Pulsars may have unlocked the door to the world of collapsed stars, but something more is needed to get inside for a better view of that world. The process by which the radio pulses are produced must be very complex, depending sensitively on details of the magnetic field and surface structure that are difficult to disentangle from the theoretical models.
The high energy pulses are a little easier to relate to the observations, but only two pulsars have been detected outside the radio band, and only one, the Crab, has been detected in X-rays. As a result, radio pulsars can give us only limited access to the realm of collapsed stars.
Other pathways to knowledge about neutron stars are available. They depend not on the rotation, but on the strong gravitational field created by the collapse. When the core of a star collapses to form a neutron star, a tremendous amount of gravitational energy is released. A similar effect occurs if you drop a book. The energy of motion gained by the book as it falls closer to the center of Earth comes from the change in the stored gravitational energy of the book. When the book hits the floor, the energy of motion goes primarily into sound and heat. In the same way, energy is released by the collapse, or infall, of a stellar core to form a neutron star. Some of this energy goes into blowing off the outer layers of the star to produce a supernova explosion. Some of it goes into energy of rotation and other forms, such as heat.
Newly formed neutron stars are expected to be very hot, with surface temperatures of 10 million degrees or more. How rapidly they cool depends on the state of the matter in the star. This will depend on the nature of the nuclear forces holding the star up, that is, on how particles interact with each other at distances of less than a trillionth of a centimeter. It is a fascinating aspect of nature that is encountered time and again: the structure of matter on a very large scale depends critically on how it behaves on a very small scale. The form of the universe and the nature of the smallest particles in the universe are tightly intertwined to form the cosmic tapestry.
Calculations based on the various theories of interactions between neutrons indicate that neutron stars formed less that 10 thousand years ago should have surface temperatures of a few million degrees. That means that they should glow primarily in the low energy portion of the X-ray band. The Einstein Observatory has surveyed a number of supernova remnants for evidence of the surface X-radiation from neutron stars. In two cases, a positive detection has been made, whereas in eight other cases no source has been detected.
 In three of the undetected objects, the lack of detection is especially bothersome. They are the remnants of Tycho's supernova, the supernova of 1006 A.D., and the source called Cassiopeia A. All these objects are less than 1000 years old, yet the neutron stars, if they exist, must have cooled well below 2 million degrees. Either the matter in the interior of the star is in an exotic, rapidly cooling state called a pion condensate, or the neutron stars are not there. On the one hand, the HEAO observations may have given us the first proof of a new state of matter. On the other hand, they may force a revision in our thinking about supernovas and neutron stars.
Until now it has been assumed that neutron stars are formed in almost every supernova explosion. A comparison of the estimates of the number of pulsars and the rate of supernova outbursts confirms this belief. Yet in the majority of remnants studied so far the neutron stars are not there.
In either event, looking for hot, newly formed neutron stars is not the most promising way to study the properties of these objects. Fortunately, there is another possibility.
If we could somehow direct a stream of particles toward a neutron star and observe the radiation produced as the particles fall toward the surface, we could learn much about the gravitational and magnetic fields on and around a neutron star. Of course, we have neither the matter nor the neutron star available to carry out this experiment. However, as is often the case in astronomy, if we can think up a cosmic experiment, we can usually find a place in the universe where that experiment is being carried out.
For neutron stars the cosmic laboratory takes the form of a double star system. The cloud of dust and gas that collapsed to form the Sun had only enough matter to form one medium-sized star with about a tenth of a percent left over for the planets. However in many systems. probably over half, the protostellar cloud breaks up into two or more stars. The Alpha Centauri system is an example; it contains two stars circling around each other in an orbit about the size of the solar system, with a third star circling around these two stars at a distance of one-sixth of a light year. Another example is the double star system of Mizar in the handle of the Big Dipper. It consists of two stars separated by a distance that is about five times the diameter of the orbit of Neptune.
The stars in the Alpha Centauri and Mizar systems are so far apart that, other than causing an orbital motion around the center of mass, they do not affect each other. However, in some double star systems, the stars are so close together that they are practically touching.
The matter on the outer layers of these stars is torn between the gravitational force of the parent star holding the material to that star and the gravitational force of the companion star. As a result, the parent star can be distorted "out of round." The same sort of thing occurs on Earth. The gravitational force of the Moon pulls Earth slightly out of round, by about a foot; this distortion shows up as the high and low tides of the ocean.
 In a similar way, one star can raise tides on another one. In extreme cases, these tides can be so great as to pull matter away from its parent star. This matter usually forms a disk around the companion star and slowly settles or accretes onto the companion.
If the companion star happens to be a neutron star, then the infalling matter will pick up a tremendous amount of energy as it falls toward the neutron star. Much of this energy will be radiated away at X-ray wavelengths, taking with it information about the conditions on or near the surface of the neutron star. Essentially the same thing would happen if the parent star had a "stellar wind" that was blowing the outer layers of the star into space. If such a star had a neutron star companion, some of the matter in the wind would eventually be captured by the neutron star, producing X-rays.
These are just the sort of experiments we wanted, and, as often happens, nature provides them for us. Several hundred strong starlike sources have been discovered. Many of the X-ray stars that have been studied in detail have turned out to be systems of the type discussed above: neutron stars accreting matter from a nearby distended companion star.
