by Isaac Asimov
Investigation of M-82 with the 200-inch Hale telescope, making use of the light of a particular wavelength, showed great jets of matter up to 1,000 light-years long emerging from the galactic center. From the amount of matter exploding outward, the distance it had traveled, and its rate of travel, it seems likely that the explosion took place about 1,500,000 years ago.
It now seems that galactic cores are generally active; that turbulent and very violent events take place there, so that the universe generally is a more exciting place than we dreamed of before the coming of radio astronomy. The apparent utter serenity of the sky as seen by the unaided eye is only the product of our limited vision (which sees only the stars of our own quiet neighborhood) over a limited time.
At the very center of our own galaxy even, there is a tiny region, only a few light-years across at most, that is an intensely active radio source.
And, incidentally, the fact that exploding galaxies exist, and that active galactic cores are common and may be universal, does not necessarily put the notion of galactic collisions out of court. In any cluster of galaxies, it seems likely that large galaxies grow at the expense of small ones; and often one galaxy is considerably larger than any of the others in the cluster. There are signs that it has achieved its size by colliding with and absorbing smaller galaxies. One large galaxy has been photographed that shows signs of several different cores, all but one of which are not its own but were once parts of independent galaxies. The phrase cannibal galaxy has thus come into use.
The New Objects
By the 1960s, it might have been easy for astronomers to suppose that there were few surprises left among the physical objects in the heavens. New theories, new insights, yes; but surely little in the way of startling new varieties of stars, galaxies, or anything else could remain after three centuries of observation with steadily more sophisticated instruments.
Any astronomers of this opinion were due for enormous shocks—the first coming as a result of the investigation of certain radio sources that looked interesting but not surprising.
QUASARS
The radio sources first studied in deep space seemed to exist in connection with extended bodies of turbulent gas: the Crab Nebula, distant galaxies, and so on. A few radio sources, however, seemed unusually small. As radio telescopes grew more refined, and as the view of the radio sources was sharpened, it began to seem possible that radio waves were being emitted by individual stars.
Among these compact radio sources were several known as 3C48, 3C147, 3C196, 3C273, and 3C286. The 3C is short for “Third Cambridge Catalog of Radio Stars,” a listing compiled by the English astronomer Martin Ryle and his co-workers; while the remaining numbers denote the placing of the source on that list.
In 1960, the areas containing these compact radio sources were combed by the American astronomer, Al1en Sandage with the 200-inch telescope; and in each case, a star did indeed seem to be the source. The first star to be detected was that associated with 3C48. In the case of 3C273, the brightest of the objects, the precise position was obtained by Cyril Hazard, in Australia, who recorded the moment of radio blackout as the moon passed before it.
The stars involved had been recorded on previous photographic sweeps of the sky and had always been taken to be nothing more than faint members of our own galaxy. Painstaking photographing, spurred by their unusual radio emission, now showed, however, that that was not all there was to it. Faint nebulosities proved to be associated with some of the objects, and 3C273 showed signs of a tiny jet of matter emerging from it. In fact, there were two radio sources in connection with 3C273: one from the star and one from the jet. Another point of interest that arose after close inspection was that these stars were unusually rich in ultraviolet light.
It would seem, then, that the compact radio sources, although they looked like stars, might not be ordinary stars after all. They eventually came to be called quasi-stellar radio sources (quasi-stellar means “star-resembling”). As the term became more important to astronomers, it became too inconvenient a mouthful and, in 1964, was shortened by the Chinese-American physicist Hong Yee Chiu to quasar, an uneuphonious word that is now firmly embedded in astronomic terminology.
Clearly, the quasars were interesting enough to warrant investigation with the full battery of astronomic techniques, including spectroscopy. Such astronomers as Allen Sandage, Jesse L. Greenstein, and Maarten Schmidt labored to obtain the spectra. When they accomplished the task in 1960, they found themselves with strange lines they could not identify. Furthermore, the lines in the spectra of one quasar did not match those in any other.
