It is a clear night, nearly all the world is asleep, when an astronomer crosses his lawn on his way to his observatory to spend the dark hours in making investigations into profound space. His brilliant mind, following the rays of light which shoot from the furthest star, will traverse immeasurable distances, while the body is forgotten. Just before entering the observatory he pauses and looks up; his eye catches sight of something that arrests him, and he stops involuntarily. Yet any stranger standing beside him, and gazing where he gazes, would see nothing unusual. There is no fiery comet with its tail stretching across from zenith to horizon, no flaming meteor dashing across the darkened sky. But that there is something unusual to be seen is evident, for the astronomer breathes quickly, and after another earnest scrutiny of the object which has attracted him, he rushes into the observatory, searches for a star-chart, and examines attentively that part of the sky at which he has been gazing. He runs his finger over the chart: here and there are the well-known stars that mark that constellation, but here? In that part there is no star marked, yet he knows, for his own eyes have told him but a few moments ago, that here there is actually blazing a star, not large, perhaps, but clear enough to be seen without a telescope—a star, maybe, which no eye but his has yet observed!

He hurries to his telescope, and adjusts it so as to bring the stranger into the field of view. A new star! Whence has it come? What does it mean?

By the next day at the latest the news has flown over the wires, and all the scientific world is aware that a new star has been detected where no star ever was seen before. Hundreds of telescopes are turned on to it; its spectrum is noted, and it stands revealed as being in a state of conflagration, having blazed up from obscurity to conspicuousness. Night after night its brilliance grows, until it ranks with the brightest stars in heaven, and then it dies down and grows dim, gradually sinking—sinking into the obscurity from whence it emerged so briefly, and its place in the sky knows it no more. It may be there still, but so infinitely faint and far away that no power at our command can reveal it to us. And the amazing part of it is that this huge disaster, this mighty conflagration, is not actually happening as it is seen, but has happened many hundreds of years ago, though the message brought by the light carrier has but reached us now.

There have not been a great many such outbursts recorded, though many may have taken place unrecorded, for even in these days, when trained observers are ceaselessly watching the sky, 'new' stars are not always noticed at once. In 1892 a new star appeared, and shone for two months before anyone noticed it. This particular one never rose to any very brilliant size. I twas situated in the constellation of Auriga, and was noticed on February 1. It remained fairly bright until March 6, when it began to die down; but it has now sunk so low that it can only be seen in the very largest telescopes.

Photography has been most useful in recording these stars, for when one is noticed it has sometimes been found that it has been recorded on a photographic plate taken some time previously, and this shows us how long it has been visible. More and more photography becomes the useful handmaid of astronomers, for the photographic prepared plate is more sensitive to rays of light than the human eye, and, what is more useful still, such plates retain the rays that fall upon them, and fix the impression. Also on a plate these rays are cumulative—that is to say, if a very faint star shines continuously on a plate, the longer the plate is exposed, within certain limits, the clearer will the image of that star become, for the light rays fall one on the top of the other, and tend to enforce each other, and so emphasize the impression, whereas with our eyes it is not the same thing at all, for if we do not see an object clearly because it is too faint, we do not see it any better, however much we may stare at the place where it ought to be. This is because each light ray that reaches our eye makes its own impression, and passes on; they do not become heaped on each other, as they do on a photographic plate.

One variable star in Perseus, discovered in 1901, rose to such brilliancy that for one night it was queen of the Northern Hemisphere, outshining all the other first-class stars.

It rose into prominence with wonderful quickness, and sank equally fast. At its height it outshone our sun eight thousand times! This star was so far from us that it was reckoned its light must take about three hundred years to reach us, consequently the great conflagration, or whatever caused the outburst, must have taken place in the reign of James I., though, as it was only seen here in 1901, it was called the new star of the new century.

