by Isaac Asimov
Naturally, there was no way of telling, at least at first, what these pulses represented. Hewish could only think of it as a pulsating star, each pulsation giving out a burst of energy. This name was shortened almost at once to pulsar, and by it the new object came to be known.
One should speak of the new objects in the plural, for once Hewish found the first, he searched for others. By February 1968, when he announced the discovery, he had located four and eventually, as a result, received a share of the 1974 Nobel Prize in physics. Other astronomers avidly began searching, and 400 pulsars are now known. It is possible there may be as many as 100,000 in our galaxy altogether. Some may be as close as 100 light-years or so. (There is no reason to suppose they do not exist in other galaxies; but at that distance they are probably too faint to detect.)
All the pulsars are characterized by extreme regularity of pulsation, but the exact period varies from pulsar to pulsar. One had a period as long as 3.7 seconds. In November 1968, astronomers at Green Bank, West Virginia, detcctcd a pulsar in the Crab Nebula that had a period of only 0.033089 seconds. It was pulsing 30 times a second.
Naturally, the question was, What can produce such short flashes with such fantastic regularity? Some astronomical body must be undergoing some very regular change at intervals rapid enough to produce the pulses. Could it be a planet that circles a star in such a way that once each revolution it moves beyond the star (as seen from the direction of earth).and, as it emerges, emits a powerful flash of radio waves? Or else could a planet be rotating and, each time it does so, would some particular spot on its surface, which leaks radio waves in vast quantity, sweep past our direction?
To do this, however, a planet must revolve about a star or rotate about its axis in a period of seconds or in fractions of a second, and this was unthinkable. For pulses as rapid as those of pulsars, some object must be rotating or revolving at enormous velocities, which would require very small size combined with huge temperatures, or huge gravitational fields, or both.
This instantly brought white dwarfs to mind, but even white dwarfs cannot revolve about each other, or rotate on their axes, or pulsate, with a period short enough to account for pulsars. White dwarfs are still too large, and their gravitational fields too weak. Thomas Gold at once suggested that a neutron star was involved. He pointed out that a neutron star is small enough and dense enough to be able to rotate about its axis in 4 seconds or less. What’s more, it had already been theorized that a neutron star would have an enormously intense magnetic field, with magnetic poles that need not be at the pole of rotation. Electrons would be held so tightly by the neutron star’s gravity that they could emerge only at the magnetic poles. As they were thrown off, they would lose energy, in the form of radio waves. Hence, there would be a steady sheaf of radio waves emerging from two opposite points on the neutron star’s surface.
If, as the neutron star rotates, one or both of those sheafs of radio waves sweeps past our direction, then we will detect a short burst of radio-wave energy once or twice each revolution. If this is so, we would detect only pulsars that happen to rotate in such a way as to sweep at least one of the magnetic poles in our direction. Some astronomers estimate that only 1 neutron star out of 100 would do so. If there are indeed as many as 100,000 neutron stars in the galaxy, then only 1000 might be detectable from earth.
Gold went on to point out that if his theory were correct, the neutron star would be leaking energy at the magnetic poles and its rate of rotation would be slowing down. Thus, the shorter the period of a pulsar, the younger it is and the more rapidly it would be losing energy and slowing down.
The most rapid pulsar at that time known was in the Crab Nebula. It might well be the youngest, since the supernova explosion that would have left the neutron star behind took place less than 1,000 years ago.
The period of the Crab Nebula pulsar was studied carefully, and it was indeed found to be slowing, just as Gold had predicted. The period was increasing by 36.48 billionths of a second each day. The same phenomenon was discovered in other pulsars as well; and as the 1970s opened, the neutron star hypothesis was widely accepted.
Sometimes a pulsar will suddenly speed up its period very slightly, then resume the slowing trend. Some astronomers suspect this may be the result of a starquake, a shifting of mass distribution within the neutron star. Or it might be the result of some sizable body plunging into the neutron star and adding its own momentum to the star’s.
There was no reason the electrons emerging from the neutron star should lose energy only as microwaves. This phenomenon should produce waves all along the spectrum. It should produce visible light, too.
Keen attention was focused on the sections of the Crab Nebula where visible remnants of the old explosion might exist. Sure enough, in January 1969, it was noted that the light of a dim star within the Nebula did flash on and off in precise time with the microwave pulses. It would have been detected earlier if astronomers had had the slightest idea that they ought to search for such rapid alternations of light and darkness. The Crab Nebula pulsar was the first optical pulsar discovered—the first visible neutron star.
The Crab Nebula pulsar released X rays, too. About 5 percent of all the X rays from the Crab Nebula emerged from that tiny flickering light. The connection between X rays and neutron stars, which seemed extinguished in 1964, thus came triumphantly back to life.
It might have seemed that no further surprises were to be expected from neutron stars; but in 1982, astronomers at the 300-meter Arecibo radio telescope in Puerto Rico located a pulsar that was pulsing at 642 times a second, twenty times faster than the Crab Nebula pulsar. It is probably smaller than most pulsars—not more than 3 miles in diameter; and with a mass of perhaps two or three times that of our sun, its gravitational field must be enormously intense. Even so, so rapid a rotation must come close to tearing it apart. Another puzzle is that its rate of rotation is not slowing nearly as fast as it ought considering the vast energies being expended.
