July 24 1964 - The Brightest Objects in the Universe

One of the most important astronomical discoveries of recent times, if not the most important of all, was that of the objects known variously as quasars, quasi-stellar objects, or simply as QSO's. Thousands of millions of times more luminous than the Sun, if present evidence is to be trusted, they look like faint and unspectacular stars, but they are in fact infinitely more dramatic. Despite our lack of knowledge as to their true nature, it was clear that a programme would have to be devoted to them, and on this occasion I was again joined by one of the regular visitors to The Sky at Night, Colin Ronan.

Our knowledge of the universe has progressed amazingly during the past half-century. In 1920, for instance, it was still not defin­itely established that the 'resolvable nebulae', now known as galaxies, lie well beyond the boundaries of our own system; leading astronomers of that time considered that they were contained in the Milky Way, and there was certainly no thought of an expanding universe. The present position is very different. There can be no doubt that the galaxies are separate objects, some of them considerably larger than the Galaxy in which we live, while there is not much reasonable doubt that all the galaxies apart from those in our Local Group are racing away from us at tremendous speeds. 3C-295, a faint galaxy in the constellation Bootes, is in the order of 5,000 million light-years away, and is receding at almost half the velocity of light, as was established by Minkowski in 1960.

Very recently, objects of a completely new type have been identified. They have been called quasars, quasi-stellar objects, or (for short) QSO's, and the evidence indicates that they are the brightest things in the whole universe. Yet they are not normal galaxies, and the source of their energy is still unknown.

A recent illustration of Quasar 3C120

The detection of QSO's would have been almost impossible s been found that galaxies of certain types are strong emitters of radio waves. According to Minkowski, over 90 per cent of these 'radio galaxies' are giant ellipticals. In some cases it has been supposed that the radio emission must be caused by collisions between galaxies, as with the famous Cen- taurus A, unfortunately too far south in the sky to be visible in Europe. When photographed with a large telescope, Centaurus A is a spectacular object, and there seemed every reason to suppose that it was in fact made up of two separate systems which were passing through each other; individual stars would seldom or never collide, but the gas and dust spread between them would be in collision all the time, so producing the radio waves picked up by the instruments at Jodrell Bank and elsewhere.

Like so many other plausible-sounding ideas, this theory has been tested, and has been found to be inadequate. The intensity of the radio emission is simply too great to be explained in this way, and it now looks as though the whole notion of galaxies in collision must be given up. Some other energy-source is in­volved, though at the moment we have to confess that we do not know what it is.

Detailed surveys carried out during the past year or two showed that there were some radio sources not associated with normal galaxies. One of these was numbered 3C-48, since it was the forty-eighth object in the third Cambridge catalogue of radio sources. The radio position could be fixed with fair accuracy, and it appeared that the waves could come only from what looked like a faint star, with a wisp of nebulosity close by. Astronomers were keenly interested, and subjected this 'star' to close examin­ation, with remarkable results. It became clear that instead of being a star, 3C-48 was something very much more dramatic. Studies of its spectrum showed that it was extremely remote, and moving away from us at high velocity. Also, it was very blue.

The real surprise came when the luminosity was worked out. 3C-48 must shine as brightly as 1,000,000,000,000 Suns put together, so that it is much more luminous than the Andromeda Spiral, the most brilliant galaxy known (twice as luminous as the Milky Way system)". Yet it gave the impression of nothing more than a 16th-magnitude star, and it was certainly much smaller than a galaxy - unless, of course, we were seeing only the brightest part of it.

Since then, eight more of these QSO's have been identified, one of which, 3C-273, far outshines even 3C-48. It is of visual magnitude 12-7, so that it is not difficult to see in an average amateur telescope, but at first sight there is nothing to single it out. In fact, it must be over 1,500 light-years away, with a luminosity 200 times as great as that of our Galaxy. Finally there is 3C-147, which lies at something like 6,000 million light- years, and has the distinction of being the most remote object so far identified by visual means.

These distances are almost incredible in view of the apparent nature of the QSO's, and yet they seem certainly to be of the right order. The estimates depend, of course, upon the Red Shift of the spectral lines; according to the Doppler principle, a shift of this kind indicates recession, the velocity depending upon the extent of the shift. There is a definite relationship between the recessional velocity and the distance of the object concerned, as has long since been established by studies of normal galaxies, so that the distances themselves can be found.

The only loophole in this argument is to suppose that the Red Shifts in the spectra of QSO's are due to some other cause, but here we run into difficulties at once. If, for instance, the QSO's were relatively close, and yet were receding very quickly, they ought to show definite proper motions — that is to say, their movement against the starry background should be detectable over a period of a year or so - but this is not the case. It has also been suggested that the Red Shifts have nothing to do with velocity, but are due to strong gravitational fields at the surfaces of what may be termed 'super-stars'. However, the American astronomers Greenstein and Schmidt have shown that a field of this intensity would collapse the super-star's atmosphere, so that the spectrum would not correspond to what is actually observed. All things considered, it seems overwhelmingly likely that the QSO's really are immensely remote and immensely lumin­ous.

