THE ORIGIN OF NEW STARS.

By Prof. Sir ROBERT S. BALL, LL.D., F.R.S.

MR. PRESIDENT, LADIES AND GENTLEMEN,-The subject of the address that I propose to give you is "The Origin of New Stars," and the title was naturally suggested by that very remarkable star-the most remarkable of its kind which has appeared for three hundred years-at which we all looked with such interest in the early months of this year.

But I am taking the opportunity to refer to the subject also in a somewhat wider manner--to speak of those processes of change and transformation which we observe actually going on at this moment in the universe around us. First of all as to what we mean by new stars. We look up into the heavens at night and see constellations, and history tells us that those constellations were the same in the days of Homer, and in the days of Job, practically, as they are at present. But we must not imagine that those constellations, and those arrangements of stars are eternal; for, as Professor Hull tells us, the mountains and other features of the earth have been in constant change during the course of geological time, though those changes are not appreciable in historic times; and in like manner these constellations of the sky are not always the same. It is very easy to show that within such a period of time as may be comparable with geological periods, the whole face of the skies, too, will have undergone a complete transformation. We have all seen in museums the ichthyosaurus, the eye of which is a most remarkable optical instrument; but if that animal could ever have glanced up to the skies I think I shall be justified in saying that not a single one of those stars that we now see were then within his ken. The heavens have gradually changed, and in the course of a certain period of time-say ten million years, from what we know of the movements of the stars, there would doubtless be a complete transformation; so that the stars which we see about us now, that may be unchangeable from our ephemeral point of view, are in a state of gradual change. When coming down Channel you see a number of ships about; you do not see much motion in them, if you look again, in

another hour you still see a number of ships about, but they are not the same ships that you saw before. So in the heavens there is a continual flux. The old stars are passing away, and new stars are coming into view.

I will try to illustrate the appearance of this new starthe way in which it came, and the evidence we have of its character. I am fortunate to be able to do this, for by the kindness of friends I have obtained the use of photographs which will show us the peculiar circumstances which mark that star from all other ordinary stars, even if we had not the remarkable circumstances in connection with its sudden outbreak and its still more remarkable decline. I will now ask for the lights to be turned down for a few minutes while we look at the pictures thrown on the

screen.

[A picture was then exhibited.]

That is merely a picture of a small part of the skies-not the whole heavens-not more than a one ten-thousandth part of the heavens. It is a very rich part of the Milky Way, containing innumerable myriads of stars. There is no spot on that diagiam upon which I could place the tip of my finger without hiding some star behind it. We take that as an example of the starry firmament as we see it.

Now we will look at another, giving us a view of a different part of the heavens.

[Exhibited on the screen.]

It would take ten thousand such pictures as you are looking at to cover the whole stars of the heavens, and here, again, there are innumerable myriads of them, and in view of what is to come, I will ask you to retain a general impression of this picture in your minds.

Now we come to a much more special picture.

[Exhibited on the screen.]

This was given to me by Mr. Stanley Williams, and it represents a photograph of a part of the sky on February 22nd. That picture represents all those stars which have, no doubt, been there for thousands of years, and then, a week later, he took this second picture and this is the new star which appeared in the meantime.

[Exhibiting on the screen.]

It shows the sudden way in which that new star burst into view. It greatly increased in lustre until it exceeded,

for a short time, any other star in the northern sky, and then there set in a decline and the star got gradually fainter. It did not go out entirely, but declined, and then every now and then there was a recrudescence. Sometimes the star could not be seen and sometimes it could. It suffered remarkable fluctuations in brightness and then gradually declined, and it had sunk down to a faint star when last seen before the advancing daylight extinguished it.

Now what is the origin of such a star as that? Let us first consider what the stars are. This is a question which depends very much on heat, or the degree of capacity for radiating heat and light that the stars present. Will you imagine this stick to be a thermometer with a long graduated scale, and where I hold my finger to represent the temperature of the bodies when red hot; when you can just see them. Up about here would be the temperature of bodies when they would be as hot as suns; and below would be a very cold temperature-the temperature of space. Such a temperature as this Professor Dewar has shown us by his most remarkable researches to be that at which air freezes to a solid lump. Below the point corresponding to redness an object sends no light that can make it visible, but above that it sends light which will make it visibleaccording to the degree of brightness and other circumstances. I exclude the moon and the planets for the moment. Jupiter is only bright because it reflects back to us a little sunlight. Were Jupiter to pass out to where the stars in that picture are, you would not see it at all; and all the objects we see in the heavens, excluding the planets, are objects which shine by their own light. They are objects of which the temperature would be on this part of the scale, above the red line. But the simplest consideration shows us that every hot body tends to get cold, and every one of those objects that we look at is radiating its heat, and is generally tending to cool, and consequently, tending to come down and pass this line. When once they get down there they may stay at that temperature to all eternity, unless some tremendous change takes place to bring them up again. There is a general tendency of bodies in space to come down to the colder temperatures. When we look up at the heavens above and their myriads of objects, we must remember that we only see the bright objects, the dark ones are invisible. The earth only has a two-thousand millionth part of the light from the sun, and therefore you

