a neat check that occurs all too seldom in astrophysics, it was shown that this is indeed the case. Observations of radio, optical, and X-ray pulses showed that the pulses were gradually getting farther apart. The neutron star is slowly spinning down, with each successive spin taking about a trillionth of a second longer. Put another way, the rotational energy of the star is gradually decreasing by just the amount required.
 Although we have a good broad brush picture explaining how the rotational energy of a neutron star is converted into high energy particles, the exact details are unknown. It is not known exactly how the particles are accelerated. Does it happen near the surface of the neutron star, where the fields are intense, or farther out, where large-scale electromagnetic waves can be effective?
The HEAO observations of the Crab Nebula will help to answer these questions. X-ray pictures made with the Einstein Observatory show the pulsar, a bright wispy region offset from the pulsar, and an overall bell-shaped nebula enveloping both the pulsar and the wisp. Comparison of these pictures with the optical and radio maps shows that the appearance of the Crab Nebula changes with the wavelength used to observe it. Since the wavelength can be related to the energy of the particles, high energy associated with X-rays, etc., this means that the high energy particles in the cloud are distributed differently from the lower energy ones. The details of the differences in these distributions tell us about the process for accelerating the high energy particles and how rotational energy is converted into particle energy. Such studies should also help us to understand high energy processes occurring on a much larger scale, in quasars and radio galaxies. These cosmic powerhouses have each poured more high energy particles into intergalactic space than a million Crab Nebulas.