In 1963, Schmidt returned to the spectrum of 3C273, which, as the brightest of these puzzling objects, showed the clearest spectrum. Six lines were present, of which four were spaced in such a way as to seem to resemble a series of hydrogen lines—except that no such series ought to exist in the place where they were found. What, though, if those lines were located elsewhere but were found where they were because they had been displaced toward the red end of the spectrum? If so, it was a large displacement, one that indicated a recession at the velocity of over 25,000 miles per second. This seemed unbelievable; and yet, if such a displacement existed, the other two lines could also be identified: one represented oxygen minus two electrons; the other magnesium minus two electrons.
Schmidt and Greenstein turned to the other quasar spectra and found that the lines there could also be identified, provided huge red shifts were assumed.
Such enormous red shifts could be brought about by the general expansion of the universe; but if the red shift was equated with distance in accordance with Hubble’s law, it turned out that the quasars could not be ordinary stars of our own galaxy at all. They had to be among the most distant objects known—billions of light-years away.
By the end of the 1960s, a concentrated search had uncovered 150 quasars. The spectra of about 110 of them were studied. Every single one of these showed a large red shift—larger indeed, than that of 3C273. The distance of a couple of them is estimated to be about 9 billion light-years.
If the quasars are indeed as far away as the red shift makes them seem, astronomers are faced with some puzzling and difficult points. For one thing, these quasars must be extraordinarily luminous to appear as bright as they do lit such a distance; they must be anywhere from 30 to 100 times as luminous as an entire ordinary galaxy.
Yet if this is so, and if the quasars have the form and appearance of a galaxy, they ought to contain up to 100 times as many stars as an ordinary galaxy and he up to 5 or 6 times as large in each dimension. Even at their enormous distances they ought to show up as distinct oval blotches of light in large telescopes. Yet they do not. They remain starlike points in even the largest telescope and thus, despite their unusual luminosity, may be far smaller in size than ordinary galaxies.
The smal1ness in size was accentuated by another phenomenon; for as early as 1963, the quasars were found to be variable in the energy they emitted, both in the visible-light region and in the radio-wave region. Increases and decreases of as much as three magnitudes were recorded over the space of a few years.
For radiation to vary so markedly in so short a time, a body must be small. Small variations might result from brightenings and dimmings in restricted regions of a body, but large variations must involve the body as a whole. If the body is involved as a whole, some effect must make itself felt across the entire width of the body within the time of variation. But no effect can travel faster than light; so that if a quasar varies markedly over a period of a few years, it cannot be more than a light-year or so in diameter. Actually, some calculations indicate quasars may be as little as a light-week (500 billion miles) in diameter.
Bodies that are at once so small and so luminous must be expending energy at a rate so great that the reserves cannot last long (unless there is some energy source as yet undreamed of, which is not impossible). Some calculations indicate that a quasar can only deliver energy at this enorm
ous rate for a million years or so. In that case, the quasars we see only became quasars a short time ago, cosmically speaking; and there must be objects that were once quasars but are quasars no longer.
Sandage, in 1965, announced the discovery of objects that may indeed be aged quasars. They seemed like ordinary bluish stars but possessed huge red shifts as quasars do. They were as distant, as luminous, as small as quasars; but they lacked the radio-wave emission. Sandage called them blue stellar objects, which can be abbreviated BSOs.
The BSOs seem to be more numerous than quasars: a 1967 estimate places the total number of BSOs within reach of our telescopes at 100,000. There are many more BSOs than quasars because the bodies last much longer in BSO form than in quasar form.
The belief that quasars are far-distant objects is not universal among astronomers. There is the possibility that the enormous red shifts of quasars are not cosmological: that is, that they are not a consequence of the general expansion of the universe; that they are perhaps relatively near objects that are hastening away from us for some local reason—having been ejected from a galactic core at tremendous velocities, for instance.