Accreting neutron stars are, in a way, the opposite of radio pulsars. Matter is drawn onto the surface of the star instead of being expelled from it, and the star's rotation speeds up instead of slowing down. This happens because the material spirals in with the same sense of rotation as the neutron star, so it adds to the star's rotational energy. The two objects are opposite in that radio pulsars are seldom X-ray sources, whereas X-ray pulsars are seldom radio sources. The radio pulses from an X-ray source are presumably quenched by the high density of infalling matter.
In two important respects, many X-ray stars and radio pulsars are similar. They are both neutron stars, and they both produce periodic pulses of radiation. In both cases, the magnetic field is responsible for the pulses. It provides a channel for the outflowing matter in the radio pulsar. For X-ray stars, the magnetic field acts as a funnel to guide the infalling matter down onto a localized hot spot on or near the surface of the neutron star. As the star rotates, this hot spot comes into view once every rotation period, producing an observed pulse of X-rays about once a second.
High energy observations of the details of the radiation from X-ray pulsars have provided the first direct measurement of the magnetic field strength near the surface of a neutron star. It is several trillion times as strong as the magnetic field on Earth and several hundred thousand times stronger than the strongest magnetic field ever produced in a laboratory.
The regular pulses produced by X-ray pulsars provide astrophysicists with an extremely useful tool. In essence it gives them a clock. By studying the variations in the time kept by the clock, they can extract information about the motion of the clock, the forces acting on it, and the structure of the clock itself.
 For example, the pulsar is in orbit around the companion star. This orbital motion will cause the clock to appear to speed up when it is moving toward our line of sight and to appear to slow down when it is moving away from us. From a careful study of this apparent speeding up and slowing down of the pulses, the motion of the pulsar can be deduced. When these observations are combined with observations of the motion of the companion star and observations of the eclipses of the X-ray pulsar by the companion star, the masses of the stars can be obtained.
Results obtained in this way represent the only reliable mass measurements of neutron stars. An exhaustive study of this type shows that the masses of neutron stars in the seven systems investigated so far are most probably in the range of 1.2 to 1.6 times the mass of the Sun. This is in good agreement with theoretical calculations of the expected mass of a neutron star and represents a major confirmation of the theoretical ideas concerning collapsed stars.
Not all of the X-ray stars are pulsars. Some of them are observed to give off a steady glow of X-radiation; others give off energetic bursts of X-rays. These latter sources, which are called X-ray bursters, give off as much X-ray energy in a few seconds as the Sun radiates at all wavelengths in two weeks. These bursts occur sporadically at intervals ranging from several hours to a few days.
A combination of detailed observations and theoretical work by many researchers has shown that these bursts are most probably caused by thermonuclear flashes on neutron stars in double star systems.
Like the X-ray pulsars, the bursters are powered by the infall of matter onto a neutron star from the companion star. The infalling material is primarily hydrogen, the most common element in the universe. The surface of the neutron star accumulates a coating of hot dense hydrogen that is about ten feet thick.
Except for the high magnetic field, the conditions in this shell are not too different from those in the center of the Sun, where the hydrogen undergoes nuclear fusion reactions that convert the hydrogen into helium.
On Earth, helium is known for being a safe, stable element that will not burn. On the surface of a neutron star, the opposite is the case. The sticking together o' nuclear fusion of helium nuclei to produce carbon nuclei requires temperatures of about 100 million degrees to work, but when it works, it works with a bang. A violent thermonuclear flash, much like what happens inside a nuclear bomb, is the result. An enormous amount of energy is released, heating the surface of the neutron star to temperatures of 15 million degrees. The hot surface glows for a few seconds, producing a burst of X-rays.
Apparently, sources that produce X-ray pulses do not burst, and sources that produce X-ray bursts do not pulse. The underlying reason for this is probably the magnetic field strength on the surface of the neutron  star. If the field is strong, the infalling matter is channeled onto a small area. The strong magnetic field and the concentration of the matter at the base of the magnetic poles tend to make the nuclear fuel burn steadily rather than in bursts.
X-ray bursters are a spectacular illustration of the energy-generating capabilities of accreting neutron stars, but even more astounding variations on the theme no doubt exist. One may be the source known as SS433. This fantastic object is spewing two narrow high-speed jets of matter in opposite directions. A large, diffuse radio and X-ray emitting cloud not unlike a supernova remnant surrounds the jets. The power needed to maintain these jets, which are moving at about a quarter of the speed of light, is several hundred thousand times the power produced by the Sun every second.
At the heart of the object is a double star system and an X-ray star. This has led to the suggestion that SS433 is an accreting neutron star gone beserk, producing so much energy that it blasts a hole in the accretion disk, producing a twin exhaust jet structure. Why is SS433 so different from other X-ray stars? No one is sure. One possibility is that it is a hybrid object, producing energy through rotation, as in the Crab Nebula pulsar, and accretion, as in normal X-ray stars. Another is that the rate of flow of gas onto the collapsed star is extraordinarily large, producing an unstable disk that explodes into the exhaust jets. Finally, the central object may not be a neutron star at all, but a massive black hole, pulling matter in at a supercritical, or unstable rate.
Whatever the answer, the data accumulated on SS433 and many other observations around the world will be studied intensively in the years to come. SS433 has a striking resemblance to other, much larger jets produced by violent radio galaxies and quasars. It could serve as the codex that reveals the secrets of some of the most enigmatic objects in the universe.