A second such fast pulsar has been detected, and astronomers are busily speculating about the reasons for its existence.
BLACK HOLES
Nor is even the neutron star the limit. When Oppenheimer worked out the properties of the neutron star in 1939, he predicted also that it was possible for a star that was massive enough (more than 3.2 times the mass of our sun) to collapse altogether to a point or singularity. When such collapse proceeded past the neutron-star stage, the gravitational field would become so intense that no matter and, in fact, not even light could escape from it. Since anything caught in its unimaginably intense gravitational field would fall into it without hope of return, it could be pictured as an infinitely deep “hole” in space. Since not even light could escape, it was a black hole—a term first used by the Amcrican physicist John Archibald Wheeler in the 1960s.
Only about one star in a thousand is massive enough to have any chance of forming a black hole on collapse; and, of such stars, most may lose enough mass in the course of a supernova explosion to avoid that fate. Even so, there may be tens of millions of such stars in existence right now; and in the course of the galaxy’s existence, there may well have been billions. Even if only one out of a thousand of these massive stars actually form a black hole on collapse, there should still be a million of them here and there in the galaxy. If so, where are they?
The trouble is that black holes are enormously difficult to detect. They cannot be seen in the ordinary way since they cannot give off light or any form of radiation. And although their gravitational field is vast in their immediate vicinity, at stellar distances the intensity of the field is no greater than for ordinary stars.
In some cases, however, a black hole can exist under specialized conditions that make detection possible. Suppose a black hole is part of a binary-star system; that it and a companion revolve about a mutual center of gravity, and that the companion is a normal star. If the two are close enough to each other, matter from the normal star may little
by little drift toward the black hole and take up an orbit about it. Such matter in orbit about a black hole is called an accretion disk. Little by little the matter in the accretion disk would spiral into the black hole and, in so doing, would (by a well-known process) give off X rays.
It is necessary, then, to search for an X-ray source in the sky where no star is visible, but a source that seems to orbit another nearby star that is visible.
In 1965, a particularly intense X-ray source was detected in the constellation Cygnus and was named Cygnus X-l. It is thought to be about 10,000 light years from us. It was just another X-ray source until an X-ray-detecting satellite was launched from the coast of Kenya in 1970 and, from space, detected 161 new X-ray sources. In 1971, the satellite detected irregular changes in the intensity of X rays from Cygnus X-l. Such irregular changes would be expected of a black hole as matter entered from an accretion disk in spurts.
Cygnus X-1 was at once investigated with great care and was found to exist in the immediate neighborhood of a large, hot, blue star about 30 times as massive as our sun. The astronomer C. T. Bolt, at the University of Toronto, showed that this star and Cygnus X-1 were revolving about each other. From the nature of the orbit, Cygnus X-1 had to be 5 to 8 times as massive as our sun. If Cygnus X-1 were a normal star, it would be seen. Since it was not seen, it had to be a very small object. Since it was too massive to be a white dwarf or even a neutron star, it had to be a black hole. Astronomers are not yet completely certain of this assumption, but most are satisfied with the evidence and believe Cygnus X-1 to be the first black hole to be discovered.
Black holes, it would seem, might most likely be formed in places where stars were most thickly strewn and where huge masses of material might most likely accumulate in one place. Because high intensities of radiation are associated with the central regions of such star accumulations as globular clusters and galactic cores, astronomers are coming more and more to the belief that there are black holes at the centers of such clusters and galaxies.
Indeed, a compact and energetic microwave source has been detected at the center of our own galaxy. Could that represent a black hole? Some astronomers speculate that it does, and that our galactic black hole has the mass of 100 million stars, or 1/1,000 that of the entire galaxy. It would have a diameter 500 times that of the sun (or equal to that of a huge red-giant star) and would be large enough to disrupt stars through tidal effects, or to gulp them down whole before they break up, if the approach were fast enough.
Actually, it now appears that it is possible for matter to escape from a black hole, although not in the ordinary way. The English physicist Stephen Hawking, in 1970, showed that the energy content of a black hole might occasionally produce a pair of subatomic particles, one of which might escape. In effect, this would mean that a black hole would evaporate. Star-sized black holes evaporate in this fashion in so slow a manner that inconceivable times would have to elapse (trillions of trillions of times the total lifetime of the universe so far) before they would evaporate totally.
The evaporation rate would increase, however, as the mass became smaller. A mini-black hole, no more massive than a planet or an asteroid (and such tiny objects could exist if they are sufficiently dense: that is, squeezed into a small enough volume) would evaporate rapidly enough to give off appreciable amounts of X rays. Furthermore, as it evaporated and grew less massive, the rate of evaporation and the rate of X-ray production would steadily increase. Finally, when the mini-black hole was small enough, it would explode and give off a pulse of gamma rays of characteristic nature.