The most ardent proponent of this viewpoint is the American astronomer Halton C. Arp, who has presented cases of quasars that seem to be physically connected with galaxies nearby in the sky. Since the galaxies have a relatively low red shift, the greater red shift of the quasars (which, if connected, must be at the same distance) cannot be cosmological.
Another puzzle has been the discovery in the late 1970s that radio sources inside quasars (which can be separately detected by present-day long baseline radio telescopes) seem to be separating at speeds that are several times the speed of light. To exceed the speed of light is considered impossible in present-day physical theory, but such a superluminal velocity would exist only if the quasars are indeed as far away as they are thought to be. If they arc actually closer, then the apparent rate of separation would translate into speeds less than that of light.
Nevertheless, the view that quasars are relatively near (which would also mean they were less luminous and produced less energy, thus relieving that puzzle) has not won over most astronomers. The general view is that the evidence in favor of cosmological distances is overwhelming, that Arp’s evidence of physical connections is insufficiently strong, and that the apparent superluminal velocities are the result of an optical illusion (and several plausi ble explanations have already been advanced).
But, then, if quasars are indeed as distant as their red shifts make them appear, if they are indeed as small and yet as luminous and energetic as such distances would make necessary, what are they?
The most likely answer dates back to 1943, when the American astronomer Carl Seyfert observed an odd galaxy, with a very bright and very small nucleus. Other galaxies of the sort have since been observed, and the entire group is now referred to as Seyfert galaxies. Though only a dozen were known by the end of the 1960s, there is reason to suspect that as many as I percent of all galaxies may be of the Seyfert type.
Can it be that Seyfert galaxies are objects intermediate between ordinary galaxies and quasars? Their bright centers show light variations that would make those centers almost as small as quasars. If the centers were further intensified and the rest of the galaxy further dimmed, they would become indistinguishable from a quasar; and one Seyfert galaxy, 3C120, is almost quasarlike in appearance.
The Seyfert galaxies have only moderate red shifts and are not enormously distant. Can it be that the quasars are very distant Seyfert galaxies—so distant that we can see only the luminous and small centers; and so distant that we can only see the largest galaxies which thus give us the impression that quasars are extraordinarily luminous, whereas we should rightly suspect that they are very large Seyfert galaxies that we can see despite their distance?
Indeed, recent photographs have shown signs of haze about quasars, seeming to indicate the dim galaxy that surrounds the small, active, and very luminous center. Presumably, then, the far reaches of the universe beyond a billion light-years are as filled with galaxies as are the nearer regions. Most of those galaxies, however, are far too dim to make out optically, and we see only the bright centers of the most active and largest individuals among them.
NEUTRON STARS
If radio-wave radiation had given rise to that peculiar and puzzling astronomical body, the quasar, research at the other end of the spectrum suggested Another body just as peculiar.
In 1958, the American astrophysicist Herbert Friedman discovered that the sun produces a considerable quantity of X rays. These could not be detected from the earth’s surface, for the atmosphere absorbs them; but rockets, shooting beyond the atmosphere and carrying appropriate instruments, could detect the radiation with ease.
For a while, the source of solar X rays was a puzzle. The temperature of the sun’s surface is only 6,000° C—high enough to vaporize any form of matter but not high enough to produce X rays. The source had to lie in the sun’s corona, a tenuous halo of gases stretching outward from the sun in all directions for many millions of miles. Although the corona delivers fully half as much light as the full moon, it is completely masked by the light of the sun itself and is visible only during eclipses, at least under ordinary circumstances. In 1930, the French astronomer Bernard Ferdinand Lyot invented a telescope that, at high altitudes and on clear days, could observe the inner corona even in the absence of an eclipse.
The corona was felt to be the X-ray source because, even before the rocket studies of X rays, it had been suspected of possessing unusually high temperatures. Studies of the spectrum of the corona (during eclipses) had revealed lines that could not be associated with any known element. A new element was suspected and named coronium. In 1941, however, it was found that the lines of coronium can be produced by iron atoms that have had many subatomic particles broken away from them. To break off all those particles, however, requires a temperature of something like a million degrees, and such a temperature would certainly be enough to produce X rays.
