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VOL. LXX

JULY, 1921

NO. 1

GIANT STARS

BY GEORGE ELLERY HALE

Director of the Mount Wilson Observatory of the Carnegie Institution of Washington

ILLUSTRATIONS FROM PHOTOGRAPHS TAKEN AT THE MOUNT WILSON OBSERVATORY

UR ancestral sun, as pictured by Laplace, originally extended in a state of luminous vapor beyond the boundaries of the solar system. Rotating upon its axis, it slowly contracted through loss of heat by radiation, leaving behind it portions of its mass, which condensed to form the planets. Still gaseous, though now denser than water, it continues to pour out the heat on which our existence depends as it shrinks imperceptibly toward its ultimate condition of a cold and darkened globe.

volume is more than a million times that of the earth.

But what of the stars, proved by the spectroscope to be self-luminous, intensely hot, and formed of the same chemical elements that constitute 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?

Prior to the application of the powerful new engine of research described in Laplace's hypothesis has been sub- this article we have had no means of jected in recent years to much criticism, measuring the diameters of the stars. and there is good reason to doubt whether We have measured their distances and his description of the mode of evolution their motions, determined their chemical of our solar system is correct in every composition, and obtained undeniable particular. All critics agree, however, evidence of progressive development, but that the sun was once enormously larger even in the most powerful telescopes their than it now is, and that the planets orig- images are so minute that they appear as inally formed part of its distended mass. points rather than as disks. In fact, the Even in its present diminished state, larger the telescope and the more perfect the sun is huge beyond easy conception. the atmospheric conditions at the obOur own earth, though so minute a frag- server's command, the smaller do these ment of the primeval sun, is nevertheless images appear. On the photographic so large that some parts of its surface have plate, it is true, the stars are recorded as not yet been explored. Seen beside the measurable disks, but these are due to sun, by an observer on one of the planets, the spreading of the light from their the earth would appear as an insignificant bright point-like images, and their diamspeck, which could be swallowed with eters increase as the exposure time is proease by the whirling vortex of a sun-spot. longed. From the images of the brighter If the sun were hollow, with the earth at stars rays of light project in straight lines, its centre, the moon, though 240,000 miles but these also are instrumental phenomfrom us, would have room and to spare ena, due to diffraction of light by the steel in which to describe its orbit, for the sun bars that support the small mirror in the is 866,000 miles in diameter, so that its tube of reflecting telescopes. In a word, Copyrighted in 1921 in United States, Canada, and Great Britain, by Charles Scribner's Sons. Printed in New York. All rights reserved.

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.

How, then, maye 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 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 Sèvres, 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.

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 un

used 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 Stéphan, 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 seconds 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 Stéphan 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 100inch 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 dis

See SCRIBNER'S MAGAZINE for October, 1920.)

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The sun, 866,000 miles in diameter, from a direct photograph showing many sun-spots. 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.

tion of war problems.* In 1919, as soon as the 100-inch telescope had been completed and tested, the work was resumed on Mount Wilson.

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 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.

The object-glass 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-glass 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

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