April 17 1963 - Exploding Stars

The appearance of a nova, or 'new star', always causes considerable interest in astronomical circles. The nova of 1963, discovered by E. Dahlgren in Sweden and independently detected by the American observer L. Peltier, was no exception, even though it never became brilliant enough to be striking. It seems to have been a perfectly ordinary nova, and faded in the conventional way, though by mid-1964 it was still observable with moderate-sized tele­scopes.

One of the more interesting astronomical events of 1963 has been the appearance of a comparatively bright nova, or 'new star', in the constellation Hercules. It was discovered by the Swedish amateur Dahlgren, who was using nothing more elaborate than a pair of binoculars, and for some time it was clearly visible to the naked eye. It has proved to be what is termed a 'slow nova', and its fading has been relatively gradual. It has now dropped below naked-eye visibility, but is still an easy telescopic object.

The nova attracted comparatively little attention - except among astronomers — partly because of the spectacular develop­ments in space research taking place during the same period, and partly because it was not well situated for observation during the evenings. Those who were anxious to see it properly had no alternative but to get up in the very early hours of the morning, and it is understandable that the bitter weather made all but the genuine enthusiasts disinclined to do anything of the sort.

However, the nova was not difficult to locate, since it lay fairly close to the brilliant star Vega, in Lyra. For some time there were doubts about whether its position was in Lyra or Hercules, since it was almost exactly on the border between the two constellations; eventually the International Astronomical Union decided that it lay just within the boundary of Hercules.

Dahlgren's star has been the brightest nova for some time. Excluding Nova Argus of 1942, which was extremely difficult to see from England because of its southerly declination, there have been only two brighter novae during the past thirty years; one in Hercules (1934) and the other in Lacerta, the Lizard (1936). There have, however, been half a dozen fainter novae, and it has become clear that such objects are not particularly uncommon. There have in fact been no fewer than eighteen observed novae in our own Galaxy during the present century.

Strictly speaking, a nova is not a 'new' star at all. What apparently happens is that a formerly obscure star suffers an out­burst which results in a striking but temporary increase in lumi­nosity. When the outburst subsides, the star returns to its old state, or basically so - from which it may be inferred that the disturbance is largely limited to the star's outer layers.

It is tempting to explain a nova as the result of a direct collision between two stars. No doubt this would cause an outburst on a grand scale, but we may be sure that the explanation is wrong. The stars are widely scattered in space, and even in the more densely populated parts of the Galaxy collisions must be exces­sively rare. Yet novae, as we have seen, are not rare; nearly thirty have been observed since the invention of the telescope in the first decade of the seventeenth century, and no doubt a great many more have escaped detection. An alternative theory, that a star passes through a dark nebula and interacts with the nebular material, is more plausible but has serious weaknesses, and there can be little doubt that disturbances in the star itself are responsible. Before saying more about the causes of novae, however, it will be as well to deal briefly with some of the more important 'stellar explosions' of modern times.

Particularly notable was Nova Persei 1901, discovered by a Scottish amateur, Dr Anderson. (This, incidentally, was Ander­son's second success; he had also discovered the less spectacular Nova Aurigae 1891.) At its maximum, Nova Persei reached zero magnitude, so that it was fully equal to Vega and Capella. Its rise was extremely rapid: two days before Anderson saw it as a prominent naked-eye object, the region had been photographed at the Harvard Observatory, and it was found that the nova was then a dim object of magnitude 13. It reached maximum a couple of days after its detection, and then began to fade; within a month it had dropped to the fourth magnitude, and continued to fall, though with fluctuations. At last, thirty years after the outburst, it had reached its pre-nova brightness. Its distance is estimated as about 1,550 light-years. The exact value is by no means certain, but we may assume that the explosion actually took place during the far-off days when Britain was still occupied by the Roman legions - though the light-rays did not reach Earth until our own century.

A few months after maximum, Nova Persei was seen to be immersed in what appeared to be an expanding nebula. The rate of expansion was calculated, and proved to be impossibly great. The Dutch astronomer Kapteyn provided what we now know to be the correct explanation. The star was immersed in a cloud of gas and dust which had previously been invisible; after the out­burst, the radiation from the nova lit up the cloud, and of course the inner parts of the cloud were illuminated first, since light travels at a finite velocity. The apparent result was a nebula expanding at the speed of light, whereas in fact there was probably no expansion at all. (Like most or all novas, the star did throw off an expanding shell, but this was a different phenomenon.)

The next really brilliant nova appeared in Aquila, the Eagle, in 1918. It exceeded even Nova Persei, and reached magnitude — 1 - i, so that of all the 'fixed stars' only Sirius outshone it. Here too there was an expanding shell; after eighteen months it had taken on the appearance of a disk about the same apparent size as the planet Neptune, and it continued to increase, at the rate of about two seconds of arc per year, until 1941. After that, the shell became so faint that it could not be observed further.

One of the difficulties about studying the full careers of novae is that we have insufficient knowledge of them in the pre-outburst stage. There are so many faint stars in the sky that we cannot hope to study them all, at least in detail, and a star which suffers a nova outburst does so without the slightest warning. Fortunately, the pre-outburst spectrum of Nova Aquilae has been recorded, and classed as of type A; in other words it had been a white star, with a spectrum not very different from that of Sirius or Vega. It is reasonable to suppose that only comparatively hot stars turn into novae. The Sun, which is a yellow dwarf of spectrum G, does not come into this category.

