APPENDIX II THE LOWELL OBSERVATORY by Professor Henry Norris Russell

The Observatory at Flagstaff is Percival Lowell’s creation. The material support which he gave it, both during his lifetime and by endowment, represents but a small part of his connection with it. He chose the site, which in its combination of excellent observing conditions and the amenities of everyday life, is still unsurpassed. He selected the permanent members of the staff and provided for the successor to the Directorship after his death. Last, but not least, he inspired a tradition of intense interest in the problems of the universe, and independent and original thought in attacking them, which survives unimpaired.

On a numerical basis—whether in number of staff, size of instruments, or annual budget—the Lowell Observatory takes a fairly modest rank in comparison with some great American foundations. But throughout its history it has produced a long and brilliant series of important discoveries and observations notable especially for originality of conception and technical skill. Percival Lowell’s own work has been fully described; it remains to summarize briefly that of the men whom he chose as his colleagues, presenting it according to its subject, rather than in chronological order.

The photography of the planets has been pursued for thirty years, mainly by the assiduous work of E. C. Slipher, and the resulting collections are unrivalled. Only a small amount of this store has been published or described in print, but among its successes may be noted the first photographs of the canals of Mars, and the demonstration by this impersonal method of the seasonal changes in the dark areas, and of the occasional appearance 207 of clouds. It is a commonplace that any astronomer who wants photographs of the planets for any illustrative purpose instinctively applies to his friends in Flagstaff, and is not likely to be disappointed.

The discovery of Pluto, and incidentally of many hundreds of asteroids, has already been described.

An important series of measurements of the radiation from the planets was made at Flagstaff in 1921 and 1922 by Dr. W. W. Coblentz of the Bureau of Standards and Dr. C. O. Lampland. Using the 40-inch reflector, and the vacuum thermocouples which the former had developed, and employed in measurements of stellar radiation at the Lick Observatory, and working with and without a water-cell (which transmits most of the heat carried by the sunlight reflected from a planet, but stops practically all of that radiated from its own surface), they found that the true “planetary heat” from Jupiter was so small that its surface must be very cold, probably below -100° Centigrade, while that from Mars was considerable, indicating a relatively high temperature. Both conclusions have been fully confirmed by later work.

Spectroscopic observation has been equally successful. In 1912 Lowell and Slipher (V. M.) successfully attacked the difficult problem of the rotation of Uranus. One side of a rotating planet is approaching us, the other receding. If its image is thrown on a spectroscope, so that its equatorial regions fall upon the slit, the lines of the spectrum will be shifted toward the violet on one edge, and the red on the other, and will cross it at a slant instead of at right angles. This method had long before been applied to Jupiter and to Saturn and its rings, but Uranus is so faint as to discourage previous observation. Nevertheless, with the 24-inch reflector, and a single-prism spectrograph, seven satisfactory plates were obtained, with an average exposure of 2½ hours, every one of which showed a definite rotation effect. The mean result indicated that Uranus rotates in 10¾ hours, with motion retrograde, as in the case of his satellites. This result was confirmed five years latter by Leon Campbell at Harvard, who observed regular variations in the planet’s brightness with substantially the same period.

It has been known since the early days of the spectroscope that the major planets exhibit in their spectra bands produced 208 by absorption by the gases of their atmospheres, and that these bands are strongest in the outer planets. Photographs showing this were first made by V. M. Slipher at the Lowell Observatory in 1902. To get adequate spectrograms of Neptune required exposures of 14 and 21 hours—occupying the available parts of the clear nights of a week. The results well repaid the effort. The bands which appear faintly in Jupiter are very strong in Uranus, and enormous in Neptune’s spectrum, cutting out great portions of the red and yellow, and accounting for the well-known greenish color of the planet. Only one band in the red was present in Jupiter alone.

For a quarter of a century after this discovery those bands remained one of the most perplexing riddles of astrophysics. The conviction gradually grew that they must be due to some familiar gases, but the first hint of their origin was obtained by Wildt in 1932, who showed that one band in Jupiter was produced by ammonia gas, and another probably by methane. These conclusions were confirmed by Dunham in the following year, but the general solution of the problem was reserved for Slipher and Adel, who, in 1934, announced that the whole series of unidentified bands were due to methane. The reason why they had not been identified sooner is that it requires an enormous thickness of gas to produce them. A tube 45 meters long, containing methane at 40 atmospheres pressure, produces bands comparable to those in the spectra of Saturn. The far heavier bands in Neptune indicate an atmosphere equivalent to a layer 25 miles thick at standard atmospheric pressure. The fainter bands though not yet observed in the laboratory, have been conclusively identified by the theory of band-spectra. Ammonia shows only in Jupiter and faintly in Saturn; the gas is doubtless liquefied or solidified at the very low temperatures of the outer planets.

The earth’s own atmosphere has also been the subject of discovery at Flagstaff. The light of a clear moonless sky does not come entirely from the stars and planets; about one-third of it originates in the upper air, and shows a spectrum of bright lines and bands. The familiar auroral line is the most conspicuous of these, but V. M. Slipher, making long exposures with instruments of remarkably great light-gathering power, has recently detected a large number of other bands, in the deep red and even the 209 infra-red. Were our eyes strongly sensitive to these wave-lengths, the midnight skies would appear ruddy.

Just as the first rays of the rising sun strike the upper layers of the atmosphere many miles above the surface, new emission bands appear in the spectrum—to be drowned out soon afterwards by the twilight reflected from the lower and denser layers; and the reverse process is observable after sunset.

The origin of these remarkable and wholly unexpected radiations is not yet determined.

