STELLAB LABORATORIES — DUNHAM 271 



than that of the sun. The atoms are rushing about more rapidly, and 

 on the average the light darts are more numerous and also more 

 energetic. The vibrations are faster and the wave lengths are shorter. 

 Because the light darts are more energetic and because the collisions 

 are more violent at this higher temperature there will be fewer nor- 

 mal atoms. More atoms will be excited and more will be ionized than 

 in the sun. Now, as we have seen, the width and the strength of a 

 spectral line depends on the number of atoms responsible for it. A 

 single atom would never show a line in our spectrograph, but when 

 several million million are doing the same thing at once enough 

 light is held back to make a noticeable dark lir^e. In the present 

 case the line in the blue corresponding to the normal atom is weaker 

 than in the sun, while the triplet in the red, corresponding to the 

 excited atoms is considerably strengthened, and so is the pair of 

 lines in the violet corresponding to the ionized atoms. What we can 

 observe is of course only the spectrum and not the atoms responsible 

 for it. So when we see a spectrum whose lines have these relative 

 intensities we must infer that we are dealing with a star that is hotter 

 than the sun. 



The left part of the diagram represents the atmosphere of a very 

 different kind of star such as y Cygni. The temperature here is the 

 same as in the sun, but the pressure is much lower. Since the tem- 

 perature is the same, the violence of the collisions is the same, the 

 number and energy of the light darts is the same, but the pressure 

 is lower, which means that the atoms are farther apart. Occasional 

 atoms will become ionized, just as in the case of the sun, because 

 collisions with light darts will be no less effective. But w4ien once an 

 electron has been torn loose from an atom it will be much more diffi- 

 cult for this free electron to find another mutilated atom with which 

 it can recombine to form a normal atom. And so it happens that in 

 this star the proportion of ionized atoms is greater than in the sun, 

 while the number of normal and excited atoms are both reduced, 

 without changing their proportion relative to one another. 



The increased number of ionized atoms results in a marked 

 strengthening of the pair of lines in the violet. The line in the blue 

 corresponding to the normal atom and the red triplet corresponding 

 to the excited atom are somewhat weaker than in the sun, but the 

 relative strengths of these lines remain unchanged. 



The net result of all this is that, when we develop a photographic 

 plate taken with our telescope and find on it a spectrum with the 

 red triplet looking stronger than the blue line, we know that the 

 light must have come from a star with an atmosphere at a high 

 temperature, while if we get a spectrum with the violet pair stronger 

 than the blue line, we know that we are dealing with a star whose 



