Formation of Neutron Stars

In 1968 a pulsar was discovered inside the Crab nebula (Figure 8.1) and the pulsar has also been detected optically and through X rays and gamma rays. It flashes at a rate of about 33 pulses per second, and its existence is confirmation of the theory that pulsars are created in exploding stars.

Whatever tire nature of the pulsar itself, it has to be spinning extremely fast. Normal stars would shatter long before they could spin as fast as pulsars. The only form of matter that is capable of accounting for the pulsar behavior is a star consisting entirely of neutrons.

Creation of a neutron star involves the catastrophic collapse of the core of a fairly normal star of at least four solar masses whose outer layers explode in a supernova. The explosion is the consequence of a sequence of events triggered when the core of the star runs out of fuel. At that stage the internal tire, which kept the star shining, is extinguished. Until then it was the internally generated heat that balanced the inward pull of gravity to keep the stars stable. When the tire dies out, the core cools, gravity suddenly dominates, and the core collapses. In this collapse, fundamental particles of matter—protons and electrons—are driven so close together that they fuse and become neutrons. As a result, a solid ball of neutrons is produced at the center of the star.

In some cases the core collapse continues with such violence that the neutrons are forced even closer together and, in turn, swallow each other in their own gravitational pull. This is how a black hole is formed. The existence of a black

FIGURE 8.1, The supernova remnant G5.4-1.2 (large scale image) called the "Duck"' Nebula because of it peculiar shape. The blight blob at the head of the duck is at the location of the pulsar B1757-24 shown in the bottom-right inset. The pulsar has traveled beyond the boundary of the shell of the supernova remnant, which lies about 15,000 light-years away in the constellation of Sagittarius. The pulsar's motion through space across the nebula has been measured from its change of position over several years as about 600 km/s. Investigators: Bry an Gaensler and Dale Frail. (Image courtesy of NRAO/AUI.)

FIGURE 8.1, The supernova remnant G5.4-1.2 (large scale image) called the "Duck"' Nebula because of it peculiar shape. The blight blob at the head of the duck is at the location of the pulsar B1757-24 shown in the bottom-right inset. The pulsar has traveled beyond the boundary of the shell of the supernova remnant, which lies about 15,000 light-years away in the constellation of Sagittarius. The pulsar's motion through space across the nebula has been measured from its change of position over several years as about 600 km/s. Investigators: Bry an Gaensler and Dale Frail. (Image courtesy of NRAO/AUI.)

hole can be recognized when it is in an orbit about a companion star. The black hole may draw matter out of that star much like a vacuum cleaner sucks dust from some distance away. The gas plummets toward the black hole and spirals inward into an ever-decreasing orbit to form an accretion disk. The gas in the disk heats up and radiates intense X rays (observed from earth) en route to disappearing into the black hole itself.

Returning to the machinations in the supernova event, when the core has collapsed to form a neutron star, layers of gas above the neutron ball suddenly find that there is nothing to hold them up against gravity, Momentarily they hang suspended and then crash downward, smashing onto the neutron mass and rebound in a violent and fiery explosion. We may see the resulting stellar cataclysm lighting up our sky.

The newly born neutron star will be spinning extremely rapidly as a natural consequence of its having contracted so much. This is due to the conservation of angular momentum, also displayed by a spinning ice skater who begins a spin with arms outstretched and then spins faster and faster as he draws his arms in. This action changes the effective radius of his spinning body, determined by how far his arms are outstretched. A diver doing somersaults uses the same principle by tucking her body in at the start of the somersaults, causing her to become a smaller object, which rotates faster. When she stretches out just before entering the water the somersaults are slowed to a near stop. In the case of the spinning neutron star, shrunk to some small size, its gravitational pull remains sufficiently large to hold the neutron ball together against the disruptive force of rotation.

Thousands of years later the ejecta from the exploded star created wonderful nebulae, such as the one seen in Figure 8.1, ''The Duck." This is a radiograph of the supernova remnant G5.4-1.2 (referring to its coordinates in the galactic longitude, 5.°4, and latitude, —1.°2) in Sagittarius and located 15,000 light-years away. The associated pulsar is B1757-24 and its immediate surroundings are shown in the insets. Measurements of the change in the pulsar position over a period of 6 years show it to be traveling at 600 km/s through space. Such a high velocity implies that it must have been torn free of a companion star at birth. The pulsar has escaped the bulk of the nebula and has dragged a trail of radio luminous material along with it.

