STELLAR EVOLUTION. 305 



We have seen that a single ghmce at the spectrum of a star is suffi- 

 cient to give us important information as to the structure of its atmos- 

 phere, while a study of the position of the lines tells what chemical 

 elements are present. We might go on to consider how the width and 

 sharpness of the lines, together with shifts in their position toward the 

 red end of the spectrum, furnish the means of estimating the density 

 of the vapors and the pressure to which they are subjected. The 

 relative intensities of certain lines also serve as a clue to the tempera- 

 ture. Thus in the spectrum of magnesium there is a pair of lines, one 

 of which is the stronger at the temperature of the electric spark, while 

 the other is the stronger at the lower temperature of the electric arc. 

 In the spectra of certain stars the greater intensity of the first line 

 indicates that the temperature is high and approximates that of the 

 electric spark, while in other stars the relative intensities are reversed, 

 indicating that the temperature is lower and corresponds more closely 

 with that of the electric arc. In addition to all this, certain easily 

 measurable changes in the position of the spectral lines are known 

 from Doppler's principle to indicate motion of the star in the direction 

 of the earth. Thus if the lines are shifted toward the red with reference 

 to their normal position, and if we have evidence that the shift is not 

 due to pressure, we may conclude that the distance between the earth 

 and the star is increasing, while if the lines are shifted toward the violet 

 we conclude that the distance between the earth and the star is decreas- 

 ing. As the earth's motion is known, the velocity of the star in the 

 line of sight can therefore be accurately determined. 



After this glance at the methods employed by the spectroscopist, 

 we may return to a further consideration of the stages of stellar evolu- 

 tion. We have seen that the long continued action of gravity tends to 

 produce condensation of a cosmical cloud. The constellation of Orion 

 contains many examples of stars in this early stage of development. As 

 the mass condenses its temperature rises, and corresponding with this 

 rise in temperature and in the density of the vapors which constitute 

 the star we find characteristic changes in the spectrum and also in the 

 star's color. Such a brilliant white or bluish-white star as Sirius or 

 Vega may be taken as representative of the next stage of stellar develop- 

 ment. Here the broad bands of hydrogen, which constitute a beautiful 

 series expressible by a simple mathematical formula, serve as the chief 

 mark of distinction. The conditions are not yet ripe for the marked 

 development of metallic lines, though doubtless the numerous elements 

 which constitute the sun and which for the most part are familiar to 

 us on the earth, are present in such stars, though they are not 

 revealed through a study of the spectrum. It is true that evidence 

 exists of the presence of iron and a few other substances, but the 

 lines are thin and few in number and would be overlooked in a 



VOL. LX.— 20. 



