122 AKNUAL REPORT SMITHSOXIAN mSTITUTIOX, 1942 



continuous background those colors it would otherwise emit. There- 

 fore the solar spectrum is a continuous spectrum on which is super- 

 posed a pattern of dark gaps or lines. These dark lines identify the 

 gases in the solar atmosphere and indicate the physical conditions 

 under which they exist. 



The nebulae are stellar systems, and their spectra resemble that of 

 the sun. Dark lines due to calcium, hydrogen, iron, and other ele- 

 ments in the atmospheres of the component stars are identified with 

 complete confidence. In the case of the nearer nebulae, these lines are 

 close to their normal positions as determined in the laboratory or in 

 the sun. In general, however, accurate measures disclose slight dis- 

 placements, either to the red or to the violet side of the exact normal 

 positions. 



Such small displacements are familiar features in the spectra of 

 stars and are known to be introduced by rapid motion in the line of 

 sight. If a star is rapidly approaching the observer, the light waves 

 are crowded together and shortened, and all the spectral lines appear 

 slightly to the violet side of the normal positions. Conversely, rapid 

 recession of a star drags out and lengthens the light waves, and the 

 spectral lines are seen to the red of their normal positions. 



The amounts of these displacements (they are called Doppler 

 shifts) indicate the velocities of the stars in the line of sight. If the 

 wave lengths are altered by a certain fraction of the normal wave 

 lengths, the star is moving at a velocity which is that same fraction 

 of the velocity of light. In this way it has been found that the stars 

 are drifting about at average speeds of 10 to 30 miles per second, 

 and, indeed, that the stellar system, our own nebula, is rotating about 

 its center at the majestic rate of one revolution in perhaps 200 mil- 

 lion years. 



Similarly, the nebulae are found to be drifting about in space at 

 average speeds of the order of 150 miles per second. Such speeds, 

 of course, are minute fractions of the velocity of light, and the cor- 

 responding Doppler shifts, which may be either to the violet or to 

 the red, are barely perceptible. 



But tlie spectra of distant nebulae show another effect as con- 

 spicuous as it is remarkable. The dark absorption lines are found 

 far to the red of their normal positions. Superposed on the small 

 red or violet shifts representing individual motions, is a systematic 

 shift to the red which increases directly with the distances of the 

 nebulae observed. If one nebula is twice as far away as another, 

 the red shift will be twice as large ; if u times as far away, the red 

 shift will be n times as large. This relation is known as the law of 

 red shifts ; it appears to be quite a general feature of the observable 

 region of space. 



