Part 11 (1/2)
In dealing with problems of structure and motion in the Galactic system, the 100-inch telescope offers especial advantages, because of its vast light-gathering power. Studies of radial velocities of the stars have hitherto been necessarily confined to the brighter stars, for the most part even to those visible to the naked eye. While some of these are very distant, most of the stars whose radial velocities are known belong to a very limited group, perhaps const.i.tuting a distinct cl.u.s.ter of which the sun is a member, but in any event of insignificant proportions when contrasted with the Galaxy. Current spectrographic work with the 60-inch telescope includes stars of the eighth magnitude, and some even fainter. But while the 60-inch has enabled Adams to measure the distances of many remote stars by his new spectroscopic method, and to double the known extent (so far as spectroscopic evidence is concerned) of the star streams of Kapteyn, a much greater advance into s.p.a.ce is necessary to find out the community of motion among the stars comprising the Galactic system. The Hooker telescope will enable us to determine accurate radial velocities to stars of the eleventh magnitude, which doubtless truly represent the Galaxy.
In order to secure a maximum return within a reasonable period of time, the stars in the selected areas of Kapteyn will be given the preference, because of the vast amount of work already done, relating to their positions, proper motions, and visual and photographic magnitudes. Such consideration as spectral type, the known directions of star-streaming, and the position of the chosen regions with reference to the plane of the Galaxy are given adequate weight, and it is of fundamental importance that the method of spectroscopic parallaxes will permit dwarf stars to be distinguished from stars that are in the giant cla.s.s, but rendered faint by their much greater distance. In addition to these problems, the stellar spectrograms will provide rich material for study of the relations.h.i.+p between stellar ma.s.s and speed, and the nature of giant stars and dwarf stars.
Shapley's recent studies of globular cl.u.s.ters have indicated the significance of these objects in both evolutional and structural problems, and the possibility of determining their parallaxes by a number of independent methods is of prime importance, both in its bearing on the structure of the universe and because it permits a host of apparent magnitudes to be at once transformed into absolute magnitudes. Here the advantage of the Hooker telescope is two-fold: at its 134-foot focus the increased scale of the crowded cl.u.s.ters makes it possible to select separate stars for spectrum photography (which could not be done with the 60-inch where the images were commingled); and the great gain in light is such that the spectra of stars to the 14th magnitude have been photographed in less than an hour.
Faint globular cl.u.s.ters, then, will comprise a large part of the early program with the 100-inch telescope: the faintest possible stars in them must be detected and their magnitudes and colors measured; spectral types must be determined, and the radial velocities of individual stars and of cl.u.s.ters as a whole; spectroscopic evidence of possible axial rotation of globular cl.u.s.ters must be searched for; and the method of spectroscopic parallaxes, as well as other methods, must be applied to ascertaining the distances of these cl.u.s.ters.
The possibility of dealing with many problems relating to the distribution and evolution of the faintest stars depends upon the establishment of photographic and photovisual magnitude scales. Below the twelfth magnitude, the only existing scale of standard visual or photovisual magnitudes is the Mount Wilson sequence, already extended by Seares to magnitude 17.5 with the 60-inch telescope.
Extension of this scale to even fainter magnitudes, and its application to the faintest stars within its range is an important task for this great telescope, as it will doubtless bring within range hundreds of millions of stars that are beyond the reach of the 60-inch. The giants among them will form for us the outer boundary of the Galactic system, while the dwarfs will be of almost equal interest from the evolutional standpoint. The photometric program of the 100-inch, then, will deal with such questions as the condensation of the fainter stars toward the Galactic plane, the color of the most distant stars, and the final settlement of the long inquiry regarding the possible absorption of light in s.p.a.ce.
[Ill.u.s.tration: GREAT SUN-SPOT GROUP, AUGUST 8, 1917. The disk in the lower left corner represents the comparative size of the earth.
(_Photo, Mt. Wilson Solar Observatory._)]
[Ill.u.s.tration: THE SUN'S DISK. The view shows the ”rice grain”
structure of the photosphere and brilliant calcium flocculi.
(_Photo, Yerkes Observatory._)]
[Ill.u.s.tration: THE LUNAR SURFACE VISIBLE DURING A TOTAL ECLIPSE OF THE MOON, FEBRUARY 8, 1906. (_Photo, Yerkes Observatory._)]
Another research of exceptional promise will be undertaken, which is of great importance in a general study of stellar evolution; and that is the determination of the spectral-energy curves of stars of various cla.s.ses, for the purpose of measuring their surface temperatures. A very few of the nebulae are found to be variable, and their peculiarities need investigation, also special problems of variable stars and temporary stars, and the spectra of the components of close double stars which are beyond the power of all other instruments to photograph.
