573RD ORDINARY GENERAL MEETING, HELD IN THE CONFERENCE HALL, THE CENTRAL HALL, SIR FRANK W. DYSON, F.R.S., ASTRONOMER ROYAL, The Minutes of the preceding Meeting were read and confirmed. The SECRETARY announced the election of Mr. William Barnett, F.R.A.S., as a Member of the Institute, and of Mr. Maurice Gregory and Dr. A. Withers Green as Associates. The CHAIRMAN introduced Professor A. S. Eddington, F.R.S., Plumian Professor of Astronomy in the University of Cambridge, and invited him to deliver his lecture on "The Movements of the Stars." The lecture was illustrated throughout by lantern slides. THE MOVEMENTS OF THE STARS. BY PROFESSOR A. S. EDDINGTON, F.R.S., Plumian Professor of Astronomy in the University of Cambridge. WH HEN you come to hear an astronomical lecture, you come prepared to quit this earth for a time and to take a long journey out into the vast territories of the sky. But the lecturer may lead you a comparatively short journey, or a long one. He may only ask you to accompany him a paltry distance of a few hundred million miles in order to show you Mars or Jupiter or the other planets of the solar system; or perhaps the comets that wander among them may be the subject of his discourse. In that case you are still more or less at home; the same sun which we see in England-sometimes— will light you on your journey, and you do not seek to quit his small empire where he rules supreme. On the other hand, the lecturer may presume further on your acquiescence. He may lead you through the midst of the universe of stars as far as the mind can conceive. That is where I ask you to accompany me to-day. As we pass through their midst, the constellations dissolve into unfamiliar forms. The sun has shrunk to a point of light, and is just one star among the crowd. As for the earth, perhaps it would be best for our sense of proportion if we could forget that so insignificant a globe ever existed. Our journey must be somewhat hurried; if we moved with the speed of light, the exploration of the universe of stars would take thousands of years. We should take four years to reach the nearest star-other than the sun. But we shall move with the speed of thought and leave the laggard light-waves far behind. We are going, then, to consider the stars-the fixed stars they are often called to distinguish them from the planets; but the name is a particularly unfortunate one, since our subject is the movements of these fixed stars." It is a numerous company with which we have to deal. A photograph of the sky shows it crowded with these tiny points of light, and each point means a body of the same character as our sun-a globe of fire which may be anything from a million times the size of the earth upwards. Many of the stars are even bigger and brighter than the sun, only they are so far off as to be reduced to mere points. We can scarcely doubt that some at least of them have families of planets circulating round them, to which they give light and heat as the sun warms and illuminates the earth, but there is no evidence whatever on this point. We ought to begin by getting some idea of the scale of this stellar universe. The stars number some hundreds of millions -a number that is quite inconceivable. I am sure that no astronomer can grasp such numbers, and I doubt whether even the Chancellor of the Exchequer can do so. But, though the number of the stars is vast, it is not a number beyond experience. If we took every star that has been seen or photographed, or indeed every star which could be photographed with the most powerful telescope yet built, and divided them among the inhabitants of the British Empire, it is unlikely that there would be enough stars to go round. But when we come to the distances of the stars, the numbers are to say the least-unusual. The nearest star is distant 24,000,000,000,000 miles, and that is only the beginning, because we must consider some of the most distant stars. However, if I were to add three more noughts on to that last number, that would represent a limit beyond which we shall not attempt to penetrate; in fact, we should be getting near the limits of the stellar system, at least in certain directions. The distance which separates the sun from the nearest star is much the same as that which separates any typical star from its nearest neighbour. To form some idea of the sparsity of the stars in space, take a sphere with the sun as centre and radius a hundred billion miles (four times the distance of the nearest star); this would contain about thirty stars-we actually know the identical stars contained, or most of them. On a smaller scale this would be represented by thirty tennis-balls distributed through a volume of space as large as the earth. Imagine thirty tennis-balls wandering about in the whole interior of the earth; that represents the fine-grainedness and sparsity of stars in a typical portion of space. There is plenty of room for the stars to move without much fear of collisions. We are often inclined to think of the celestial bodies as moving under nicely-balanced forces, each with its own path arranged to prevent disaster; however that may be in the solar system, there is no need for regulation of the traffic in interstellar space. There is any amount of room for each star to take its own course, and the duty of a look-out man would be a sinecure. That being the case, are we to regard each star as an independent islet in space, unrelated to any other? That is the grand question of stellar astronomy. With thirty tennis-balls distributed through the whole terrestrial globe, can we imagine anything more unlikely than that any connection should bind one to another? Is each star an independent entity; its birth, its motion and its history having no relation to any other? are there signs of some community of origin by which we can group the stars into relationship? In fact, is the universe a chaos or a system? I use the word chaos in no depreciatory sense, for it is one of the beautiful discoveries of science that out of chaos proceed some of the most simple and uniform laws of Nature. Various suggestions were at one time made that the stars revolved around some