Another problem concerns the short-term variations in brightness shown by at least four of the QSO's, including 3C- 273. It must be emphasized that QSO's are not recent discoveries ; they have been known for a long time, since they are not parti­cularly faint when photographed with large telescopes - it is simply that not until now has it been realized that there is anything remarkable about them. A. Sandage and H. Smith, in America, have studied photographs of 3C-273 taken during the last sixty years, and have found that there are fairly regular changes of from 0-2 to 0-3 magnitude with a period of about thirteen years, with possible 'flashes' of more than half a magni­tude lasting for a few weeks. 3C-273 is a double object, each radio source being about 1,500 light-years in diameter, which is impossibly small for a normal galaxy and impossibly large for an ordinary star. It is hard to see how a QSO can virtually double its luminosity in a few days, when light takes several hundred times as long to travel from one part of it to another. In fact, most of the light from a QSO must be coming from a body whose diameter is less than half a light-year.

Clearly, a QSO cannot be simply a galaxy with more than its fair quota of highly-luminous supergiant stars. In this case, the angular diameters of the objects would be much greater than they actually are, and they would not appear as stellar points. Rather more rational, but profoundly unconvincing, is the idea that the energy source may be due to chains of supernovae.

A supernova is a star which suffers a cataclysmic outburst and destroys itself, shining for a brief period with a luminosity millions of times that of the Sun. Only three have been recorded in our own Galaxy - the stars of 1054, 1572, and (probably) 1604; the first of these has left the mass of gas known as the Crab Nebula. Supernova wrecks are known to be radio emitters, and the Crab Nebula itself is a particularly strong source. On the other hand, it would need a good many supernovae to produce as much energy as is sent out by a QSO. No mechanism is known whereby one supernova could 'trigger off' another, and in any case the period of brilliance would be relatively brief. There is no reason to suppose that the QSO's are temporary features, and few authorities have much faith in the supernova theory. It is worth noting, however, that G. Field has proposed that star formation would occur suddenly in the case of a non-rotating mass of gas which is condensing into a galaxy, so that many supernovae might explode at much the same time several millions of years after the main period of star formation.

More plausible, though highly tentative, is the idea of gravi­tational collapse. Here we begin with a 'star' of fantastic size, perhaps 100 million times as massive as the Sun, and lying inside a galaxy. If it collapsed under its own gravity, the collapse would be catastrophic, so that the super-star would explode inwards. Immense quantities of energy would be produced, certainly enough to account for the strong radio emissions received from QSO's. Significantly, F. Hoyle and A. Fowler had actually discussed the properties of such collapsing heavy bodies about a year before the first QSO's were identified.

This is all very well, but it leads to further difficulties. Though the normal stars show a great range in size and luminosity, they are much more uniform in mass; generally speaking a large star is rarefied, while a small star is dense. A star 100 times as massive as the Sun must be classed as a real freak. If the gravitational collapse theory of QSO's is valid, the original bodies must be entirely different from anything previously suspected, and we can have no clue as to their origin, so that in trying to explain one set of peculiar facts we have merely introduced another. It follows, incidentally, that on this hypothesis a QSO would be comparatively short-lived, and after a period of from 100,000 to 1,000,000 years would evolve into an as yet unknown state. (In itself this would not, of course, be an obstacle, since the changes would still be much too slow to become evident over a great many generations of astronomers!)

It would be much more straightforward to suppose that a QSO is nothing more than the nucleus of a strange sort of galaxy. This is quite possible, since the great distances involved would mean that the fainter parts of the system would be too dim to be observed. Yet as we have seen, there are indications that the majority of the light comes to us from a body which is relatively small, and which may well be gaseous. If this is so, there is no escape from the conclusion that the energy is being produced in some way that we cannot yet visualize. Ordinary nuclear processes, such as those which operate inside stars, are hopelessly inadequate.

The identification of QSO's has come as an astronomical bombshell, and may lead to a drastic revision of many of our cherished ideas about the universe. The problem is obviously linked with that of the origin of the galaxies. At present, the arguments between the supporters of the 'steady-state' and 'evolutionary' theories continue unabated; some authorities consider that the universe has always existed, and that new matter is being continually created out of nothingness, while others hold that the universe began at a set moment in time, is now evolving in a definite pattern, and will eventually die. Radio astronomy holds out the greatest hope of deciding between these rival theories, but up to now it has been supposed that all the very faint and presumably remote radio sources must be galaxies of some sort or other. If a QSO is not a galaxy in the proper sense of the word, then the situation may be very much altered.

At least there can be little doubt that the QSO's are the most extraordinary objects ever discovered. Apparently small by cosmical standards, extremely remote, and of incredible lumin­osity, their nature remains a mystery. Further studies of them may provide at least some of the answers, but the detection of a whole new class of objects, previously unsuspected, brings home to us how little we really know about the universe.