Part 2 (1/2)
Laplace's hypothesis has been subjected in recent years to much criticism, and there is good reason to doubt whether his description of the mode of evolution of our solar system is correct in every particular. All critics agree, however, that the sun was once enormously larger than it now is, and that the planets originally formed part of its distended ma.s.s.
Even in its present diminished state, the sun is huge beyond easy conception. Our own earth, though so minute a fragment of the primeval sun, is nevertheless so large that some parts of its surface have not yet been explored. Seen beside the sun, by an observer on one of the planets, the earth would appear as an insignificant speck, which could be swallowed with ease by the whirling vortex of a sun-spot. If the sun were hollow, with the earth at its centre, the moon, though 240,000 miles from us, would have room and to spare in which to describe its...o...b..t, for the sun is 865,000 miles in diameter, so that its volume is more than a million times that of the earth.
[Ill.u.s.tration: Fig. 19. Gaseous prominence at the sun's limb, 140,000 miles high (Ellerman).
Photographed with the spectroheliograph, using the light emitted by glowing calcium vapor. The comparative size of the earth is indicated by the white circle.]
But what of the stars, proved by the spectroscope to be self-luminous, intensely hot, and formed of the same chemical elements that const.i.tute the sun and the earth? Are they comparable in size with the sun? Do they occur in all stages of development, from infancy to old age?
And if such stages can be detected, do they afford indications of the gradual diminution in volume which Laplace imagined the sun to experience?
[Ill.u.s.tration: Fig. 20. The sun, 865,000 miles in diameter, from a direct photograph showing many sun-spots (Whitney)
The small black disk in the centre represents the comparative size of the earth, while the circle surrounding it corresponds in diameter to the orbit of the moon.]
STAR IMAGES
Prior to the application of the powerful new engine of research described in this article we have had no means of measuring the diameters of the stars. We have measured their distances and their motions, determined their chemical composition, and obtained undeniable evidence of progressive development, but even in the most powerful telescopes their images are so minute that they appear as points rather than as disks. In fact, the larger the telescope and the more perfect the atmospheric conditions at the observer's command, the smaller do these images appear. On the photographic plate, it is true, the stars are recorded as measurable disks, but these are due to the spreading of the light from their bright point-like images, and their diameters increase as the exposure time is prolonged.
From the images of the brighter stars rays of light project in straight lines, but these also are instrumental phenomena, due to diffraction of light by the steel bars that support the small mirror in the tube of reflecting telescopes. In a word, the stars are so remote that the largest and most perfect telescopes show them only as extremely minute needle-points of light, without any trace of their true disks.
[Ill.u.s.tration: Fig. 21. Great sun-spot group, August 8, 1917 (Whitney).
The disk in the corner represents the comparative size of the earth.]
How, then, may we hope to measure their diameters? By using, as the man of science must so often do, indirect means when the direct attack fails. Most of the remarkable progress of astronomy during the last quarter-century has resulted from the application of new and ingenious devices borrowed from the physicist. These have multiplied to such a degree that some of our observatories are literally physical laboratories, in which the sun and stars are examined by powerful spectroscopes and other optical instruments that have recently advanced our knowledge of physics by leaps and bounds. In the present case we are indebted for our star-measuring device to the distinguished physicist Professor Albert A. Michelson, who has contributed a long array of novel apparatus and methods to physics and astronomy.
THE INTERFEROMETER
The instrument in question, known as the interferometer, had previously yielded a remarkable series of results when applied in its various forms to the solution of fundamental problems. To mention only a few of those that have helped to establish Michelson's fame, we may recall that our exact knowledge of the length of the international metre at Sevres, the world's standard of measurement, was obtained by him with an interferometer in terms of the invariable length of light-waves. A different form of interferometer has more recently enabled him to measure the minute tides within the solid body of the earth--not the great tides of the ocean, but the slight deformations of the earth's body, which is as rigid as steel, that are caused by the varying attractions of the sun and moon. Finally, to mention only one more case, it was the Michelson-Morley experiment, made years ago with still another form of interferometer, that yielded the basic idea from which the theory of relativity was developed by Lorentz and Einstein.