X-ray radiation increases sharply when a solar flare erupts into the corona. The X-ray intensity at that time implies a temperature as high as 100 million degrees in the corona above the flare. The reason for such enormous temperatures in the thin gas of the corona is still a matter of controversy. (Temperature here has to be distinguished from heat. The temperature is a measure of the kinetic energy of the atoms or particles in the gas; but since the particles are few, the actual heat content per unit of volume is low. The X rays are produced by collisions between the extremely energetic particles.)
X rays come from beyond the solar system, too. In 1963, rocket-borne instruments were launched by Bruno Rossi and other astronomers to see whether solar X rays were reflected from the moon’s surface. They detected, instead, two particularly concentrated X-ray sources elsewhere in the sky. The weaker (Tau X-1, because it is in the constellation Taurus) was quickly associated with the Crab Nebula. In 1966, the stronger, in the constellation Scorpio (Sco X-1) was found to be associated with an optical object which seemed the remnant (like the Crab Nebula) of an old nova. Since then, many other X-ray sources have been detected in the sky.
To be giving off energetic X rays with an intensity sufficient to be detected across an interstellar gap required a source of extremely high temperature and large mass. The concentration of X rays emitted by the sun’s corona would not do at all.
To be at once massive and have a temperature of a million degrees suggested something even more condensed and extreme than a white dwarf. As long ago as 1934, Zwicky had suggested that the subatomic particles of a white dwarf might, under certain conditions, combine into uncharged particles called neutrons. These could then be forced together until actual contact was made. The result would be a sphere no more than 10 miles across which would yet retain the mass of a full-sized star. In 1939, the properties of such a neutron star were worked out in some detail by the American physicist J. Robert Oppenheimer. Such
an object would attain so high a surface temperature, at least in the initial stages after its formation, as to emit X rays in profusion.
The search by Friedman for actual evidence of the existence of such neutron stars centered on the Crab Nebula, where it was felt that the terrific explosion that had formed it might have left behind, not a condensed white dwarf, but a supercondensed neutron star. In July 1964, the moon passed across the Crab Nebula, and a rocket was sent beyond the atmosphere to record the X-ray emission. If it were coming from a neutron star, then the X-ray emission would be cut off entirely and at once as the moon passed before the tiny object. If the X-ray emission were from the Crab Nebula generally, then it would drop off gradually as the moon eclipsed the nebula bit by bit. The latter proved to be the case, and the Crab Nebula seemed to be but a larger and much more intense corona.
For a moment, the possibility that neutron stars might actually exist and be detectable dwindled; but in the same year that the Crab Nebula failed its test, a new discovery was made in another direction. The radio waves from certain sources seemed to indicate a very rapid fluctuation in intensity. It was as though there were “radio twinkles” here and there.
Astronomers quickly designed instruments that were capable of catching very short bursts of radio-wave radiation, and that they felt would make it possible to study these fast changes in greater detail. One astronomer making use of such a radio telescope was Anthony Hewish at Cambridge University Observatory. He supervised the construction of 2,048 separate receiving de vices spread out in an array that covered an area of nearly 3 acres; and in July 1967, the array was put to work. Within a month, a young British graduate student, Jocelyn Bell, who was at the controls, detected bursts of radio-wave energy from a place midway between Vega and Altair. It was not difficult to detect and would have been found years earlier if astronomers had expected to find such short bursts and had developed the equipment to detect them. The bursts were, as it happened, astonishingly brief, lasting only one-thirtieth of a second. Even more astonishing, the bursts followed one another with remarkable regularity at intervals of 1.33 seconds. The intervals were so regular, in fact, that the period could be worked out to one hundred-millionth of a second: it was 1.33730109 seconds.
