Читать книгу Practical Talks by an Astronomer - Harold Jacoby - Страница 7

Table of Contents

One of the most brilliant observations of the last few years is Campbell's recent discovery of the triple character of this star. Centuries and centuries ago, when astronomy, that venerable ancient among the sciences, was but an infant, the pole-star must have been considered the very oldest of observed heavenly bodies. In the beginning it was the only sure guide of the navigator at night, just as to this day it is the foundation-stone for all observational stellar astronomy of precision. There has never been a time in the history of astronomy when the pole-star might not have been called the most frequently measured object in the sky of night. So it is indeed strange that we should find out something altogether new about it after all these ages of study.

But the importance of the discovery rests upon a surer foundation than this. The method by which it has been made is almost a new one in the science. A generation ago, men thought the "perfect science," for so we love to call astronomy, could advance only by increasing a little the exact precision of observation. The citadel of perfect truth might be more closely invested; the forces of science might push forward step by step; the machinery of research might be strengthened, but that a new engine of investigation would be discovered capable of penetrating where no telescope can ever reach, this, indeed, seemed far beyond the liveliest hope of science. Even the discoverer of the spectroscope could never have dreamed of its possibilities, could never have foreseen its successes, its triumphs.

The very name of this instrument suggests mystery to the popular mind. It is set down at once among the things too difficult, too intricate, too abstruse to understand. Yet in its essentials there is nothing about the spectroscope that cannot be made clear in a few words. Even the modern "undulatory theory" of light itself is terrible only in the length of its name. Anyone who has seen the waves of ocean roll, roll, and ever again roll in upon the shore, can form a very good notion of how light moves. 'Tis just such a series of rolling waves; started perhaps from some brilliant constellation far out upon the confining bounds of the visible universe, or perhaps coming from a humble light upon the student's table; yet it is never anything but a succession of rolling waves. Only, unlike the waves of the sea, light waves are all excessively small. We should call one whose length was a twenty-thousandth of an inch a big one!

Now the human eye possesses the property of receiving and understanding these little waves. The process is an unconscious one. Let but a set of these tiny waves roll up, as it were, out of the vast ocean of space and impinge upon the eye, and all the phenomena of light and color become what we call "visible." We see the light.

And how does all this find an application in astronomy? Not to enter too much into technical details, we may say that the spectroscope is an instrument which enables us to measure the length of these light waves, though their length is so exceedingly small. The day has indeed gone by when that which poets love to call the Book of Nature was printed in type that could be read by the eye unaided. Telescope, microscope, and spectroscope are essential now to him who would penetrate any of Nature's secrets. But measurements with a telescope, like eye observations, are limited strictly to determining the directions in which we see the heavenly bodies. Ever since the beginning of things, when old Hipparchus and Ulugh Beg made the first rude but successful attempts to catalogue the stars, the eye and telescope have been able to measure only such directions. We aim the telescope at a star, and record the direction in which it was pointed. Distances in astronomy can never be measured directly. All that we know of them has been obtained by calculations based upon the Newtonian law of gravitation and observations of directions.

Now the spectroscope seems to offer a sort of exception to this rule. Suppose we can measure the wave-lengths of the light sent us from a star. Suppose again that the star is itself moving swiftly toward us through space, while continually setting in motion the waves of light that are ultimately to reach the waiting astronomer. Evidently the light waves will be crowded together somewhat on account of the star's motion. More waves per second will reach us than would be received from a star at rest. It is as though the light waves were compressed or shortened a little. And if the star is leaving us, instead of coming nearer, opposite effects will occur. We have then but to compare spectroscopically starlight with some artificial source of light in the observatory in order to find out whether the star is approaching us or receding from us. And by a simple process of calculation this stellar motion can be obtained in miles per second. Thus we can now actually measure directly, in a certain sense, linear speed in stellar space, though we are still without the means of getting directly at stellar distances.

But the most wonderful thing of all about these spectroscopic measures is the fact that it makes no difference whatever how far away is the star under observation. What we learn through the spectroscope comes from a study of the waves themselves, and it is of no consequence how far they have travelled, or how long they have been a-coming. For it must not be supposed that these waves consume no time in passing from a distant star to our own solar system. It is true that they move exceeding fast; certainly 180,000 miles per second may be called rapid motion. But if this cosmic velocity of light is tremendous, so also are cosmic distances correspondingly vast. Light needs to move quickly coming from a star, for even at the rate of motion we have mentioned it requires many years to reach us from some of the more distant constellations. It has been well said that an observer on some far-away star, if endowed with the power to see at any distance, however great, might at this moment be looking on the Crusaders proceeding from Europe against the Saracen at Jerusalem. For it is quite possible that not until now has the light which would make the earth visible had time to reach him. Yet distant as such an observer might be, light from the star on which he stood could be measured in the spectroscope, and would infallibly tell us whether the earth and star are approaching in space or gradually drawing farther asunder.

The pole-star is not one of the more distant stellar systems. We do not know how far it is from us very exactly, but certainly not less than forty or fifty years are necessary for its light to reach us. The star might have gone out of existence twenty years ago, and we not yet know of it, for we would still be receiving the light which began its long journey to us about 1850 or 1860. But no matter what may be its distance, Campbell found by careful observations, made in the latter part of 1896, that the pole-star was then approaching the earth at the rate of about twelve miles per second. So far there was nothing especially remarkable. But in August and September of the present year twenty-six careful determinations were made, and these showed that now the rate of approach varied between about five and nine miles per second. More astonishing still, there was a uniform period in the changes of velocity. In about four days the rate of motion changed from about five to nine miles and back again. And this variation kept on with great regularity. Every successive period of four days saw a complete cycle of velocity change forward and back between the same limits. There can be but one reasonable explanation. This star must be a double, or "binary" star. The two components, under the influence of powerful mutual gravitational attraction, must be revolving in a mighty orbit. Yet this vast orbit, as a whole, with the two great stars in it, must be approaching our part of the universe all the time. For the spectroscope shows the velocity of approach to increase and diminish, indeed, but it is always present. Here, then, is this great stellar system, having a four-day revolution of its own, and yet swinging rapidly through space in our direction. Nor is this all. One of the component stars must be nearly or quite dark; else its presence would infallibly be detected by our instruments.

And now we come to the most astonishing thing of all. How comes it that the average rate of approach of the "four-day system," as a whole, changed between 1896 and 1899? In 1896 only this velocity of the whole system was determined, the four-day period remaining undiscovered until the more numerous observations of 1899. But even without considering the four-day period, the changing velocity of the entire system offers one of those problems that exact science can treat only by the help of the imagination. There must be some other great centre of attraction, some cosmic giant, holding the visible double pole-star under its control. Thus, that which we see, and call the pole-star, is in reality threading its path about the third and greatest member of the system, itself situated in space, we know not where.

Spiral Nebula in Constellation Leo.

Photographed by Keeler, February 24, 1900.

Exposure, three hours, fifty minutes.