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,
surpassing all others in power, directs especial attention to the
extensive activities of this institution, 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 relationship 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 clusters; 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 massive 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 titanium 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 circumstances 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 surpasses the 60-inch telescope in every
class of work. For many months most of the observations and photographs
have been made with the Cassegrain 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 clusters, 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 masses, dealt with incompressible matter,
which assumes 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 mass, 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 masses, 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 mass of the sun) if they are sufficiently
massive, but will diffuse away if their gravitational attraction is
small. The mass 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 principal 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.
In dealing with problems of structure and motion in the Galactic system,
the 100-inch telescope offers especial advantages, because of its vast
light-gathering power. Studies of radial velocities of the stars have
hitherto been necessarily confined to the brighter stars, for the most
part even to those visible to the naked eye. While some of these are
very distant, most of the stars whose radial velocities are known belong
to a very limited group, perhaps constituting a distinct cluster of
which the sun is a member, but in any event of insignificant proportions
when contrasted with the Galaxy. Current spectrographic work with the
60-inch telescope includes stars of the eighth magnitude, and some even
fainter. But while the 60-inch has enabled Adams to measure the
distances of many remote stars by his new spectroscopic method, and to
double the known extent (so far as spectroscopic evidence is concerned)
of the star streams of Kapteyn, a much greater advance into space is
necessary to find out the community of motion among the stars comprising
the Galactic system. The Hooker telescope will enable us to determine
accurate radial velocities to stars of the eleventh magnitude, which
doubtless truly represent the Galaxy.
In order to secure a maximum return within a reasonable period of time,
the stars in the selected areas of Kapteyn will be given the preference,
because of the vast amount of work already done, relating to their
positions, proper motions, and visual and photographic magnitudes. Such
consideration as spectral type, the known directions of star-streaming,
and the position of the chosen regions with reference to the plane of
the Galaxy are given adequate weight, and it is of fundamental
importance that the method of spectroscopic parallaxes will permit dwarf
stars to be distinguished from stars that are in the giant class, but
rendered faint by their much greater distance. In addition to these
problems, the stellar spectrograms will provide rich material for study
of the relationship between stellar mass and speed, and the nature of
giant stars and dwarf stars.
Shapley's recent studies of globular clusters have indicated the
significance of these objects in both evolutional and structural
problems, and the possibility of determining their parallaxes by a
number of independent methods is of prime importance, both in its
bearing on the structure of the universe and because it permits a host
of apparent magnitudes to be at once transformed into absolute
magnitudes. Here the advantage of the Hooker telescope is two-fold: at
its 134-foot focus the increased scale of the crowded clusters makes it
possible to select separate stars for spectrum photography (which could
not be done with the 60-inch where the images were commingled); and the
great gain in light is such that the spectra of stars to the 14th
magnitude have been photographed in less than an hour.
Faint globular clusters, then, will comprise a large part of the early
program with the 100-inch telescope: the faintest possible stars in them
must be detected and their magnitudes and colors measured; spectral
types must be determined, and the radial velocities of individual stars
and of clusters as a whole; spectroscopic evidence of possible axial
rotation of globular clusters must be searched for; and the method of
spectroscopic parallaxes, as well as other methods, must be applied to
ascertaining the distances of these clusters.
The possibility of dealing with many problems relating to the
distribution and evolution of the faintest stars depends upon the
establishment of photographic and photovisual magnitude scales. Below
the twelfth magnitude, the only existing scale of standard visual or
photovisual magnitudes is the Mount Wilson sequence, already extended by
Seares to magnitude 17.5 with the 60-inch telescope.
Extension of this scale to even fainter magnitudes, and its application
to the faintest stars within its range is an important task for this
great telescope, as it will doubtless bring within range hundreds of
millions of stars that are beyond the reach of the 60-inch. The giants
among them will form for us the outer boundary of the Galactic system,
while the dwarfs will be of almost equal interest from the evolutional
standpoint. The photometric program of the 100-inch, then, will deal
with such questions as the condensation of the fainter stars toward the
Galactic plane, the color of the most distant stars, and the final
settlement of the long inquiry regarding the possible absorption of
light in space.
[Illustration: GREAT SUN-SPOT GROUP, AUGUST 8, 1917. The disk in the
lower left corner represents the comparative size of the earth.
(_Photo, Mt. Wilson Solar Observatory._)]
[Illustration: THE SUN'S DISK. The view shows the "rice grain"
structure of the photosphere and brilliant calcium flocculi.
(_Photo, Yerkes Observatory._)]
[Illustration: THE LUNAR SURFACE VISIBLE DURING A TOTAL ECLIPSE OF
THE MOON, FEBRUARY 8, 1906. (_Photo, Yerkes Observatory._)]
Another research of exceptional promise will be undertaken, which is of
great importance in a general study of stellar evolution; and that is
the determination of the spectral-energy curves of stars of various
classes, for the purpose of measuring their surface temperatures. A very
few of the nebulae are found to be variable, and their peculiarities need
investigation, also special problems of variable stars and temporary
stars, and the spectra of the components of close double stars which are
beyond the power of all other instruments to photograph.
Such a program of research conveys an excellent idea of many of the
great problems that are under investigation by astronomers to-day, and
gives some notion of the instrumental means requisite in executing
comprehensive plans of this character. It will not escape notice that
the climax of instrumental development attained at Mount Wilson has only
been made possible by an unbroken chain of progress, link by link, each
antecedent link being necessary to the successful forging of its
following one. In very large part, and certainly indispensable to these
instrumental advances, has the art of working in glass and metals been
the mainstay of research. As we review the history of astronomical
progress, from Galileo's time to our own, the consummate genius of the
artisan and his deft handiwork compel our admiration almost equally with
the keen intelligence of the astronomer who uses these powerful engines
of his own devising to wrest the secrets of nature from the heavens.