central sun. Alcyone in the Pleiades was suggested for this centre, for reasons which seem to have been more sentimental than scientific; but we now know there is no simple arrangement of that sort. Very recently, however, there have been discovered anomalies in the way stars are moving, which, however they may be interpreted, forbid us to think of the universe as a pure chaos of stars. There seems to be some sort of association between even the most widely separated stars. The clue to these associations is in the movements of the stars. In the early days of astronomy the fixed stars were regarded as marking out a definite background against which we could record the movements of the wandering stars or planets. They were like the figures on the dial of a clock by which we tell how the hands are moving. But in 1718 Halley, just before he became Astronomer Royal, made the discovery that some at least of the fixed stars were in motion. The star on which in particular he based this conclusion was Arcturus; there was no doubt that this star was changing its position with respect to the surrounding stars. It is interesting that this famous old star-mentioned, as we know, in the Book of Job— should be the one to open up a new branch of astronomy. Now what does the change of position amount to? We now know that Arcturus is exceptionally fast-moving, but not the fastest; in fact, about twenty stars are known to exceed it in speed. I am speaking here of apparent rate of progress across the sky, not the actual velocity in miles per hour. The apparent rate is, of course, influenced by the nearness or distance of the star. The quickest of all is a telescopic star in the Southern Hemisphere (C.Z. 5h. 243), which travels at the rate of nine seconds of arc per year. As that may not convey much impression to you, I will put it another way. You know Orion and the three stars that form his belt. I will use the belt as a sort of standard race-track in the sky. The fastest star would take 1,050 years to travel from one end to the other of Orion's belt. That does not seem a very rapid rate, but still it is something quite appreciable without need for specially refined measures. Arcturus would take about 3,000 years to do the same course. But speeds so great as this are quite exceptional; a sort of average motion would be about one-twentieth of a second per year, or Orion's belt in 180,000 years. That is getting down to something very minute, but still it is quite practicable to detect this and even considerably smaller movements with certainty. We have now at our disposal the measured movements of some thousands of stars, which we may proceed to examine. There are a number of cases in which these motions reveal at once connections between stars which are certainly widely separated from one another, and between which we should scarcely have expected that any relation could exist. Incidentally we find that many pairs of stars near together in the sky move along together, and in such a group as the Pleiades all the stars have a common motion; but this tendency to a common motion is found in some much more widely scattered stars. we select a certain region of the sky comprising Perseus and parts of the surrounding constellations, and take, not all the stars, but those characterized as particularly white-hot-as we should say, stars of the helium type of spectrum-it is found that these stars by their movements are sorted out into two distinct groups. The stars of one set are moving moderately If fast, and the motion is the same in amount and direction for all of them (within the limits of observational error); these must clearly be associated. When we have picked out the stars of this group by their characteristic movements, and turn to see where they really are in the sky, we find that they form a long, rather open, chain stretching over quite a large arc in the skylike a row of skirmishers advancing together. Turning for a moment to the other set, we find that they are characterised by extremely small motions, scarcely detectable. We cannot infer that there is any relationship between these. The small motion may, and probably does, mean that all of these stars are extremely distant; the actual movements may be quite diverse and unconnected, but distance has diminished the scale so that we can scarcely observe their differences of motion. We have, therefore, detected a "moving cluster" of helium stars in the Perseus region, each moving with the same velocity across a background of much more distant stars of the same type. Another case of this kind is afforded by the Great Bear. Of the well-known stars forming the Plough, the five middle ones share just the same motion. The tip star of the tail and one of the Pointers do not belong-their presence is only accidentalbut the other five are moving in exactly the same direction with the same speed as accurately as we can detect. But there is a still more curious fact: the Dog-star, Sirius, is also a member of this system. We happen to know the motion of Sirius thoroughly -not merely its apparent progress across the sky, but its actual linear motion in three-dimensional space-and it fits in exactly. The evidence is the more convincing because the system happens to be proceeding in a direction which is very unusual. (We shall see later that some directions of motion are much more common than others.) Very few stars are taking a course at all approximating to that of the Great Bear system; so when we find Sirius going in just this direction with just the right speed it is a pretty clear case. There are a few other stars in different parts of the sky which also seem to belong to this system, but we are not so certain of them as we are of Sirius. You now see that the constellations in the sky do not correspond well with the real relations of the stars. Taking the Great Bear, we have had to cut off two stars which are not really of that system, whereas Sirius, which is quite the other side of the sky, apparently in the Great Dog, must really (according to physical relations) count as part of the Great Bear. If only this had been known to the old mythologists I am sure they would D |