Adams, Walter Sydney

(b. Kessab, near Antioch, Syria, 20 December 1876; d. Pasadena, California, 11 May 1956)

astrophysics.

The son of Lucien Harper Adams and Nancy Dorrance Francis, missionaries in Syria, Adams was brought up in a strict, though broad-minded, environment. He received his early schooling from his mother and from his father’s classical and historical library. His childhood was spent near Antioch, crossroads of the Crusades, and at the age of six he knew more of the history of Athens and Rome and the campaigns of Alexander the Great and Hannibal than of the United States. Here, too, his interest in astronomy was aroused when his father pointed out the constellations in the clear Syrian skies.

In 1885 the family returned to Derry, New Hampshire. Adams graduated with the A.B. from Dartmouth College in 1898 and, on the advice of Edwin B. Frost, his teacher of astronomy, entered the University of Chicago. There he earned a reputation as a skillful mathematician. He gained his first practical observing experience under George Ellery Hale, founder and director of the Yerkes Observatory, which, with its forty-inch refracting telescope, was then the largest in the world. After receiving his M.A. in 1900, he went to the University of Munich; the following year he returned to Yerkes as computer and general assistant.

Adams was married twice, first to Lillian Wickham, who died in 1920, and in 1922 to Adeline L. Miller, by whom he had two sons, Edmund and John.

Inspired by Hale’s vision of the future of astrophysics and by his belief in the importance of an observatory as a physical laboratory, Adams followed his path enthusiastically. In 1947, fifty years after the dedication of the Yerkes Observatory, he described the revolution in astronomy:

It opened at a time when visual observations were still a major factor in observatory activities, photographic methods were in their infancy, the spectrum was studied empirically and cosmogony was almost a completely sealed book. The period ends with visual observations greatly reduced in amount, although still holding an important place, with photographic methods applied almost universally and enormously improved and extended, with the spectrum analyzed and used as an extraordinarily powerful tool to seek out physical processes in the sun and stars, and with a clear and logical picture of a physical universe beyond the imagination of the astronomer of fifty years ago.¹

In this transformation Adams played a leading role, first at the Yerkes, then at the Mount Wilson Observatory. As acting director of Mount Wilson at various periods, then as director from 1923 to 1946, he contributed significantly to the design of instruments, especially of the 100-inch and of the 200-inch Hale telescopes on Palomar Mountain. Through his leadership he helped to make the Mount Wilson and Palomar observatories preeminent, so that astronomers the world over came to use these, the most powerful astronomical instruments on earth, to push back the frontiers of the universe.

Adams’ observations ranged from planetary atmospheres to interstellar gases, from sunspot spectra to his greatest achievement—the discovery of a method for determining stellar distances. His research was characterized by notable skill in observation and precision in measurement.

His influence on astronomical development, both nationally and internationally, was reflected in his positions in the American Astronomical Society (president, 1931–1934), the Astronomical Society of the Pacific (president, 1923), and the International Astronomical Union (vice-president, 1935–1948, acting secretary, 1940–1945). His broader interests in science were reflected in his membership in the American Philosophical Society (elected 1915) and the National Academy of Sciences (1917) and by his election as foreign associate to many academies of science, including those of France, Sweden, and the Soviet Union, and the Royal Society of London (foreign member).

His earliest work, on the polar compression of Jupiter, was followed by research with Frost on radial velocities in B-type, or helium, stars. By 1903, despite the difficulty of observing the diffuse spectral lines, measurement of the velocities in twenty such stars showed the average motion to be exceptionally small. This result would prove important in discussions of stellar motions, especially in the recognition by W.W. Campbell of the so-called K-term. In April 1904, Adams joined Hale on the Yerkes expedition to Pasadena, California; this led to the establishment by the Carnegie Institution of Washington of the Mount Wilson Solar Observatory on 20 December 1904. In his vivid “Early Days at Mount Wilson,” he described the wild and primitive conditions, the joys and difficulties of a pioneer time when transportation of equipment was wholly by pack train and the only means of reaching the peak was by mule or burro, or on foot. He had a wiry, athletic build and an indefatigable spirit, and often climbed the steep, twisting, eight-mile trail; prepared the telescope for observing; worked the night through; then walked down the mountain the following morning.