[Ill.u.s.tration: Fig. 22. Photograph of the hydrogen atmosphere of the sun (Ellerman).
Made with the spectroheliograph, showing the immense vortices, or whirling storms like tornadoes, that centre in sun-spots. The comparative size of the earth is shown by the white circle traced on the largest sun-spot.]
The history of the method of measuring star diameters is a very curious one, showing how the most promising opportunities for scientific progress may lie unused for decades. The fundamental principle of the device was first suggested by the great French physicist Fizeau in 1868. In 1874 the theory was developed by the French astronomer Stephan, who observed interference fringes given by a large number of stars, and rightly concluded that their angular diameters must be much smaller than 0.158 of a second of arc, the smallest measurable with his instrument. In 1890 Michelson, unaware of the earlier work, published in the _Philosophical Magazine_ a complete description of an interferometer capable of determining with surprising accuracy the distance between the components of double stars so close together that no telescope can separate them.
He also showed how the same principle could be applied to the measurement of star diameters if a sufficiently large interferometer could be built for this purpose, and developed the theory much more completely than Stephan had done. A year later he measured the diameters of Jupiter's satellites by this means at the Lick Observatory. But nearly thirty years elapsed before the next step was taken. Two causes have doubtless contributed to this delay. Both theory and experiment have demonstrated the extreme sensitiveness of the ”interference fringes,” on the observation of which the method depends, and it was generally supposed by astronomers that disturbances in the earth's atmosphere would prevent them from being clearly seen with large telescopes. Furthermore, a very large interferometer, too large to be carried by any existing telescope, was required for the star-diameter work, though close double stars could have been easily studied by this device with several of the large telescopes of the early nineties. But whatever the reasons, a powerful method of research lay unused.
The approaching completion of the 100-inch telescope of the Mount Wilson Observatory led me to suggest to Professor Michelson, before the United States entered the war, that the method be thoroughly tested under the favorable atmospheric conditions of Southern California. He was at that time at work on a special form of interferometer, designed to determine whether atmospheric disturbances could be disregarded in planning large-scale experiments. But the war intervened, and all of our efforts were concentrated for two years on the solution of war problems.[*] In 1919, as soon as the 100-inch telescope had been completed and tested, the work was resumed on Mount Wilson.
[Footnote *: Professor Michelson's most important contribution during the war period was a new and very efficient form of range-finder, adopted for use by the U. S. Navy.]
A LABORATORY EXPERIMENT
The principle of the method can be most readily seen by the aid of an experiment which any one can easily perform for himself with simple apparatus. Make a narrow slit, a few thousandths of an inch in width, in a sheet of black paper, and support it vertically before a brilliant source of light. Observe this from a distance of 40 or 50 feet with a small telescope magnifying about 30 diameters.
The object-gla.s.s of the telescope should be covered with an opaque cap, pierced by two circular holes about one-eighth of an inch in diameter and half an inch apart. The holes should be on opposite sides of the centre of the object-gla.s.s and equidistant from it, and the line joining the holes should be horizontal. When this cap is removed the slit appears as a narrow vertical band with much fainter bands on both sides of it. With the cap in place, the central bright band appears to be ruled with narrow vertical lines or fringes produced by the ”interference”[*] of the two pencils of light coming through different parts of the object-gla.s.s from the distant slit. Cover one of the holes, and the fringes instantly disappear. Their production requires the joint effect of the two light-pencils.
[Footnote *: For an explanation of the phenomena of interference, see any encyclopaeedia or book on physics.]
Now suppose the two holes over the object-gla.s.s to be in movable plates, so that their distance apart can be varied. As they are gradually separated the narrow vertical fringes become less and less distinct, and finally vanish completely. Measure the distance between the holes and divide this by the wavelength of light, which we may call 1/50000 of an inch. The result is the angular width of the distant slit. Knowing the distance of the slit, we can at once calculate its linear width. If for the slit we subst.i.tute a minute circular hole, the method of measurement remains the same, but the angular diameter as calculated above must be multiplied by 1.22.[*]
[Footnote *: More complete details may be found in Michelson's Lowell Lectures on ”Light-Waves and Their Uses,” University of Chicago Press, 1907.]