Let us turn again to chemistry, and see where experiments performed in cosmic laboratories can serve as a guide to the investigator.
A spinning solar tornado, incomparably greater in scale than the devastating whirlwinds that so often cut narrow paths of destruction through town and country in the Middle West, gradually gives rise to a sun-spot. The expansion produced by the centrifugal force at the centre of the storm cools the intensely hot gases of the solar atmosphere to a point where chemical union can occur. Titanium and oxygen, too hot to combine in most regions of the sun, join to form the vapor of titanium oxide, characterized in the sunspot spectrum by fluted bands, made up of hundreds of regularly spaced lines. Similarly magnesium and hydrogen combine as magnesium hydride and calcium and hydrogen form calcium hydride. None of these compounds, stable at the high temperatures of sun-spots, has been much studied in the laboratory. The regions in which they exist, though cooler than the general atmosphere of the sun, are at temperatures of several thousand degrees, attained in our laboratories only with the aid of such devices as powerful electric furnaces.
[Illustration: Fig. 34. Splitting of spectrum lines by a magnetic field (Babcock).
The upper and lower strips show lines in the spectrum of chromium, observed without a magnetic field. When subjected to the influence of magnetism, these single lines are split into several components.
Thus the first line on the right is resolved by the field into three components, one of which (plane polarized) appears in the second strip, while the other two, which are polarized in a plane at right angles to that of the middle component, are shown on the third strip. The next line is split by the magnetic field into twelve components, four of which appear in the second strip and eight in the third. The magnetic fields in sun-spots affect these lines in precisely the same way.]
It is interesting to follow our line of reasoning to the stars, which differ widely in temperature at various stages in their life-cycle.[*] A sun-spot is a solar tornado, wherein the intensely hot solar vapors are cooled by expansion, giving rise to the compounds already named. A red star, in Russell's scheme of stellar evolution, is a cooler sun, vast in volume and far more tenuous than atmospheric air when in the initial period of the "giant" stage, but compressed and denser than water in the "dwarf" stage, into which our sun has already entered as it gradually approaches the last phases of its existence. Therefore we should find, throughout the entire atmosphere of such stars, some of the same compounds that are produced within the comparatively small limits of a sun-spot. This, of course, on the correct assumption that sun and stars are made of the same substances. Fowler has already identified the bands of titanium oxide in such red stars as the giant Betelgeuse, and in others of its class. It is safe to predict that an interesting chapter in the chemistry of the future will be based upon the study of such compounds, both in the laboratory and under the progressive temperature conditions afforded by the countless stellar "giants"
and "dwarfs" that precede and follow the solar state.
[Footnote *: See Chapter II.]
[Illustration: Fig. 35. Electric furnace in the Pasadena laboratory of the Mount Wilson Observatory.
With which the chemical phenomena observed in sun-spots and red stars are experimentally imitated.]
It is precisely in this long sequence of physical and chemical changes that the astrophysicist and the astrochemist can find the means of pushing home their attack. It is true, of course, that the laboratory investigator has a great advantage in his ability to control his experiments, and to vary their progress at will.
But by judicious use of the transcendental temperatures, far out ranging those of his furnaces, and extreme conditions, which he can only partially imitate, afforded by the sun, stars, and nebulae, he may greatly widen the range of his inquiries. The sequence of phenomena seen during the growth of a sun-spot, or the observation of spots of different sizes, and the long series of successive steps that mark the rise and decay of stellar life, resemble the changes that the experimenter brings about as he increases and diminishes the current in the coils of his magnet or raises and lowers the temperature of his electric furnace, examining from time to time the spectrum of the glowing vapors, and noting the changes shown by the varying appearance of their lines.
[Illustration: Fig. 36. Titanium oxide in red stars.
The upper spectrum is that of titanium in the flame of the electric arc, where its combination with oxygen gives rise to the bands of titanium oxide (Fowler). The lower strip shows the spectrum of the red star Mira (Omicron Ceti), as drawn by Cortie at Stonyhurst.
The bands of titanium oxide are clearly present in the star.]
[Illustration: Fig. 37. Titanium oxide in sun-spots.