see that these stars, or sunlighted bodies and planets around them, only get a two-thousand millionth part of the light and are not seen. The brilliant objects that we see, though they are overwhelmingly numerous, yet they must be absolutely as nothing in comparison with the myriads of dark objects which are totally invisible to us, except when certain very remarkable circumstances arise. The probability is, when we look at the stars, we do not see a thousandth, part of the actual amount of matter up there, because it is much more usual for a body to be dark than it is for that body to be bright. Brightness is a temporary stage, and darkness is a stage of indefinitely long duration.

When we look up at the heavens, I sometimes think that the view we get may be paralleled with that a being from some other part would get of this city of London if, instead of looking at us in daylight, he came and stood on the Monument, at night, and looked down on the sleeping city. What would he see? He would not see the buildings, and of the wondrous life of that city he would see nothingmerely lights here and there dissipating the gloom slightly. If that being were to go away thinking he had seen London, simply because he had seen those lights, how very inadequate would be his knowledge. In a somewhat similar way we must interpret the lights in heaven. It is the dark things that are most numerous, and we have only become acquainted with a very few. Occasionally it happens that by some remarkable incident in the heavens, the dark objects, or some of them, become known to us. We are no longer left merely to conjecture as to their existence, but they became actually apparent; and such an instance we found in that new star which broke out last February in the constellation of Perseus. For the examination of stars in these modern days, methods are provided by the spectroscope. We now analyze them in a way which was impossible before the spectrum analysis was available. I will illustrate this by showing some photographs that were taken by Father Sidgreaves of Stonyhurst College, to whose kindness I am so greatly indebted.

I had the privilege of looking at the spectrum of this new star in Perseus, and it was a most striking sight. If there were nothing else than that spectrum, if nobody knew anything further about the history of the star he would at once have said, "Surely this is a star of the most remarkable character and quite unlike ordinary stars"; for there are

several brilliant lines crossing the spectrum. Here we get nature to speak for herself-we are looking at the actual photographs. In these days there is no excuse for giving illustrations which are not absolute photographs.*

*

The spectroscope shows in Nova Persei the presence of a great mass of blazing incandescent hydrogen, and when you accompany this with the fact that the star suddenly broke out and declined again, it becomes of great interest indeed. I pointed out this statement how in the spectrum of this star the dark lines and bright lines of hydrogen are close side by side. It would take too long to go into more detail; but there is no doubt, from the evidence that these photographs contain, that there were two bodies concerned. Those two bodies were moving with different velocities. The dark line belongs to one and the bright line (speaking generally) may be said to belong to the other. Taking that into account, and taking into account the suddenness of its outbreak, and the indications of blazing hydrogen, it is quite easy for us to form a supposition, which is not an unnatural one, as to what was the cause of that remarkable star. I mentioned that, of the stars above us, the dark stars are in all probability incomparably more numerous than the brilliant ones. We do not see those dark stars under ordinary circumstances; but, in their myriads, it does sometimes happen that one of those dark stars, hurrying along, comes into collision with another. I have no doubt that the occurrence of such a collision is excessively rare. We must remember that these bodies are moving at enormously high velocities in vast numbers, and there is just a possibility of two of them striking. It is not unlike supposing that two rifle shots fired at random, in the air, should strike in the course of their flight. I need not tell you that if one man were firing a rifle in one place and another man a mile away were firing in an opposite direction, that it is most unlikely the two bullets should strike each other. If myriads of rifle bullets were being fired in every sort of direction then it would be conceivable that some pair of those bullets would strike each other. It is not so very unlikely-in fact there is the very best reason for believing that it may sometimes happen. I believe it is recorded that on a field of battle one riffe bullet has been known to pierce another. I think I have

* For spectra of Nova Persei, 1901, see "Observations on the star of Nova Persei," by Sir Norman Lockyer, Proc. Royal Society for Feb. 28, March 7, and June 20, 1901.