Passing over the novae of 1920 (in Cygnus, the Swan) and 1925 (in the southern constellation of Pictor, the Painter, invisible in England) we come to the interesting Nova Herculis 1934. It was discovered in December of that year by J. P. M. Prentice, a British amateur who was (and, happily, still is) a celebrated observer of meteors.

Prentice was not searching for novae at the time. He had been studying the Geminid meteor shower, and after a spell of observing he went for a casual stroll; on looking up at the region of the Dragon's head, he suddenly noticed an unfamiliar star of between the third and fourth magnitudes. This nova, too, lay just

within Hercules, not far from the spot where Dahlgren's star appeared a few weeks ago.

Nova Herculis 1934 brightened up to the first magnitude, so that it surpassed the famous Deneb in Cygnus. It had an unusually long maximum, and remained visible to the naked eye for some time. By now it has become extremely faint, but is still visible in moderate telescopes, and is a most interesting object. Apparently it is a binary system; that is to say, there are two stars, moving round their common centre of gravity. One, the old nova, is surrounded by a compact nebula; the other is thought to be a dim Red Dwarf, much feebler than our Sun. The distance is in the region of 750 light-years, so that the outburst actually took place before the signing of the Magna Carta.

Two more conspicuous novae have been seen since then. The 1936 star in the little northern group of Lacerta became as bright as the Pole Star, but faded rapidly; it too threw out a shell of gas which expanded at the unprecedented rate of about 2,400 miles per second. Then, in 1942, came the nova in Argo Navis, the Ship. It lay in that part of Argo known as Puppis (the Poop), but, as noted earlier, was very badly placed for European observers. At its most energetic it is thought to have been at least 1,500,000 times more luminous than the Sun. Mention should also be made of the 1960 nova, again in Hercules. It was discovered by Olaf Hassell, of Norway, and was just visible without a telescope when at maximum.

There seems to be a definite link between novae and some types of irregular variable stars. We even know of 'recurrent novae', which have suffered more than one outburst; T Coronae, for instance, blazed up to the second magnitude in 1866, faded in the usual way, and then had another maximum of the third magnitude in 1946. Entirely different are the rare supernovae, of which three have been seen in our Galaxy during the past 1,000 years.

In 1054 Chinese and Japanese observers noted a strikingly brilliant object in Taurus, the Bull, not far from Orion. It became bright enough to be visible in broad daylight, and lasted for months, but when it faded below naked-eye visibility all track of it was naturally lost. Later, in the telescopic era, a curious gas-cloud was found in precisely the position of the 1054 star; it is now known as the Crab Nebula, and is visible with a very small instrument, though photographs taken with powerful telescopes are necessary to show it properly. There is not the slightest doubt that the Nebula represents the wreck of the old star, and it is equally certain that we are dealing with something much more significant than a normal nova. The gas-cloud is still expanding from the old explosion-centre, and has now attained a diameter of three light- years, or well over 17,000,000,000,000 miles.

Another supernova appeared in 1572 (Tycho's Star, so called because great attention was paid to it by the famous Danish astronomer Tycho Brahe), and it is probable that a star studied by Johann Kepler in 1604 was also a supernova. Since then, none has been recorded in our Galaxy - which is a matter for regret, since the telescopic era has been entirely devoid of them. However, supernovae have often been seen in external galaxies. The most celebrated of these appeared in the Andromeda Galaxy in 1885, and was close to naked-eye visibility even at its colossal distance of over 2,000,000 light-years. At maximum, it may have been 200,000,000 times as luminous as the Sun. Even so, it is not the most luminous supernova ever recorded; this distinction must go to an object which was seen in 1937 in a much more remote system, and seems to have equalled 350,000,000 Suns.

The essential difference between a nova and a supernova is that a nova returns eventually to its old state, whereas a supernova does not. In fact, a supernova outburst destroys the star in its old form. It is significant, too, that the Crab Nebula is an energetic source of radio waves, and it is extremely probable that many of the radio sources in our Galaxy are nothing more nor less than supernova wrecks.

When we return to the more normal novae, and try to find an explanation for them, we are faced with the general problem of how a star radiates. Basically, it is known that the source of stellar energy is to be found in nuclear reactions; in the Sun, for instance, hydrogen is being transformed into helium, with the release of energy and a steady loss of mass. The whole process is delicately balanced, and an average star continues to shine with a more or less constant output for a very long period. It may well be that if the nuclear processes become 'out of control', so to speak, a violent disturbance results, with a nova-like outburst affecting the outer layers. A supernova outburst must presumably affect the whole star, and not merely the outer portion.

Much remains to be learned about these remarkable stellar explosions; modern theories appear to have a solid foundation, but many of the details remain to be worked out. This is why astronomers are anxious to study any conveniently bright novae as closely as possible. The appearance of a supernova in our Galaxy would be a welcome event, and by the law of averages it seems that one is due, but we cannot tell when it will occur; it may be tomorrow, it may be next month, it may not be for centuries to come. At any rate, we may be certain of one thing: when the next galactic supernova blazes forth, it will do so with dramatic suddenness, and will take us by surprise.