The atoms of different elements, and atoms of the same element in different states of ionization, all have distinctive sets of lines which are shown when the light is examined through a spectroscope. Under certain conditions (as in the nebulae) these appear as bright lines; but more often they are imprinted as dark lines on a continuous background. In either case the lines enable us to identify the element, unless they happen to belong to an atom in a state of which we have had no terrestrial experience. The rash prophecy that knowledge of the composition of the heavenly bodies must be for ever beyond our reach has long been disproved; and the familiar elements, hydrogen, carbon, calcium, titanium, iron, and many others, can be recognized in the most distant parts of the universe. The thrill of this early discovery has now passed. But meanwhile stellar spectroscopy has greatly extended its scope; it is no longer chemical analysis, but physical analysis. When we meet an old acquaintance there is first the stage of recognition; the next question is ‘How are you?’ After recognizing the stellar atom we put this question, and the atom answers, ‘Quite sound’ or ‘Badly smashed’, as the case may be. Its answer conveys information as to its environment--the severity of the treatment to which it is being subjected--and hence leads to a knowledge of the conditions of temperature and pressure in the object observed. Surveying the series of stars from the coolest to the hottest, we can trace how the calcium atoms are at first whole, then singly ionized, then doubly ionized--a sign that the battering becomes more severe as the heat becomes more intense. (The last stage is indicated by the disappearance of all visible signs of calcium, because the ion with two electrons missing has no lines in the observable part of the spectrum.) The progressive change of other elements is shown in a similar way. A great advance in this study was made in 1920 by Professor M. N. Saha, who first applied the quantitative physical laws which determine the degree of ionization at any given temperature and pressure. He thereby struck out a new line in astrophysical research which has been widely developed. Thus, if we note the place in the stellar sequence where complete calcium atoms give place to atoms with one electron missing, the physical theory is able to state the corresponding temperature or pressure.[16] Saha’s methods have been improved by R. H. Fowler and E. A. Milne. One important application was to determine the surface temperatures of the hottest types of stars (12,000°—25,000°), since alternative methods available for cooler stars are not satisfactory at these high temperatures. Another rather striking result was the discovery that the pressure in the star (at the level surveyed by the spectroscope) is only ¹⁄₁₀₀₀₀th of an atmosphere; previously it had been assumed on no very definite evidence to be about the same as that of our own atmosphere.