At Mount Wilson he joined Hale in an intensive study of that “typical star,” our sun, first with the horizontal Snow reflector, then with the sixty-foot and 150-foot tower solar telescopes. Visually it had been observed that the spectrum of a sunspot differs from that of the solar disk, but little was known of the nature of the spot spectrum, and nothing of its cause. Now, for the first time, it became possible to study spot spectra photographically with adequate apparatus. In 1906 Adams and Hale took the first photograph of a spot spectrum at Mount Wilson and undertook a detailed comparison of spot spectra with those of the solar disk. Simultaneously experiments were begun in the primitive laboratory on the mountain to imitate the conditions observed in the sun. Working with Henry Gale on arc spectra, Adams and Hale were able to show that temperature must be the cause of the differences observed between spots and disk, and thus to prove that sunspots are cooler than the surrounding solar surfaces. From further laboratory studies of pressure and density, they also showed that “enhanced lines” (those lines identified and named by Norman Lockyer to denote lines that are much stronger in the electric spark spectrum than in that of the arc) are the result of a lower density of the gases, while in sunspots the density proved to be higher. These and other results found a rational explanation when, in 1920, M.N. Saha published his theory of ionization.

In the course of this investigation Adams became interested in the problem of solar rotation. From a study of the minute Doppler displacements at various solar levels, he found that higher levels in the sun showed a higher rate of rotation and a smaller equatorial acceleration than the lower levels. In 1909, with Hale, he succeeded in photographing the flash spectrum without eclipse.

Gradually Adams turned from studies of the sun to other, larger stars. Yet, as he was often to show, the early sunspot studies played an important role in the understanding of other stars, and especially in the determination of their distances. In “Sunspots and Stellar Distances” he described the fascinating chain of events that led from the classification of groups of spectral lines according to temperature, to show how unexpected the ramifications of scientific investigation can be. “The study attained its primary objectives, but in addition it provided in the field of physics the first clues to the analysis of complex spectra according to energy levels in the atom, in solar physics the discovery of magnetism in the sun, and in astrophysics a new and fundamental method for determining the distances of the stars.”²

In 1906, using the Snow telescope, Adams had succeeded in taking a twenty-three-hour exposure, on five successive nights, of the line spectrum of the cool star Arcturus. When he compared this spectrum with that of a sunspot, he found them to be similar. In 1908, after the sixty-inch reflecting telescope was set up on Mount Wilson, he extended his comparative studies to other stars—their motions, spectral classifications, magnitudes, and the distances of those too far away to be measured trigonometrically. These studies included the first thorough investigation of the differences in the spectra of the large and massive stars of high luminosity called giants and the comparatively dense bodies of very low luminosity known as dwarfs.

In 1914, working at first with Arnold Kohlschütter, Adams compared pairs of stars of nearly the same spectral type, and therefore of nearly the same temperature, but of very different luminosity.

It soon appeared that a few lines were stronger in the spectrum of the highly luminous star, and others in that of the intrinsically faint star. With the use of all the available material for well-determined luminosities it then became possible to establish numerical correlations between luminosity and the intensities of these sensitive lines. The process could then be reversed and in the case of a star of unknown luminosity its value could be derived from the intensities of the lines; the distance of the star is then readily calculated from the simple relationship connecting apparent brightness, luminosity and distance.³

This ingenious method of obtaining “spectroscopic parallaxes,” applied to thousands of stars, has become a fundamental astronomical tool of immense value in gaining knowledge of giant and dwarf stars and of galactic structure. Otto Struve commented, “It is not an exaggeration to say that almost all our knowledge of the structure of the Milky Way which has developed during the past quarter of a century has come from the Mount Wilson discovery of spectroscopic luminosity criteria.”⁴

