At an early period of spectroscopic dis covery, Secchi, the Roman astronomer, made an exact classification of the spectra of stars, which he connected with their respective colors. The bluish stars he designated as of the first type; those which were almost pure white or slightly yellowish were of the second type, while those approaching toward red were mostly of the third type. Speaking in a rough way, it would seem that these gradations correspond to the absorption of the light by atmospheres surrounding all the stars. The more dense and the deeper this atmosphere the more blue light it absorbs, and the more red the remaining light appears. It seems, therefore, to be a general rule that the redder stars have the densest and most absorbing atmosphere. From many such stars a large part of the blue and green light is cut off. This deficiency in the blue light may also be due, in some cases, to the stars which show it being not so hot as the others; because we know that the hotter an object the more blue light it radiateS in propor tion to its red light. But everything we know of the stars leads us to believe that these cases are exceptional.
A very remarkable fact in connection with this classification is that the blue or first type largely predominates in the stars of the Milky Way, while stars distant from the Milky Way mostly belong to the other two types. Quite possibly it may be found that this distinction will enable us to determine in what part of the universe a star is situated, by means of its spec trum. When, at Harvard Observatory, the spec tra of the stars were photographed on a large scale, and the number studied brought up into the thousands, it was found that Secchi's classifi cation was quite insufficient. We now have a great number of such classes designated by letters of the alphabet and numerals which are too technical to be set forth in the present article. We shall mention but a single type known as Orion stars, because many of the stars showing it belong to the constellation Orion. A remarkable peculiarity of this type is the great number of fine lines in the spectra, many of which do not belong to any known substance. What gives great Interest to it is that the stars which show it belong mostly or entirely to the Milky Way, and have markedly slower proper motions than the stars in general.
One result of spectral analysis is that most of the stars are composed in the main of the same chemical elements which we know to exist upon the earth: iron, calcium and hydro gen seem to be present throughout the whole universe of stars. It cannot yet be said with certainty to what extent elements unknown on the earth exist in the stars, for the reason that the spectrum of an element depends so much upon the conditions of pressure or temperature to which the substance is exposed. We may regard it as certain that, whether elements un known to us do or do not exist in the stars, the substances which form the stars, and especially the nebulae, exist in many cases, in a form dif ferent from any we have yet produced in the laboratory.
Proper Motions of the One of the most remarkable features of the universe is that every so-called fixed star is moving for ward on an undeviating path which, so far as we can yet determine, is a straight line. This is called the proper motion of the star. idea ing is better calculated to give us an dea of the extent of the universe than the contrast be tween the speeds of these motions as we know that they really are and the apparent speeds as we observe them from the earth. The actual speed is enormous when compared with any that we can produce by artificial means. The speed of a shot from the most powerful gun that can be made can scarcely, if at all, exceed half a mile per second. But if the motion of any star is as slow as one mile a second, it is only in very rare and extraordinary cases. The average speed of the stars is about 20 miles a second; and this motion, it must be remem bered, is not, so far as yet determined, motion round and round in an orbit, but a straight ahead motion, never relaxing and never swerv ing. Almost every star, therefore, travels hun dreds of millions of miles every year, century after century. And yet, so slow do the motions appear to us that the naked eye can see no change in the configuration of the constella tions during a period of thousands of years. A remarkable instance of this kind is afforded by Arcturus, which is, so far as we know, one of the swiftest moving stars in the heavens. It seems quite certain that its speed exceeds a hundred miles a second, and it may be much greater than this. And yet, if Job could again come to life and study the constellation Bootis, in which Arcturus is situated, he would scarcely notice any change in its appearance. There is not a star in the constellation Orion moving so fast that any change would be noticed by the naked eye in 100,000 years. The result is that the motions in question have been detected only by the most refined observations of modern times, extended through long periods. Every i star in the heavens appears in the same posi tion when observed night after night. There are very few in which the astronomer can detect any motion by one year of observations. Accurate determinations of position commenced with the observations of Bradley, Astronomer Royal of England, in the middle of the 18th century, who determined the position of more than 3,000 of the brighter stars. Since his time the position of several hundred thousand stars have been accurately fixed. Yet, so small is the apparent proper motion in most cases that it has actually been detected in the case of only a few thousand stars. Even now there are scarcely a hundred stars of which the motions are known to exceed one second in a year. To under stand what this means we must reflect that it would take a good eye to see that two stars in the sky, 200 seconds apart, were not a single object.