8.5. Binary Pulsars—Nature's Fabulous Space Labs

During 1874 a major pulsar search was launched at Arecibo observatory. Joe Taylor and Russell Hulse devised an elegant technique that allowed them to discriminate against interference and quickly recognize a pulsar. They found 40 new candidates. One of these, in the constellation Aquilla and labeled PSR 1813+16, turned out to be very peculiar, even for pulsars. The pulses occurred on average every 0.05803000 s, but this rate was not constant, unlike all the other pulsars observed before. Its period showed a 7-h 45-min cyclical change. PSR1813-1-16 appeared to be binary pulsar, a neutron star in orbit about another object. Pulse rate changes were produced by the Doppler effect, which caused the arrival time of pulses to speed up or slow down (by 16.84 pulses/s with respect to the average) as the pulsar moved either toward or away from the earth din ing its orbit.

Since at least half the stars in the Galaxy are locked in binaries, a binary pulsar should have come as no surprise. However, the nature of this binary was extraordinary. Careful timing observations enabled the variations in the pulse arrival time to be interpreted with sufficient accuracy to allow the precise orbits and the masses of the pair of stars to be estimated. PSR 1813+16 consists of two objects, each of about 1.4 solar masses, traveling around each other at hundreds of kilometers per second in orbits so close that the distance between them ranges from 1.1 to 4.8 times the radius of the sun (which is about 650,000 km). The maximum diameter of the pulsar's orbit is only a million kilometers.

The binary pulsar provided a fabulous additional bonus. It is a perfect clock in orbit about a massive object, the ideal laboratory for testing Einstein's general theory of relativity. In 1915 Einstein had developed an elegant way to describe gravity and its effects and had explained an observation made during the previous century, that Mercury's orbit about the sun shows an anomaly not accounted for by other theories. Mercury's point of closest approach to the sun, known as its perihelion, moves slowly around the sun at a rate of 43 arcseconds per century. This is called the "precession" of the perihelion. Einstein's theory explained this phenomenon and now the discovery of the binary pulsar provided a further test.

The pulsar is in an elliptical orbit about another object and their point of closest approach (known as the periastron) also precesses. Changes in the pulse arrival times (due to the Doppler shift) should show tiny variations as the pulsar's orbit itself slowly swings around in space. The effect was measured to be 4° per year, precisely as predicted by relativity theory.

The binary pulsar, however, turned out to offer an even more exciting prospect for the radio astronomers, an opportunity unique in the history of the science. The pulsar presented a novel way to test one of Einstein's most important predictions, that objects accelerating in a strong gravitational field should emit a form of radiation called gravitational waves. Just as radio waves are produced by accelerating electrons, gravity waves should be produced by accelerating matter. In the case of the binary pulsar the conditions for radiating gravitational waves appeared to be perfect. Two massive objects moving around one another are constantly accelerating within each other's gravitational influence.

Einstein had stated that he believed gravitational waves would never be detected on earth because they are far too feeble to produce any measurable effects. Despite his caution, however, several laboratories have, with a notorious lack of success, attempted to detect gravitational waves. Now the radio astronomers realized they could search for the effect on the pulsar orbit as a consequence of the radiation of gravitational waves. They would not directly search for the waves, but see what happened to the binary orbit as the system lost energy in the form of gravitational radiation. The energy loss should be manifested as a very small change in the orbital period. This is the consequence of conservation of angular momentum, discussed before. As the system loses energy its orbit shrinks; the pulsar w ill move a little faster through space, hence the time taken to complete one orbit decreases.

Six years later, after extensive monitoring of the radio pulses from the invisible object in Aquilla, the pulsar orbital period was found to be slowing down by 6.7 x 10-s s/year, equivalent to a shrinkage of 3.1 mm/orbit or 3.5 m/year. This was just the amount that should result from the radiation of gravitational waves. This remarkable measurement, confirming a prediction of a theory proposed 66 years earlier, has proven to be one of the most exciting bonuses produced by radio astronomy research. In 1983, Taylor and Hulse were awarded the Nobel Prize in Physics for this discovery.

But why are these two objects, the pulsar and its invisible companion, so close together in space? The other object is likely to be a neutron star, perhaps a pulsar, but its beam of radio waves does not happen to sweep past the earth. The tw o ob jects could not have been so close when they were normal stars. The explanation runs something as follows. Once these were two normal albeit quite massive members of a binary star system. The more massive one evolved quickly, consumed its fuel, and died in a violent supernova explosion. The neutron star stayed in orbit about the other star, which, in turn, reached old age and began to expand to form a red giant. The neutron star then became enveloped in the red giant's atmosphere, where it experienced frictional drag, slowed down, and slowly spiraled deeper into the giant star. The neutron star would not suffer undue hardship at this point, but this would produce severe reactions in the giant star. In due course the star exploded and produced the second neutron core. Today the two neutron stars are in close orbit in the binary pulsar PSR 1813-1-16, nature's most remarkable laboratory in space.

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