When these new stars die down they sometimes continue to shine faintly for a long time, so that they are visible with a telescope, but in other cases they may die out altogether. We know very little about them, and have but small opportunity for observing them, and so it is not safe to hazard any theories to account for their peculiarities. At first men supposed that the great flame was made by a violent collision between two bodies coming together with great velocity so that both flared up, but this speculation has been shown by the spectroscope to be improbable, and now it is supposed by some people that two stars journeying through space may pass through a nebulous region, and thus may flare up, and such a theory is backed up by the fact that a very great number of such stars do seem to be mixed up in some strange way with a nebulous haze.

All these new stars that we have been discussing so far have only blazed up once and then died down, but there is another class of stars quite as peculiar, and even more difficult to explain, and these are called variable stars. They get brighter and brighter up to a certain point, and then die down, only to become bright once more, and these changes occur with the utmost regularity, so that they are known and can be predicted beforehand. This is even more unaccountable than a sudden and unrepeated outburst, for one can understand a great flare-up, but that a star should flare and die down with regularity is almost beyond comprehension. Clearly we must look further than before for an explanation. Let us first examine the facts we know. Variable stars differ greatly from each other. Some are generally of a low magnitude, and only become bright for a short time, while others are bright most of the time and die down only for a short time. Others become very bright, then sink a little bit, but not so low as at first; then they become bright again, and, lastly, go right down to the lowest point, and they keep on always through this regular cycle of changes. Some go through the whole of these changes in three days, and others take much longer. The periods, as the intervals between the complete round of changes are called, vary, in fact, between three days and six hundred! It may seem impossible that changes covering so long as six hundred days could be known and followed, but there is nothing that the patience of astronomers will not compass.

One very well-known variable star you can see for yourselves, and as an ounce of observation is worth a pound of hearsay, you might take a little trouble to find it. Go out on any clear starlight night and look. Not very far from Cassiopeia (W.), to the left as you face it, are three bright stars running down in a great curve. These are in the constellation called Perseus, and a little to the right of the middle and lowest one is the only variable star we can see in the sky without a telescope.

This is Algol. For the greater part of three days he is a bright star of about the second magnitude, then he begins to fade, and for four and a half hours grows steadily dimmer. At the dimmest he remains for about twenty minutes, and then rises again to his ordinary brightness in three and a half hours. How can we explain this? You may possibly be able to suggest a reason. What do you say to a dark body revolving round Algol, or, rather, revolving with him round a common centre of gravity? If such a thing were indeed true, and if such a body happened to pass between us and Algol at each revolution, the light of Algol would be cut off or eclipsed in proportion to the size of such a body. If the dark body were the full size of Algol and passed right between him and us, it would cut off all the light, but if it were not quite the same size, a little would still be seen. And this is really the explanation of the strange changes in the brightness of Algol, for such a dark body as we are imagining does in reality exist. It is a large dark body, very nearly as large as Algol himself, and if, as we may conjecture, it is a mighty planet, we have the extraordinary example of a planet and its sun being nearly the same size. We have seen that the eclipse happens every three days, and this means, of course, that the planetary body must go round its sun in that time, so as to return again to its position between us and him, but the thing is difficult to believe. Why, the nearest of all our planets to the sun, the wee Mercury, takes eighty-seven days to complete its orbit, and here is a mighty body hastening round its sun in three! To do this in the time the large dark planet must be very near to Algol; indeed, astronomers have calculated that the surfaces of the two bodies are not more than about two million miles apart, and this is a trifle when we consider that we ourselves are more than forty-six times as far as that from the sun. At this distance Algol, as observed from the planet, will fill half the sky, and the heat he gives out must be something stupendous. Also the effects of gravitation must be queer indeed, acting on two such huge bodies so close together. If any beings live in such a strange world, the pull which draws them to their mighty sun must be very nearly equal to the pull which holds them to their own globe; the two together may counteract each other, but the effect must be strange!

From irregularities in the movements of Algol it has been judged that there may be also in the same system another dark body, but of it nothing has been definitely ascertained.

But all variable stars need not necessarily be due to the light being intercepted by a dark body. There are cases where two bright stars in revolving round each other produce the same effect; for when seen side by side the two stars give twice as much light as when one is hidden behind the other, and as they are seen alternately side by side and in line, they seem to alter regularly in lustre.