Such a program of research conveys an excellent idea of many of the great problems that are under investigation by astronomers to-day, and gives some notion of the instrumental means requisite in executing comprehensive plans of this character. It will not escape notice that the climax of instrumental development attained at Mount Wilson has only been made possible by an unbroken chain of progress, link by link, each antecedent link being necessary to the successful forging of its following one. In very large part, and certainly indispensable to these instrumental advances, has the art of working in gla.s.s and metals been the mainstay of research. As we review the history of astronomical progress, from Galileo's time to our own, the consummate genius of the artisan and his deft handiwork compel our admiration almost equally with the keen intelligence of the astronomer who uses these powerful engines of his own devising to wrest the secrets of nature from the heavens.
CHAPTER XXIV
OUR SOLAR SYSTEM
Now let us go upward in imagination, far, far beyond the tops of the highest mountains, beyond the moon and sun, and outward in s.p.a.ce until we reach a point in the northern heavens millions and millions of miles away, directly above and equally distant from all points in the ecliptic, or path in which our earth travels yearly round the sun. Then we should have that sort of comprehensive view of the solar system which is necessary if we are to visualize as a whole the working of the vast machine, and the motions, sizes, and distances of all the bodies that comprise it. Of such stupendous mechanism our earth is part.
Or in lieu of this, let us attempt to get in mind a picture of the solar system by means of Sir William Herschel's apt ill.u.s.tration: ”Choose any well-leveled field. On it place a globe two feet in diameter. This will represent the sun; Mercury will be represented by a grain of mustard seed on the circ.u.mference of a circle 164 feet in diameter for its...o...b..t; Venus, a pea on a circle of 284 feet in diameter; the Earth also a pea, on a circle of 430 feet; Mars a rather larger pin's head on a circle of 654 feet; the asteroids, grains of sand in orbits of 1,000 to 1,200 feet; Jupiter, a moderate sized orange in a circle of nearly half a mile across; Saturn, a small orange on a circle of four-fifths of a mile; Ura.n.u.s, a full-sized cherry or small plum upon the circ.u.mference of a circle more than a mile and a half; and finally Neptune, a good-sized plum on a circle about two miles and a half in diameter....
To imitate the motions of the planets in the above mentioned orbits, Mercury must describe its own diameter in 41 seconds; Venus in 4 minutes, 14 seconds; the Earth in 7 minutes; Mars in 4 minutes 48 seconds; Jupiter in 2 minutes 56 seconds; Saturn in 3 minutes 13 seconds; Ura.n.u.s in 2 minutes 16 seconds; and Neptune in 3 minutes 30 seconds.”
Now, let us look earthward from our imaginary station near the north pole of the ecliptic. All these planetary bodies would be seen to be traveling eastward round the sun, that is, in a counter-clockwise direction, or contrary to the motions of the hands of a timepiece. Their orbits or paths of motion are very nearly circular, and the sun is practically at the center of all of them except Mercury and Mars; of Venus and Neptune, almost at the absolute center. The planes of all their orbits are very nearly the same as that of the ecliptic, or plane in which the earth moves. These and many other resemblances and characteristics suggest a uniformity of origin which comports with the idea of a family, and so the whole is spoken of as the solar system, or the sun and his family of planets.
In addition to the nine bodies already specified, the solar system comprises a great variety of other and lesser bodies; no less than twenty-six moons or satellites tributary to the planets and traveling round them in various periods as the moon does round our earth. Then between the orbits of Mars and Jupiter are many thousands of asteroids, so called, or minor planets (about 1,000 of them have actually been discovered, and their paths accurately calculated). And at all sorts of angles with the planetary orbits are the paths of hundreds of comets, delicate filmy bodies of a wholly different const.i.tution from the planets, and which now and then blaze forth in the sky, their tails appearing much like the beam of a searchlight, and compelling for the time the attention of everybody. Connected with the comets and doubtless originally parts of them are uncounted millions of millions of meteors, which for the time become a part of the solar system, their minute ma.s.ses being attracted to the planets, upon which they fall, those hitting the earth being visible to us as familiar shooting stars.
We next follow the story of astronomy through the solar system, beginning with the sun itself and proceeding outward through his family of planets, now much more numerous and vastly more extended than it was to the ancient world, or indeed till within a century and a half of our own day.
CHAPTER XXV
THE SUN AND OBSERVING IT
As lord of day, king of the heavens, mankind in the ancient world adored the sun. By their researches into the epoch of the a.s.syrians, Hitt.i.tes, Phoenicians and other early peoples now pa.s.sed from earth, archaeologists have unearthed many monuments that evidence the veneration in which the early peoples who inhabited Egypt and Asia Minor many thousand years ago held the sun. A striking example is found in the architecture of early Egyptian temples, on the lintels of which are carved representations of the winged globe or the winged solar disk, and there is a bare possibility that the wings of the globe were suggested by a type of the solar corona as glimpsed by the ancients.