Part 10 (2/2)
The working of astronomical instruments from within air-tight compartments does not present any insurmountable difficulties, either mechanical or physical. Since the time these experiments were made, the Guayaquil-Quito railway has been constructed over a saddle of Chimborazo, at an elevation of 12,000 feet; and only six miles of railway would need to be built from this station to the point where the tunnel would enter the mountain.
Only by the execution of some such plan as this can astronomers hope to overcome the baleful effects of an ever mobile atmosphere, and secure the advantages contemplated by Sir Isaac Newton in that tranquillity of atmosphere, which he conceived as perpetually surrounding the summits of the highest mountains.
In Russell's theory of the progressive development of the stars, from the giant cla.s.s to the dwarf, an element of verification from observation is lacking, because hitherto no certain method of measuring the very minute angular diameters of the stars has been successfully applied. The apparent surface brightness corresponding to each spectral type is pretty well known, and by dividing it into the total apparent brightness, we have the angular area subtended by the star, quite independent of the star's distance. This makes it easy to estimate the angular diameter of a star, and Betelgeuse is the one which has the greatest angular diameter of all whose distances we know. Antares is next in order of angular diameter, 0”.043, Aldebaran 0”.022, Arcturus 0”.020, Pollux 0”.013, and Sirius only 0”.007.
Can these theoretical estimates be verified by observation? Clearly it is of the utmost importance and the exceedingly difficult inquiry has been undertaken with the 100-inch reflector on Mount Wilson, employing the method of the interferometer developed by Michelson and described later on, an instrument undoubtedly capable of measuring much smaller angles than can be measured by any other known method. Unquestionably the interference of atmospheric waves, or in other words what astronomers call ”poor seeing,” will ultimately set the limit to what can be accomplished. ”But even if,” says Eddington, ”we have to send special expeditions to the top of one of the highest mountains in the world, the attack on this far-reaching problem must not be allowed to languish.”
CHAPTER XXIII
THE PROGRAM OF A GREAT OBSERVATORY
The Mount Wilson Observatory has now been in operation about fifteen years. The novelty in construction of its instruments, the investigations undertaken with them and the discoveries made, the interpretation of celestial phenomena by laboratory experiment, and the recent addition to its equipment of a telescope 100 inches in diameter, surpa.s.sing all others in power, directs especial attention to the extensive activities of this inst.i.tution, whose budget now exceeds a million dollars annually. Results are only achieved by a carefully elaborated program, such as the following, for which the reader is mainly indebted to Dr. Hale, the director of the observatory, who gives a very clear idea of the trend of present-day research on the magnetic nature of the sun, and the structure and evolution of the sidereal universe.
The purpose of the observatory, as defined at its inception, was to undertake a general study of stellar evolution, laying especial emphasis upon the study of the sun, considered as a typical star; physical researches on stars and nebulae; and the interpretation of solar and stellar phenomena by laboratory experiments. Recognizing that the development of new instruments and methods afforded the most promising means of progress, well-equipped machine shops and optical shops were provided with this end in view.
The original program of the observatory has been much modified and extended by the independent and striking discovery by Campbell and Kapteyn of an important relations.h.i.+p between stellar speed and spectral type; the demonstration by Hertzsprung and Russell of the existence of giant and dwarf stars; the successful application of the 60-inch reflector by Van Maanen to the measurement of minute parallaxes of stars and nebulae; the important developments of Shapley's investigation of globular star cl.u.s.ters; the possibilities of research resulting from Seares's studies in stellar photometry; and the remarkable means of attack developed by Adams through the method of spectroscopic parallaxes.
By this method the absolute magnitude, and hence the distance of a star is accurately determined from estimates of the relative intensities of certain lines in stellar spectra. Attention was first directed toward lines of this character in 1906, when it was inferred that the weakening of some lines in the spectra of sun spots and the strengthening of others was the result of reduced temperature of the spot vapors. On testing this hypothesis by laboratory experiments, it was fully verified.
Subsequently Adams, who had thus become familiar with these lines and their variability, studied them extensively in the spectra of other stars. In this way was discovered the dependence of their relative intensities on the star's absolute magnitude, so providing the powerful method of spectroscopic parallaxes.
This method, giving the absolute magnitude as well as the distance of every star (excepting those of the earliest type) whose spectrum is photographed, is no less important from the evolutional than from the structural point of view.
Investigations in solar physics which formerly held chief place in the research program have developed along unexpected lines. It could not be foreseen at the outset that solar magnetic phenomena might become a subject of inquiry, demanding special instrumental facilities, and throwing light on the complex question of the nature of the sun spots and other solar problems of long standing. It is obvious that these researches, together with those on the solar rotation and the motions of the solar atmosphere, developed by Adams and St. John, must be carried to their logical conclusion, if they are to be utilized to the fullest in interpreting stellar and nebular phenomena.
The discovery of solar magnetism, like many other Mount Wilson results, was the direct outcome of a long series of instrumental developments.
