SPECTRUM ANALYSIS
A new period in the history of astronomy followed upon the discovery of spectrum analysis by Bunsen and Kirchhoff. At the outset the chemical composition of the sun revealed itself. Later that of the stars became known; still later it became possible to classify the stars on the basis of their spectra, and at length it has become evident that variations in spectra are at least largely due to differences in the age of suns (the length of time during which cooling has gone on), that all stars are probably very much alike both chemically and physically, and that our sun is probably very much like all other stars. The geological doctrine of uniformity has been extended to astronomy.
This results in renewed interest in the nebular hypothesis and in novel speculations regarding the origin of the solar system. In like manner, the problem of the physicochemical nature of the sun, and of the processes which take place within it, assumes great interest; for, if the universe be homogeneous, we may extend our local discoveries to the utmost confines of space. These, however, have themselves turned out not so unapproachable as a few years ago they seemed to be. Certain peculiarities of star spectra enable astronomers to judge of the motion of stars both relative to the earth and in rotation. The behavior of variable stars can also in part be accounted for by ingenious hypotheses.
Thus the old science preserves its youth and promises to continue its contributions to the growth of human understanding.