bands is therefore the reverse of that in subdivi

sion a.

This classification of Vogel is still generally followed in Germany and elsewhere. It is found, however, that there are star spectra of types intermediate to all these defined. Moreover, in each type the individual differences are so considerable that there is no welldefined limit to the number of classes that may be recognised. Other designations frequently occur in literature. The stars of type II are sometimes termed Capellan stars, or solar stars. The stars which show the lines of helium are known as helium stars.

A classification far more minute than either of the preceding was made by Miss Antonio C. Maury, of the Harvard Observatory, and has been adopted in the Draper Memorial work of that institution.1 The classification is too extended for us to give more than its principal features. In the main it recognises a regular progression in the character of the spectra. The principal feature is the addition of an extended type called the Orion type, because the stars showing it abound in the constellation Orion, though not confined to it. It is marked principally by what are called Orion lines, which include most of the lines of hydrogen, and nearly one hundred others. Few or none of the latter can be recognised as solar lines, nor can they certainly be ascribed to any known substances. The peculiar feature of the type is that the Orion lines are strong and numerous, declining in the

1 Annals Harvard Observatory, vol. xxviii., No. 1.

later groups. The hydrogen lines are of moderate intensity, inclining toward those of the first type. Of the two main calcium lines, K is often, and H generally, absent.

This Orion type is divided into five groups: type I into five, types II and III each into four. Besides these there are several intermediate groups, and a group each for the fourth and fifth types, the whole number of such groups being twenty-two. Each group is still further subdivided into classes.

There are many star spectra which cannot be included in any of the classes we have described. Up to the present time these are generally described as stars of peculiar spectra.

As the present chapter is confined to the more general side of the subject, we shall not attempt any description of special spectra. These, especially the peculiar spectra of the nebulæ, of new stars, of variable stars, etc., will be referred to, so far as necessary, in the chapters relating to those objects.

Results of

The most interesting conclusion drawn from observations with the spectroscope is that the stars are composed, in the main, of elements similar to those found in our sun. As the latter Spectrum contains most of the elements found on the Analysis. earth and few or no others, we may say that earth. and stars seem to be all made out of like matter. It is, however, not yet easy to decide to what extent elements unknown on the earth exist in the heavens. It would scarcely be safe to assume that, because the line of some terrestial substance is found in the

spectrum of a star, it is produced by that substance. It is quite possible that an unknown substance might show a line in appreciably the same position as that of some substance known to us. The evidence becomes conclusive only in the case of those elements of which the spectral lines are so numerous that when they all coincide with lines given by a star there can be no doubt of the identity.

CHAPTER VI

PROPER MOTIONS OF THE STARS

I'm constant as the Northern Star,

Of whose true-fixed and vesting quality

There is no fellow in the firmament.-SHAKESPEARE.

WE may assume that the stars are all in motion.

It is true that only a comparatively small number of stars have been actually seen to be in motion; but as some motion exists in nearly every case where observations would permit of its being determined, we may assume the rule to be universal. Moreover, if a star were at rest at any time it would be set in motion by the attraction of other stars.

In dealing with the subject, the astronomer commonly expresses the motion in angular measurement, as so many seconds per year or per century. The keenest eye would not, without telescopic aid, be able to distinguish between two stars whose apparent distance is less than 2' or 120" of arc. The pair of stars known as Epsilon Lyræ are over 3' apart; yet to ordinary vision they appear as a single star. To ap

preciate what" of arc means we must conceive that the distance between these two stars is divided by 200. Yet this minute space is easily distinguished and accurately measured by the aid of a telescope of ordinary power.

Apparent and Real

Statements of the motion from different points of view illustrate in a striking way the vast distance of the stars and the power of modern telescopic research. If Hipparchus or Ptolemy should Motions. rise from his sleep of two thousand years -nay, if the earliest priests of Babylon should come to life again and view the heavens, they would not perceive any change to have taken place in the relative positions of the stars. The general configurations of the constellations would be exactly that to which they were accustomed. Had they been exact observers they might notice a slight change in the position of Arcturus; but not in that of any other star.

Slow as the angular motion is, our telescopic power in the course of a few years makes its detection frequently possible-in the case of Arcturus even in a few weeks. As accurate determinations of positions of the stars have been made only during a century and a half, no motions can be positively determined except those which would become evident to telescopic vision in that period. Only about three thousand stars have been accurately observed so long as this. In the large majority of cases the interval of observation is so short or the motion so slow that nothing can be asserted respecting the law of the motion.

Contrast these apparently slow motions with the actual motions. Swift indeed are these when meas. ured by terrestrial standards. Arcturus has been moving ever since the time of Job at the rate of probably more than two hundred miles per second