Fig. 8. THE RING NEBULA IN LYRA
Fig. 9. HYDROGEN—THE BALMER SERIES
And what is more, the element carries its number-plate so conspicuously that a physicist is able to read it. He can, for instance, see that iron is No. 26 without having to count up how many known elements precede it. The elements have been called over by their numbers, and up to No. 84 they have all answered ‘Present’.[15]
The element helium (No. 2) was first discovered by Lockyer in the sun, and not until many years later was it found on the earth. Astrophysicists are not likely to repeat this achievement; they cannot discover new elements if there aren’t any. The unknown source of the two rings close together on the right of the photograph (a bright ring and a fainter ring) has been called nebulium. But nebulium is not a new element. It is some quite familiar element which we cannot identify because it has lost several of its electrons. An atom which has lost an electron is like a friend who has shaved off his moustache; his old acquaintances do not recognize him. We shall recognize nebulium some day. The theoretical physicists are at work trying to find laws which will determine exactly the kind of light given off by atoms in various stages of mutilation—so that it will be purely a matter of calculation to infer the atom from the light it emits. The experimental physicists are at work trying more and more powerful means of battering atoms, so that one day a terrestrial atom will be stimulated to give nebulium light. It is a great race; and I do not know which side to back. The astronomer cannot do much to help the solution of the problem he has set. I believe that if he would measure with the greatest care the ratio of intensity of the two nebulium lines he would give the physicists a useful hint. He also provides another clue—though it is difficult to make anything of it—namely, the different sizes of the rings in the photograph, showing a difference in the distribution of the emitting atoms. Evidently nebulium has a fondness for the outer parts of the nebula and helium for the centre; but it is not clear what inference should be drawn from this difference in their habits.
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.
We commonly use the method of spectrum analysis when we wish to determine which elements are present in a given mineral on the earth. It is equally trustworthy in examining the stars since it can make no difference whether the light we are studying comes from a body close at hand or has travelled to us for hundreds of years across space. But one limitation in stellar work must always be remembered. When the chemist is looking, say, for nitrogen in his mineral, he takes care to provide the conditions which according to his experience are necessary for the nitrogen spectrum to show itself. But in the stars we have to take the conditions as we find them. If nitrogen does not appear, that is no proof that nitrogen is absent; it is much more likely that the stellar atmosphere does not hit off the right conditions for the test. In the spectrum of Sirius the lines of hydrogen are exceedingly prominent and overwhelm everything else. We do not infer that Sirius is composed mainly of hydrogen; we infer instead that its surface is at a temperature near 10,000°, because it can be calculated that that is a temperature most favourable for a great development of these hydrogen lines. In the sun the most prominent spectrum is iron. We do not infer that the sun is unusually rich in iron; we infer that it is at a comparatively low temperature near 6,000° favourable for the production of the iron spectrum. At one time it was thought that the prominence of hydrogen in Sirius and of metallic elements in the sun indicated an evolution of the elements, hydrogen turning into heavier elements as the star cools from the Sirian to the solar stage. There is no ground for interpreting the observations in that way; the fading of the hydrogen spectrum and the increase of the iron spectrum would occur in any case as the result of the fall of temperature; and similar spurious appearances of evolution of elements can be arranged in the laboratory.
It is rather probable that the chemical elements have much the same relative abundance in the stars that they have on the earth. All the evidence is consistent with this view; and for a few of the commoner elements there is some positive confirmation. But we are limited to the outside of the star as we are limited to the outside of the earth in computing the abundance of the elements, so that this very provisional conclusion should not be pressed unduly.