But what could compress small amounts of matter of the fearfully high densities required for mini-black hole formation? Massive stars can be compressed by their own gravitational fields, but that will not work for a planet sized object, and the latter would require greater densities for black-hole formation than the former would.
ln 1971, Hawking suggested that mini-black holes were formed at the time of the big bang when conditions were far more extreme than they have been at any other time. Some of those mini-black holes may have been of such a size that only now, after 15 billion years of existence, have they evaporated to the point of explosion, and astronomers might detect gamma-ray bursts that would serve as evidence for their existence.
The theory is attractive, but so far no such evidence has been reported.
“EMPTY” SPACE
But if there are objects in the universe that surprise us, there are also surprises in the vast not-so-empty spaces between the stars. The non-emptiness of “empty” space has proven to be a matter of difficulty for astronomers in observations relatively close to home.
In a sense, the galaxy hardest for us to see is our own. For one thing, we are imprisoned within it, while the other galaxies can be viewed as a whole from outside. It is like the difference between trying to view a city from the roof of a low building and seeing it from an airplane. Furthermore, we are far out from the center and, to make matters worse, lie in a spiral arm clogged with dust. In other words, we are on a low roof on the outskirts of the city on a foggy day.
The space between stars, generally speaking, is not a perfect vacuum under the best of conditions. There is a thin gas spread generally through interstellar space within galaxies. Spectral absorption lines due to such interstellar gas were first detected in 1904 by the German astronomer Johannes Franz Hartmann. In the outskirts of a galaxy, the concentration of gas and dust becomes much thicker. We can see such dark fogs of dust rimming the nearer galaxies.
We can actually “see” the dust clouds, in a negative way, within our own galaxy as dark areas in the Milky Way. Examples are the dark Horsehead Nebula, outlined starkly against the surrounding brilliance of millions of stars, and the even more dramatically named Coalsack in the Southern Cross, a region of scattered dust particles 30 light-years in diameter and about 400 light-years away from us.
Although the gas and dust clouds hide the spiral arms of the galaxy from direct vision, they do not hide the structure of the arms from the spectroscope. Hydrogen atoms in the clouds are ionized (broken up into electrically charged subatomic particles) by the energetic radiation from the bright Population I stars in the arms. Beginning in 1951, streaks of ionized hydrogen were found by the American astronomer William Wilson Morgan, marking out the lines of the blue giants—that is, the spiral arms. Their spectra were similar to the spectra shown by the spiral arms of the Andromeda galaxy.
The nearest such streak of ionized hydrogen includes the blue giants in the constellation of Orion, and this streak is therefore called the Orion Arm. Our solar system is in that arm. Two other arms were located in the same way. One lies farther out from the galactic center than our own and includes giant stars in the constellation Perseus (the Perseus Arm). The other lies closer to the galactic center and contains bright clouds in the constellation Sagittarius (the Sagittarius Arm). Each arm seems to be about 10,000 light-years long.
Then radio came along as a still more powerful tool. Not only could it pierce through the obscuring clouds, but it made the clouds themselves tell their story—through their own voice. This came about as a result of the work of the Dutch astronomer Hendrik Christoffel Van de Hulst. In 1944, the Netherlands was ground under the heavy boot of the Nazi army, and astronomic observation was nearly impossible. Van de Hulst, confining himself to pen and paper work, studied the characteristics of ordinary hydrogen atoms, of which most of the interstellar gas is composed.
He suggested that, every once in a while, such atoms, on colliding, might change their energy state and, in so doing, emit a weak radiation in the radio part of the spectrum. A particular hydrogen atom might do so only once in 11 million years; but among the vast numbers present in intergalactic space, enough would be radiating each moment to produce a continuously detectable emission. Van de Hulst calculated that the wavelength of the radiation should be 21 centimeters. Sure enough, with the development of new radio techniques after the war, this “song of hydrogen�
�� was detected in 1951 by Edward Mills Purcell and Harold Irving Ewen at Harvard University.
By tuning in on the 21-centimeter radiation of collections of hydrogen, astronomers were able to trace out the spiral arms and follow them for long distances—in most cases, nearly all the way around the galaxy. More arms were found, and maps of the concentration of hydrogen show half a dozen or more streaks.
What is more, the song of hydrogen told something about its movements. Like all waves, this radiation is subject to the Doppler-Fizeau effect. It allows astronomers to measure the velocity of the moving hydrogen clouds and, thereby, to explore, among other things, the rotation of our galaxy. This new technique confirmed that the galaxy rotates in a period (at our distance from the center) of 200 million years.
In science, each new discovery unlocks doors leading to new mysteries. And the greatest progress comes from the unexpected—the discovery that over throws previous notions. An interesting example at the moment is a puzzling phenomenon brought to light by radio study of a concentration of hydrogen at the center of our galaxy. The hydrogen seems to be expanding yet is confined to the equatorial plane of the galaxy. The expansion itself is surprising, because there is no theory to account for it. And if the hydrogen is expanding, why has it not all dissipated away during the long lifetime of the galaxy? Is it a sign perhaps that, some 10 million years ago, as Oort suspects, its center exploded, as that of M-82 did much more recently? Then, too, the plane of hydrogen is not perfectly flat. It bends downward on one end of the galaxy and upward on the other. Why? No good explanation has yet been offered.
