STELLAR LABORATORIES DUNHAM 273 



different, even when the temperature of the two is the same, the 

 difference being due entirely to pressure, which means that the dif- 

 ference is connected with the sizes, and so with the luminosities of 

 the stars. 



When enough stars of known brightness had been studied to 

 show just how the spectrum depends on intrinsic brightness, it was 

 quite simple to match with these the spectra of other stars and thus 

 find their true brightness and from that their distance. It is the 

 distance which we particularly want for mapping out the arrange- 

 ment of the stars in space, and this method for deriving it looked at 

 first too good to be true. 



But the severest tests have only increased our confidence in its 

 reliability^ The distances of several thousand stars have been de- 

 termined in this way, including many much too far away to investi- 

 gate by the direct surveying method. We must not forget, how- 

 ever, that the calibration of this far-reaching spectroscopic method 

 depends entirely on the surveyed distances of representative stars. 



So much for temperature and pressure revealed to us by the atoms. 

 They tell us three other things. First, the velocity of a star to- 

 ward us or away from us. The positions of the lines in the spectrum 

 of a stationary star are well known and can be depended upon not 

 to vary by one part in a million. But if the star is moving toward us 

 at any considerable speed the waves of light tend to pile up slightly, 

 and more of them enter our spectrograph in a second than would if 

 the star were at rest. This results in displacing all the spectral lines 

 to the violet of their normal positions, and the amount of the dis- 

 placement is a measure of the velocity of the star. This method is 

 not adapted to measuring the velocity of a man walking down the 

 street; in fact it could just detect the motion of a racing car headed 

 toward the telescope and carrying a neon lamp as a source of light. 

 But it is well adapted to measuring stellar velocities, which are often 

 20 or 30 miles a second. 



When a star rotates and the axis of rotation does not happen to 

 point directly toward us, one side of the star will approach us while 

 the other recedes. The approaching side of the star tends to dis- 

 place all spectral lines to the blue, while the other side displaces these 

 same lines to the red. The result is that the lines are greatly widened 

 and no longer look sharp. Plate I (A) and (B) shows the spectrum 

 of a Aquilae, a rapidly rotating star, with the spectrum of a Cygni 

 for comparison. The distinguishing feature of the spectrum of a 

 rotating star is that all the lines are widened. 



It has long been known that if an atom emits light while in a 

 strong electric field some of the lines are widened. In some of the 

 hotter stars, the hydrogen lines are extremely wide, while the metal- 