The upper strip shows a portion of the spectrum of a sun-spot (Ellerman); the lower one the corresponding region of the spectrum of titanium oxide (King). The fluted bands of the oxide spectrum are easily identified in the spot, where they indicate that titanium and oxygen, too hot to combine in the solar atmosphere, unite in the spot because of the cooling produced by expansion in the vortex.]
Astronomical observations of this character, it should be noted, are most effective when constantly tested and interpreted by laboratory experiment. Indeed, a modern astrophysical observatory should be equipped like a great physical laboratory, provided on the one hand with telescopes and accessory apparatus of the greatest attainable power, and on the other with every device known to the investigator of radiation and the related physical and chemical phenomena. Its telescopes, especially designed with the aims of the physicist and chemist in view, bring images of sun, stars, nebulae, and other heavenly bodies within the reach of powerful spectroscopes, sensitive bolometers and thermopiles, and the long array of other appliances available for the measurement and analysis of radiation. Its electric furnaces, arcs, sparks, and vacuum tubes, its apparatus for increasing and decreasing pressure, varying chemical conditions, and subjecting luminous gases and vapors to the influence of electric and magnetic fields, provide the means of imitating celestial phenomena, and of repeating and interpreting the experiments observed at the telescope.
And the advantage thus derived, as we have seen, is not confined to the astronomer, who has often been able, by making fundamental physical and chemical discoveries, to repay his debt to the physicist and chemist for the apparatus and methods which he owes to them.
NEWTON AND EINSTEIN
Take, for another example, the greatest law of physics--Newton's law of gravitation. Huge balls of lead, as used by Cavendish, produce by their gravitational effect a minute rotation of a delicately suspended bar, carrying smaller balls at its extremities. But no such feeble means sufficed for Newton's purpose. To prove the law of gravitation he had recourse to the tremendous pull on the moon of the entire mass of the earth, and then extended his researches to the mutual attractions of all the bodies of the solar system.
Later Herschel applied this law to the suns which constitute double stars, and to-day Adams observes from Mount Wilson stars falling with great velocity toward the centre of the galactic system under the combined pull of the millions of objects that compose it. Thus full advantage has been taken of the possibility of utilizing the great masses of the heavenly bodies for the discovery and application of a law of physics and its reciprocal use in explaining celestial motions.
[Illustration: Fig. 38. The Cavendish experiment.
Two lead balls, each two inches in diameter, are attached to the ends of a torsion rod six feet long, which is suspended by a fine wire. The experiment consists in measuring the rotation of the suspended system, caused by the gravitational attraction of two lead spheres, each twelve inches in diameter, acting on the two small lead balls.]
Or consider the Einstein theory of relativity, the truth or falsity of which is no less fundamental to physics. Its inception sprang from the Michelson-Morley experiment, made in a laboratory in Cleveland, which showed that motion of the earth through the ether of space could not be detected. All of the three chief tests of Einstein's general theory are astronomical--because of the great masses required to produce the minute effects predicted: the motion of the perihelion of Mercury, the deflection of the light of a star by the attraction of the sun, and the shift of the lines of the solar spectrum toward the red--questions not yet completely answered.
But it is in the study of the constitution of matter and the evolution of the elements, the deepest and most critical problem of physics and chemistry, that the extremes of pressure and temperature in the heavenly bodies, and the prevalence of other physical conditions not yet successfully imitated on earth, promise the greatest progress.
It fortunately happens that astrophysical research is now at the very apex of its development, founded as it is upon many centuries of astronomical investigation, rejuvenated by the introduction into the observatory of all the modern devices of the physicist, and strengthened with instruments of truly extraordinary range and power. These instruments bring within reach experiments that are in progress on some minute region of the sun's disk, or in some star too distant even to be glimpsed with ordinary telescopes.
