270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



other, so that if at any one instant a snapshot photograph, such as 

 this diagram, could be taken, we should find that a considerable num- 

 ber of the calcium atoms had just undergone collisions which had 

 lifted one of their electrons into a larger or excited orbit. 



At this temperature light darts are also very plentiful, rushing 

 about in all directions among the atoms. By striking normal atoms, 

 they produce still more excited atoms. Wlien an atom which has al- 

 ready been excited by one collision is struck a second time before it 

 has recovered from the first collision, the electron may be knocked 

 entirely off the atom. The mutilated atom which remains is called 

 an " ionized " atom. 



We have, then, in the solar atmosphere at any one instant three 

 kinds of calcium atoms: The normal atom, the excited atom, and 

 the ionized atom. 



Now, we have seen that any particular atom can absorb a light 

 dart if the energy of the light dart is of exactly the right amount 

 to raise one of the electrons in the atom into one of its possible 

 orbits. If this happens, that particular light dart is lost forever as 

 far as we and our spectrographs are concerned, because the atom, in 

 unwinding, will either send out a second dart in a random direction 

 or else will use the stored energy to kick one of its neighbors, in 

 which case the energy goes into heat and is again diverted. 



Thus the atoms in the atmosphere stand at the gateway between 

 the star and outer space and each one sidetracks light darts of a 

 particular energy and color. The result is that each type of atom 

 is responsible for characteristic gaps or dark lines in the otherwise 

 continuous band of color. 



The normal calcium atoms absorb blue light, causing a single 

 dark line in the blue part of the spectrum. The excited atoms 

 absorb red light, causing primarily a strong group of three lines 

 close together in the red part of the spectrum, while the ionized 

 atoms which have lost an electron absorb violet light, causing a pair 

 of conspicuous lines in the violet. 



Now, the strength of these spectral lines depends in a striking way 

 on the number of atoms causing them, so that if with our spectro- 

 graphs we can measure the strength of the lines it means that we 

 can count the number of atoms of each kind. When many atoms 

 cause a line, the line is wide and when the atoms are few the line 

 is narrow. This we speak of as differences in line-intensity. 



In the sun the lines we are discussing are of very different intensi- 

 ties, but they have intentionally been made equal in Figure 4 so as 

 to bring out more clearly the variations when we pass to other stars. 



In the right-hand section of the diagram we have the atmos- 

 phere of a star, such as Procyon, which is at a higher temperature 