The spectrograph of the Observatory was also employed in observations of stars, and again led to unexpected discoveries. In 1908, while observing the spectroscopic binary Beta Scorpii, V. M. Slipher found that the K line of calcium was sharp on his plates, while all the others were broad and diffuse. Moreover, while the broad lines shifted in position as the bright star moved in its orbit, the narrow line remained stationery. Hartmann, in 1904, had observed a similar line in the spectra of Delta Orionis, and suggested that it was absorbed in a cloud of gas somewhere between the sun and the star. Slipher, extending his observations to other parts of the heavens, found that such stationery calcium lines were very generally present (in spectra of such types that they were not masked by heavier lines arising in the stars themselves), and made the bold suggestion that the absorbing medium was a “general veil” of gas occupying large volumes of interstellar space.

This hypothesis, which appeared hardly credible at that time, has been abundantly confirmed—both by the discovery of similar stationery lines of sodium, and by the theoretical researches of Eddington,—and no one now doubts that interstellar space is thinly populated by isolated metallic atoms presumably ejected from some star in the remote past, but now wandering in the outer darkness, with practically no chance of returning to the stars.

To secure satisfactory spectroscopic observations of nebulae is often very difficult. Though some of these objects are of considerable brightness, they appear as extended luminous surfaces in the heavens, and in the focal plane of the telescope. The slit of a spectroscope, which must necessarily be narrow to permit good resolution of the lines, admits but a beggarly fraction of the nebula’s light. To increase the size of the telescope helps very 210 little, for, though more light is collected in the nebular image, this image is proportionately increased in area, and no more light enters the slit than before.

For the gaseous nebulae, whose spectra consist of separate bright lines, there is no serious difficulty; but the majority of nebulae have continuous spectra, and when the small amount of light that traverses the slit is spread out into a continuous band, it becomes so faint that prohibitively long exposures would be required to photograph it. It was at the Lowell Observatory that Dr. V. M. Slipher first devised a way of meeting this difficulty.

By employing in the camera of the spectrograph (which forms the image of the spectrum on the plate) a lens of short focus, this image became both shorter and narrower, thereby increasing the intensity of the light falling on a given point of the plate in a duplicate ratio. Moreover, since with this device the image of the slit upon the plate is much narrower than the slit itself, it became possible to open the slit more widely and admit much more of the light of the nebula, without spoiling the definition of the spectral lines.

This simple but ingenious artifice opened up a wholly new field of observation, and led to discoveries of great importance.

Within the cluster of the Pleiades, and surrounding it, are faint streaky wisps of nebulosity, which have long been known. One might have guessed that the spectrum, like that of some other filamentous nebulae, would be gaseous. But when Slipher photographed it in December 1912 (with an exposure of 21 hours, on three successive nights) he found a definite continuous spectrum, crossed by strong dark lines of hydrogen and fainter lines of helium—quite unlike the spectrum of any previously observed nebula, but “a true copy of that of the brighter stars in the Pleiades.” Careful auxiliary studies showed that the light which produced this spectrum came actually from the nebula. This suggested at once that this nebula is not self-luminous, but shines by the reflected light of the stars close to it. This conclusion has been fully verified by later observations, at Flagstaff and elsewhere. It is only under favorable conditions that one of these vast clouds (probably of thinly scattered dust) lies near enough to any star to be visibly illuminated. The rest 211 reveal themselves as dark markings against the background of the Milky Way.

Similar observations of the Great Nebula of Orion showed that the conspicuous “nebular” lines found in its brighter portions faded out in its outer portions, leaving the hydrogen lines bright, while, at the extreme edge, only a faint continuous spectrum appeared. This again has been fully explained by Bowen’s discovery of the mechanism of excitation of nebular radiation by the ultra-violet light from exceedingly hot stars, and affords a further confirmation of it.

But the most important contribution of the new technique was in the observation of the spiral nebulae. Their spectra are continuous and so faint that previous instruments brought out only tantalizing suggestions of dark lines. With the new spectrograph, beautiful spectra were obtained, showing numerous dark lines, of just the character that might have been expected from vast clouds of stars of all spectral types. This provided the first definite indication of one of the greatest of modern astronomical discoveries—that the white nebulae are external galaxies, of enormous dimensions, and at distances beyond the dreams of an earlier generation.

By employing higher dispersion, spectra were secured which permitted the measurement of radial velocity. The first plates, of the Andromeda Nebula, revealed the almost unprecedented speed of 300 kilometers per second toward the Sun. Later measures of many other nebulae showed that this motion was, for a nebula, unusually slow, but remarkable in its direction, for practically all the others were receding.

Similar measures upon globular star-clusters showed systematic differences in various parts of the heavens, which indicated that, compared with the vast system of these clusters, the Sun is moving at the rate of nearly 300 kilometers per second—a motion which is now attributed to its revolution, in a vast orbit, about the center of the Galaxy, as a part of the general rotation of the latter.

The velocities of the nebulae reveal substantially the same solar motion, but, over and above this, an enormous velocity of recession, increasing with the faintness and probable distance of the nebulae.

This, again, was a discovery of primary importance. It has 212 been confirmed at other observatories and observations with the largest existing telescope have revealed still greater velocities of recession in nebulae too faint to observe at Flagstaff. How this has led to the belief that the material universe is steadily expanding and that its ascertainable past history covers only some two thousand millions of years, can only be mentioned here.

This is a most remarkable record for thirty years’ work of a single observatory with a regular staff never exceeding four astronomers. But its distinction lies less in the amount of the work than in its originality and its fertile character in provoking extensive and successful researches at other observatories as well.

All this is quite in the spirit of its Founder, and, to his colleagues in the science, makes the Observatory itself seem his true monument. His body lies at rest upon the hill, but, in an unquenched spirit of eager investigation, his soul goes marching on.