Indeed, the huge astronomical lenses and mirrors now available serve for these remote light-sources exactly the purpose of the lens or mirror employed by the physicist to project upon the slit of his spectroscope the image of a spark or arc or vacuum tube within which atoms and molecules are exposed to the influence of the electric discharge. The physicist has the advantage of complete control over the experimental conditions, while the astrophysicist must observe and interpret the experiments performed for him in remote laboratories. In actual practice, the two classes of work must be done in the closest conjunction, if adequate utilization is to be made of either. And this is only natural, for the trend of recent research has made clear the fact that one of the three greatest problems of modern astronomy and astrophysics, ranking with the structure of the universe and the evolution of celestial bodies, is the constitution of matter. Let us see why this is so.
TRANSMUTATION OF THE ELEMENTS
The dream of the alchemist was to transmute one element into another, with the prime object of producing gold. Such transmutation has been actually accomplished within the last few years, but the process is invariably one of disintegration--the more complex elements being broken up into simpler constituents. Much remains to be done in this same direction; and here the stars and nebulae, which show the spectra of the elements under a great variety of conditions, should help to point the way. The progressive changes in spectra, from the exclusive indications of the simple elements hydrogen, helium, nitrogen, possibly carbon, and the terrestrially unknown gas nebulium in the gaseous nebulae, to the long list of familiar substances, including several chemical compounds, in the red stars, may prove to be fundamentally significant when adequately studied from the standpoint of the investigator of atomic structure. The existing evidence seems to favor the view, recently expressed by Saha, that many of these differences are due to varying degrees of ionization, the outer electrons of the atoms being split off by high temperature or electrical excitation. It is even possible that cosmic crucibles, unrivalled by terrestrial ones, may help materially to reveal the secret of the formation of complex elements from simpler ones. Physicists now believe that all of the elements are compounded of hydrogen atoms, bound together by negative electrons.
Thus helium is made up of four hydrogen atoms, yet the atomic weight of helium (4) is less than four times that of hydrogen (1.008).
The difference may represent the mass of the electrical energy released when the transmutation occurred.
[Illustration: Fig. 39. The Trifid Nebula in Sagittarius (Ritchey).
The gas "nebulium," not yet found on the earth, is the most characteristic constituent of irregular nebulae. Nebulium is recognized by two green lines in its spectrum, which cause the green color of nebulae of the gaseous type.]
Eddington has speculated in a most interesting way on this possible source of stellar heat in his recent presidential address before the British Association for the Advancement of Science (see _Nature_, September 2, 1920). He points out that the old contraction hypothesis, according to which the source of solar and stellar heat was supposed to reside in the slow condensation of a radiating mass of gas under the action of gravity, is wholly inadequate to explain the observed phenomena. If the old view were correct, the earlier history of a star, from the giant stage of a cool and diaphanous gas to the period of highest temperature, would be run through within eighty thousand years, whereas we have the best of evidence that many thousands of centuries would not suffice. Some other source of energy is imperatively needed. If 5 per cent of a star's mass consists originally of hydrogen atoms, which gradually combine in the slow process of time to form more complex elements, the total heat thus liberated would more than suffice to account for all demands, and it would be unnecessary to assume the existence of any other source of heat.
[Illustration: Fig. 40. Spiral nebula in Ursa Major (Ritchey).
Luminous matter, in every variety of physical and chemical state, is available for study in the most diverse celestial objects, from the spiral and irregular nebulae through all the types of stars.
Doctor van Maanen's measures of the Mount Wilson photographs indicate outward motion along the arms of spiral nebulae, while the spectroscope shows them to be whirling at enormous velocities.]
This, it may fairly be said, is very speculative, but the fact remains that celestial bodies appear to be the only places in which the complex elements may be in actual process of formation from their known source--hydrogen. At least we may see what a vast variety of physical conditions these cosmic crucibles afford. At one end of the scale we have the excessively tenuous nebulae, the luminosity of which, mysterious in its origin, resembles the electric glow in our vacuum tubes. Here we can detect only the lightest and simplest of the elements. In the giant stars, also extremely tenuous (the density of Betelgeuse can hardly exceed one-thousandth of an atmosphere) we observe the spectra of iron, manganese, titanium, calcium, chromium, magnesium, vanadium, and sodium, in addition to titanium oxide.