The progressive improvement and advance in size of the tools of research was absolutely necessary. Hale's first spectroheliograph at Kenwood in 1890 was attached to a 12-inch refractor, and the solar image was but two inches in diameter. It was soon found that a larger solar image was essential, and a spectrograph of much greater linear dispersion; in fact, the spectrograph must be made the prime element in the combination, and the telescope so designed as to serve as a necessary auxiliary.
Accordingly, successive steps have led through spectrographs of 18 and 30 feet dimension to a vertical spectrograph 75 feet in focal length.
The telescope is the 150 feet tower telescope, giving a solar image of 16.5 inches in diameter. Its spectrograph is ma.s.sive in construction, and by extending deep into the earth, it enjoys the stability and constancy of temperature required for the most exacting work.
Another direct outgrowth of the work of sun-spot spectra is a study of the spectra of red stars, where the chemistry of these coolest regions of the sun is partially duplicated. The combination of t.i.tanium and oxygen, and the significant changes of line intensity already observed in both instances, and also in the electric furnace at reduced temperatures, give indication of what may be expected to result from an attack on the spectra of the red stars with more powerful instrumental means, which is now provided by the 100-inch telescope and its large stellar spectrograph.
Other elements in the design of the 100-inch Hooker telescope have the same general object in view--that of developing and applying in astronomical practice the effective research methods suggested by recent advances in physics. Fresh possibilities of progress are constantly arising, and these are utilized as rapidly as circ.u.mstances permit.
The policy of undertaking the interpretations of celestial phenomena by laboratory experiments, an important element in the initial organization of Mount Wilson, has certainly been justified by its results. Indeed, the development of many of the chief solar investigations would have been impossible without the aid of special laboratory studies, going hand in hand with the astronomical observations. So indispensable are such researches, and so great is the promise of their extension, that the time has now come for advancing the laboratory work from an accessory feature to full equality with the major factors in the work of the observatory. Accordingly a new instrument now under installation is an extremely powerful electro-magnet, designed by Anderson for the extension of researches on the Zeeman effect, and for other related investigations. Within the large and uniform field of this magnet, which is built in the form of a solenoid, a special electric furnace, designed for this purpose by King, is used for the study of the inverse Zeeman effect at various angles with the lines of force. This will provide the means of interpreting certain remarkable anomalies in the magnetic phenomena of sun spots.
The 100-inch telescope is now in regular use. All the tests so far applied show that it greatly surpa.s.ses the 60-inch telescope in every cla.s.s of work. For many months most of the observations and photographs have been made with the Ca.s.segrain combination of mirrors, giving an equivalent focal length of 134 feet and involving three reflections of light. The 100-inch telescope is found to give nearly 2.8 times as much light as the 60-inch telescope, and therefore extends the scope of the instrument to all the stars an entire magnitude fainter. This is a very important gain for research on the faint globular cl.u.s.ters, as well as the small and faint spiral and planetary nebulae, providing a much larger scale for these objects and sufficient light at the same time.
Photographs of the moon and many other less critical tests have been made with very satisfactory results. Those of the moon appear to be decidedly superior in definition to any previously taken with other instruments.
Another investigation is of great importance in the light of recent advances in theoretical dynamics. Darwin, in his fundamental researches on the dynamics of rotating ma.s.ses, dealt with incompressible matter, which a.s.sumes the well-known pear-shaped figure, and may ultimately separate into two bodies. Roche on the other hand discussed the evolution of a highly compressible ma.s.s, which finally acquires a lens-shaped form and ejects matter at its periphery. Both of these are extreme cases. Jeans has recently dealt with intermediate cases, such as are actually encountered in stars and nebulae. He finds that when the density is less than about one-fourth that of water, a lens-shaped figure will be produced with sharp edges, as depicted by Roche. Matter thrown off at opposite points on the periphery, under the influence of small tidal forces from neighboring ma.s.ses, may take the form of two symmetric filaments, though it is not yet entirely clear how these may attain the characteristic configuration of spiral nebulae. The preliminary results of Van Maanen indicate motion outward along the arms, in harmony with Jeans's views.
Jeans further discusses the evolution of the arms, which will break up into nuclei (of the order of ma.s.s of the sun) if they are sufficiently ma.s.sive, but will diffuse away if their gravitational attraction is small. The ma.s.s of our solar system is apparently not great enough, according to Jeans, to account for its formation in this way. As is apparent, these investigations lead to conclusions very different from those derived by Chamberlin and Moulton from the planetesimal hypothesis.
This is a critical study of spiral nebulae for which the 100-inch telescope is of all instruments in existence the best suited. The spectra of the spirals must be studied, as well as the motions of the matter composing the arms. Their parallaxes, too, must be ascertained.
A photographic campaign including spiral nebulae of various types will settle the question of internal motions. The large scale of the spiral nebulae at the princ.i.p.al focus of the Hooker telescope, and the experience gained in the measurement of nebular nuclei for parallax determination, will help greatly in this research. A multiple-slit spectrograph, already applied at Mount Wilson, will be employed, not only on spiral nebulae whose plane is directed toward us, but also on those whose plane lies at an angle sufficient to permit both components of motion to be measured by the two methods.
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