One of the greatest problems that confronts astronomers at the present time is an accurate knowledge of the distances of stars. From the earliest times, the relative positions of stars or their apparent directions in space have been observed and measured with increasing accuracy. Before anything definite could be learned, however, as to the actual structure of the sidereal universe in which the earth plays its ultra-microscopic part, the actual distance to many of the brighter stars had to be measured. When Copernicus promulgated the heliocentric theory in 1543 through the publication of his epoch-making book. 'De Revolution-ibus Orbium Coelestium", it was seen that the motion of the earth in its orbit about the sun should produce an apparent displacement or change in direction of a star as the observer was carried in the space of six months from one side to the other of the orbit 186,000,000 miles across. In spite of this very great length of base line, the shift in a stars position is so slight that it took 300 years of patient endeavor before instruments were perfected sufficiently exact to discover this so-called "parallax" which gave the clue to stellar distances. The first star to have its distance determined in this way is a faint star numbered 61 in the constellation Cygnus now visible in the eastern sky. Its distance was found to be about 60,000,000,000,000 miles, or so far away that the light from it consumed ten years in reaching the earth. This happened in 1840. Since that time hundreds of stars have had their distance measured similarly, but less than a dozen are known to be nearer than 61 Oygni. The nearest is Alpha Centauri, a star in the southern skies, a little over four "light years" distant. The number of stars whose direct parallax can be measured is very limited.
There has been developed, however, in recent years, at the Mount Wilson Observatory a new method of determining parallaxes which may be applied to the more remote stars and which unlocks great possibilities for a study of stellar distances from the hundreds of plates in the photographic library of the Harvard Observatory. The newer method rests upon careful interpretation of small differences in the spectra of stars. A systematic study shows a definite relation between the actual iuminosity or radiating power of known stars of certain classes and the relative intensity of the absorbtion lines in their spectra.
Brightness Determines Distance
In the case of a star of unknown distance whose spectrum has been photographed, the relative intensity of the lines is compared with a standard reduction curve, and the star is at once recognized as of a certain luminosity or absolute magnitude. The apparent brightness of this star, however, as seen from the earth will depend on its distance from us. The apparent brightness or magnitude is then determined either visually or photographically; and, knowing the actual brightness, the distance of the star is computed on the basis of the well known law of physics that the intensity of light varies inversely as the square of the distance. Since the method was put into operation, parallaxes have been published by the Mount Wilson Observatory for some sixteen hundred stars.
The Harvard Observatory is peculiarly fortunate in having the large collection of plates of the Henry Draper Memorial, from which the spectra of 225,000 stars distributed over the entire sky have been catalogued by Miss Cannon, curator of astronomical photographs. Many of these plates were made at the South American station of the Observatory, and afford valuable material for the determination of the "spectroscopic parallax" by the Mount Wilson method, as there is a great paucity of data concerning stars in the Southern hemisphere. Professor Shapley, Director of the Observatory, has just recently published in Circulars 228 and 232 of the Observatory Publications the parallaxes and absolute magnitude for 137 stars from an examination of their spectra.
The question of stellar distances is but one of the many problems that are presented to the professional astronomer, but the distances in miles which correspond to these parallax determinations tax the imagination to conceive. The contributions of astronomy in interpreting the sky and the earth's plane in the universe have exerted a profound influence on men's thinking in all ages'. For this reason, perhaps, astronomy makes so the casual star gazer who has learned to recognize familiar groups and the brighter planets.
The month of June, when nature is at her best, brings this year many attractions astronomically. Four of the five conspicuous planets occupy prominent places in the evening sky. Venus in the west shortly after sunset is by far the most brilliant object. It is during the last year that the word comes from the Mount Wilson Observatory in California that definite evidence has been obtained of the absence of oxygen and water vapor bands in the spectrum of Venus, thus indicating an apparent lack on that planet of what we regard as life essentials.
Jupiter in South at Sundown
In the south at sundown is Jupiter, the brightest object in its neighborhood. A short distance to the west of Jupiter is Saturn. These two planets may be distinguished from the bright star Spica, which lies below Jupiter and Saturn and a little to the eastward, by the steadiness of the light emitted,--a characteristic feature of planets as compared with the more distant stars. Jupiter and Saturn are the two largest planets of the solar system, and are well situated for observation with telescopes of moderate size.
Mars now rises in the southwest about 9 P. M. and is conspicuous as a bright red star. On the tenth of June it will be in opposition with the sun, and will then be at the nearest distance to the earth that it has been for thirteen years. On that date the planet will be 42,000,000 miles from the earth. The nearest possible approach of the earth and Mars is 35,000,000 miles, and the farthest distance separating these two planets may be as great as 240,000,000 miles. This present opposition and the next following in 1924 will give the most favorable opportunities for observation for a period of fifteen years.
With the approach of summer we lose, to be sure, some of the most brilliant constellations of the northern skies: notably, Orion, Taurus, Gemini, and Ganis Major; but, if the skies of summer are less brilliant they are not less interesting. High up in the north is the "dipper", the catch figure of Ursa Major chased by the huntsman Bootes. The brightest star of Bootes is Arcturus, found by following the bend of the handle of the "dipper" to the eastward. Arcturus is one of the few stars whose diameter has actually been measured, although, like all "fixed stars", it is so far distant that with no telescope of any size however great can we even hope to see size or shape directly. A little more than a year ago, the diameter of Arcturus was measured at Mount Wilson by the interferometer method familiar to the physicist, and found to be 22,000,000 miles or about two and a half times the diameter of our own sun. Its distance is known to be about twelve light years. To the east of Arcturus and Bootes is Lyra, a small constellation marked by the bright star Vega, from which the light emitted is a hundred and thirty years reaching the earth. It is toward the region of the sky marked by this bright star that the spectroscope shows the sun and the solar system to be travelling 12 miles a second, or 400,000,000 miles each year.
This motion of the sun is not peculiar, for, contrary to the earlier conceptions, all "fixed" stars appear in motion. The static universe of the ancients has become dynamic, and the astronomer may wait centuries for correct conclusions in regard to those motions of the more distant stars. Three things, however, seem well established as a result of study of both proper motions and radial velocities. These are: (1) the motion of the sun in space above mentioned; (2) the common motion of certain groups or clusters of stars, the two most notable examples of which are the Taurus and the Ursa Major groups; and (3) leaving the sun's motion out of account, the vast majority of stars appear to be streaming in one or the other of two opposite directions, as though two gigantic clusters of stars were passing each other in space, the distances between individuals of these two clusters being so great that there is complete intermingling of the members of one system with that of the other. Whether such intermingling of two streams is a correct interpretation, or whether time will show a vast rotating system, we cannot toll. The fact that some of the large spiral nebulae have recently shown indications of rotation in no way proves the probability or the improbability of rotation in the galactic system as a whole. If recent estimates of the very great dimensions of the Milky Way stand correct, we should be led to suppose that these spiral nebulae must be relatively near in order to make possible any evidence of their rotation at all. If relatively near compared with the dimensions of the galactic system, they should be well within its limits, and are therefore on this assumption not objects at all comparable in size with the galaxy itself, whose probable diameter has been set by Professor Shapley at about 300,000 light years