IN THE DEPTHS OF SPACE.*
• The Sustirns of the Stars. By Agnes N. Clarke. London : A. and C. Blook. [20s, net.]
THE advances of astronomy during the last half-century are nowhere more remarkable than in what they have taught us about the:nature and movements of the so-called "fixed stars." This part of astronomy is entirely modern. The early observers—the shepherds who watched their flocks and the skies by night on Chaldean plains, and the pyramid-builders who oriented their vast work by the stars—discovered a great deal about the solar system by patient observation of the planetary movements. Ptolemy was thus able to construct a system of astronomy which, in spite of all the fun that has been poked at it, was quite adequate to explain the apparent motions of the planets, though it lacked the beautiful sim- plicity of the Newtonian or gravitational theory. But the idea that the stars which bedeck the heavens so richly on a clear night are themselves suns like our own—like it moving through space, and perhaps lighting and warming unknown and invisible planets, but at so vast a distance that our most powerful telescope still shows them as mere points of light —is quite modern. One of the strongest objections brought against the epoch-making theory of Copernicus was that, if the earth really travelled round the sun in a vast orbit, the fixed stars ought to present different appearances at half- yearly intervals, just as the landscape seen from the window of a railway carriage keeps changing its apparent form as the observer is carried onward. It took some time for men to familiarise themselves with the idea that the fixed stars are at such immense distances from the earth that scarcely any per- ceptible change is made in their relative positions by our orbital motion. Now we realise that our sun is only one amongst the stars, separated by an almost inconceivable depth of space from his nearest neighbours. Miss Clerke, with her usual power of accurate and lucid exposition, has given us a most fascinating account of all that astronomers have thus far discovered about these immensely distant stars. Her book was first published fifteen years ago, but so many advances have been made since then that this second edition is prac- tically a new work. To the student of astronomy it is an admirable introductionto the sidereal branch of the science, while the intelligent general reader will find it a fascinating account of some of the greatest wonders of Nature.
The first thing to realise about the stars is the immensity of the distances which separate them from one another. Our sun exists in splendid isolation. The only way in which we can measure the distance of a star is by a kind of trigono, metrical survey which takes for its base-line the diameter of the earth's orbit,—the distance between two positions of the earth at intervals of half a year. This base-line is over one hundred and eighty millions of miles in length. Yet it is only by the most minute accuracy of measurement that astronomers have been able to find out that the apparent direction of some
stars does vary as the -earth moves from end to end-of this base-line. If we were to construct a triangle with this line fbr its base, and its sides formed by lines- drawn from either end of the earth's orbit to the nearest star, the angle opposite the base of such a triangle would be less than one second. To all the early observers such an angle was quite imperceptible, imd consequently they were obliged to conclude either that the stars -were at infinite distances, or that the earth was the real centre of their movements. Nowadays, however, it has been found possible to measure the change in direction, or parallax, of a good many stars at half-yearly intervals. Once this angle is known, it is an easy matter to calculate the distance of a star from the earth. The star which is our nearest neighbour has a parallax of about three-quarters of a second. This means, as Miss Clerke puts it, that the earth's orbit as seen from this nearest star would look rather smaller than a sixpence on the weather-cock of the Tate Gallery would look to an observer at Charing Cross. That such minute quantities should be measurable is a con- slam-able tribute to the labours of modern science. If we attempt to express such distances as those of the stars in ordinary units of measurement, we have to deal in numbers which are quite beyond any power of mental realisation. In order to get over this difficulty astronomers have introduced a new unit for the measurement of stellar distances. This is known as the light-year, or the distance which light, travelling at one hundred and eighty-six thousand miles per second, would traverse in a year. The nearest star, Alpha Centauri, is rather more than four light-years from the earth. There are not many stars within twenty or thirty light-years. Some of the most brilliant orbs, like Canopus, Rigel, and Spica, are so distant that they display no measurable parallax, and must be at least three hundred light-years away. It can readily be conceived how gigantic an outflow of light there must be from these distant suns ; Canopus, for instance, must be equal at least to twenty-two thousand stars like our sun. It is rather a good exercise for the mind which is tempted to exaggerate the importance of human affairs to try to picture the con- ditions of existence on one of the planets which may circle such an orb as Canopus.
One very curious consequence follows from this way of measuring stellar distances. When we look at the star- strewn sky we are fathoming the depths, not merely of space, but of time. We see none of the stars as it exactly is at this moment, but as it was when the light which affects our retina set out upon its journey. And, as almost all visible stars are set at very various distances from the earth, we see them all at different periods. If we look at the constellation of Orion, which shines so resplendent in the southern sky during the frosty evenings of winter, we see the bright star Betelgeux at the head of the constellation as it was one hundred and twenty-six years ago, whilst Rigel at the foot is visible by light which must have started at least three centuries before it reaches us. Sirius, however, the brightest star visible in our latitudes, which "bickers into red and emerald" to the south-east of Orion, we see by light which only started eight or nine years ago. It is plausibly suggested that some of the faintest stars visible in our largest telescopes may be as much as thirty thousand light-years away. The new star which recently displayed itself in Perseus was shown by an ingenious train of reasoning to be three hundred light-years distant, so that in 1902 we were able to watch the progress of a stellar conflagration which really occurred about the time of the Spanish Armada. An ingenious French astronomer has based on this fact the pleasant fantasy in which he imagines that a disembodied spirit, able to move with the speed of thought, and endowed with supernatural powers of vision, may at will behold any incident which had ever taken place on the earth under an open sky, by transporting itself through space to the point which the light-waves emitted by that incident have reached in their endless journey. Such a being, placed at the distance of Canopus, might now be watching the massacre of St. Bartholo- mew; and. by travelling thence in a straight line towards the earth it would-be able to pass in panoramic view the whole subse- qpent course of the world's history. Of course, there would be considerable gaps, due to clouds, to the rotation of the earth, and to the fact that a great part of the earth's history has been conducted indoors. But the general idea is perfectly sound. The old superstition of the Recording Angel might be
replaced by this modern discovery of the light-waves which travel for ever out into boundless space with their story of human actions and sufferings.
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