158 STELLAR EVOLUTION. 



We have seen tliat a single glance 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 w^ith shifts in their position toward the 

 ]'ed end of the spectrum, furnish the means of estimating the density 

 of the vapors and the pressure to which the}" are subjected. The rela- 

 tive intensities of certain lines also serve as a clew to the temperature. 

 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 

 may indicate that the temperature is high and approximates that of the 

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

 suggesting that the temperature is lower and corresponds more closely 

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

 measureable 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 refer- 

 ence to their normal position, and if we haA^e 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 betw een the earth and the star 

 is decreasing. 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 evolution. 

 We have seen that the long-continued action of gravity tends to pro- 

 duce 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 consti- 

 tute 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 

 development. 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 3^et ripe for 

 the marked development of metallic lines, though doubtless the numer- 

 erous eUmKMits 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 evi- 

 dence exists of the presence of iron and a few other su])stances. ])ut 

 the lines are thin and few in number, and might be overlooked in a 

 casual examination of the spectrum. The period for their greatest 

 development has not yet arrived. The light gas hydrogen, reaching 