The outer part of these bodies, from which light reaches us, must therefore be at a temperature of only a few thousand degrees, but vastly higher temperatures must prevail at their centres. In passing up the temperature curve more and more elements appear, the surface temperature rises, and the internal temperature may reach millions of degrees. At the same time the pressure within must also rise, reaching enormous figures in the last stages of stellar life. Cook has calculated that the pressure at the centre of the earth is between 4,000 and 10,000 tons per square inch, and this must be only a very small fraction of that attained within larger celestial bodies. Jeans has computed the pressure at the centre of two colliding stars as they strike and flatten, and finds it may be of the order of 1,000,000,000 tons per square inch--sufficient, if their diameter be equal to that of the sun--to vaporize them 100,000 times over.
Compare these pressures with the highest that can be produced on earth. If the German gun that bombarded Paris were loaded with a solid steel projectile of suitable dimensions, a muzzle velocity of 6,000 feet per second could be reached. Suppose this to be fired into a tapered hole in a great block of steel. The instantaneous pressure, according to Cook, would be about 7,000 tons per square inch, only 1/150000 of that possible through the collision of the largest stars.
[Illustration: Fig. 41. Mount San Antonio as seen from Mount Wilson.
Michelson is measuring the velocity of light between stations on Mount Wilson and Mount San Antonio. Astronomical observations afford the best means, however, of detecting any possible difference between the velocities of light of different colors. From studies of variable stars in the cluster Messier 5 Shapley concludes that if there is any difference between the velocities of blue and yellow light in free space it cannot exceed two inches in one second, the time in which light travels 186,000 miles.]
Finally, we may compare the effects of light pressure on the earth and stars. Twenty years ago Nichols and Hull succeeded, with the aid of the most sensitive apparatus, in measuring the minute displacements produced by the pressure of light. The effect is so slight, even with the brightest light-sources available, that great experimental skill is required to measure it. Yet in the case of some of the larger stars Eddington calculates that one-half of their mass is supported by radiation pressure, and this against their enormous gravitational attraction. In fact, if their mass were as great as ten times that of the sun, the radiation pressure would so nearly overcome the pull of gravitation that they would be likely to break up.
But enough has been said to illustrate the wide variety of experimental devices that stand at our service in the laboratories of the heavens.
Here the physicist and chemist of the future will more and more frequently supplement their terrestrial apparatus, and find new clues to the complex problems which the amazing progress of recent years has already done so much to solve.
PRACTICAL VALUE OF RESEARCHES ON THE CONSTITUTION OF MATTER
The layman has no difficulty in recognizing the practical value of researches directed toward the improvement of the incandescent lamp or the increased efficiency of the telephone. He can see the results in the greatly decreased cost of electric illumination and the rapid extension of the range of the human voice. But the very men who have made these advances, those who have succeeded beyond all expectation in accomplishing the economic purposes in view, are most emphatic in their insistence upon the importance of research of a more fundamental character. Thus Vice-President J. J. Carty, of the American Telephone and Telegraph Company, who directs its great Department of Development and Research, and Doctor W. J. Whitney, Director of the Research Laboratory of the General Electric Company, have repeatedly expressed their indebtedness to the investigations of the physicist, made with no thought of immediate practical return. Faraday, studying the laws of electricity, discovered the principle which rendered the dynamo possible. Maxwell, Henry, and Hertz, equally unconcerned with material advantage, made wireless telegraphy practicable. In fact, all truly great advances are thus derived from fundamental science, and the future progress of the world will be largely dependent upon the provision made for scientific research, especially in the fields of physics and chemistry, which underlie all branches of engineering.
The constitution of matter, therefore, instead of appealing as a subject to research only to the natural philosopher or to the general student of science, is a question of the greatest practical concern. Already the by-products of investigations directed toward its elucidation have been numerous and useful in the highest degree.
Helium has been already cited; X-rays hardly require mention; radium, which has so materially aided sufferers from cancer, is still better known. Wireless telephony and transcontinental telephony with wires were both rendered possible by studies of the nature of the electric discharge in vacuum tubes. Thus the "practical man," with his distrust of "pure" science, need not resent investments made for the purpose of advancing our knowledge of such fundamental subjects as physics and chemistry. On the contrary, if true to his name, he should help to multiply them many fold in the interest of economic and commercial development.