In 1917 the 100-inch telescope went into operation. Adams had made the Hartmann tests of the mirror which were vital to its successful figuring. For it he built the powerful Coudé spectrograph that would provide higher dispersion and make the penetration of hitherto unknown regions possible. The following year, in studies that stemmed from his investigation of giants and dwarfs, he became interested in Sirius B, the companion of Sirius that he had first identified as a tiny white-hot star, or white dwarf, in 1915. He found that while the companion is small, it has a mass not much less than that of the sun (actually four-fifths of that mass). It proved, almost incredibly, to be about 50,000 times as dense as water. A ton of such material could be squeezed into a matchbox. Sir Arthur Eddington predicted that, since the Einstein effect is proportional to the mass divided by the radius of the star and the radius of the companion of Sirius is very small, the relativity effect should be large. In 1925 Adams performed the difficult feat of taking a spectrogram of the faint companion, which is 10,000 times fainter than its neighbor, yet only twelve arc seconds away. He confirmed Eddington’s prediction when he found a displacement to the red of 21 km./sec., a result he later modified to 19 km./sec. Eddington wrote: “Prof. Adams has thus killed two birds with one stone. He has carried out a new test of Einstein’s general theory of relativity, and he has shown that matter at least 2,000 times denser than platinum is not only possible, but actually exists in the stellar universe.”⁵

In the 1920’s and 1930’s Adams also applied the spectrograph to studies of the atmospheres of Venus and Mars—those observations were difficult because of the problem of identifying such substances as oxygen. carbon dioxide, and water vapor, which are also continued in the earth’s atmosphere. In 1932, with Theodore Dunham. Jr., he identified carbon dioxide in the infrared spectrum of Venus. In 1934 similar observations of Mars indicated that the amount of free oxygen above a given area of the surface of Mars cannot exceed one-tenth of one percent.

Over the years other investigations included Cepheids, spectroscopic binaries, and, from 1901 to 1936, the spectra of novae that he felt might be explained by an expanding shell or succession of shells. In his last extensive research on the clouds of interstellar gas he had the arduous task of sorting out the lines in a star’s own spectrum from those belonging to the tenuous interstellar gases. He found double or multiple interstellar lines in 80 percent of the stars examined, identified two classes of clouds, and observed four clouds moving with radial velocities up to 100 km./sec. The highly accurate velocities provided good values for the relative motions caused by the rotation of the galaxy.

These, then, were the far-ranging programs through which Adams contributed to our knowledge of the nature of the universe and profoundly influenced the development of cosmogony.

NOTES

1. “Some Reminiscences of the Yerkes Observatory,” p. 196.

2.Cooperation in Solar Research, pp. 135–137.

3. “Biographical Notes—Walter S. Adams,” written for the National Academy of Sciences (Jan. 1954). p. 9 (unpublished).

4. “Fifty Years of Progress in Astronomy,” p. 6.

5.Stars and Atoms, p. 52.

BIBLIOGRAPHY

I. Original Works. Adams’ bibliography in Joy’s biographical article (see below) includes 270 papers, in addition to his annual Mount Wilson reports. Among these papers are “The Polar Compression of Jupiter,” in Astronomical Journal, 20 (1899), 133, written while he was still a graduate student; “Radial Velocities of Twenty Stars Having Spectra of the Orion Type,” in Publications of the Yerkes Observatory, 2 (1904), 143–250, written with E.B. Frost; “Photographic Observations of the Spectra of Sunspots,” in Astrophysical Journal, 23 (1906), 11–44, written with G.E. Hale; “Preliminary Paper on the Cause of the Characteristic Phenomena of Sunspot Spectra,” ibid., 24 (1906), 185–213, written with G.E. Hale and H.G. Gale: “Sunspot Lines in the Spectrum of Arcturus,” ibid., 69–77; “Spectroscopic Observations of the Rotation of the Sun,” ibid., 26 (1907), 203–224; “Photography of the Flash Spectrum Without an Eclipse,” ibid., 30 (1909), 222–230, written with G.E. Hale; “The Radial Velocities of 100 Stars With Measured Parallaxes,” ibid., 39 (1914), 341–349, written with A. Kohlschütter; “The Spectrum of the Companion of Sirius,” in Publications of the Astronomical Society of the Pacific, 27 (1915), 236–237; “A Spectroscopic Method of Determining Parallaxes,” in Proceedings of the National Academy of Sciences, 2 (1916), 147–152; “Address of the Retiring President...,” in Publications of the Astronomical Society of the Pacific, 36 (1924), 2–9; “The Relativity Displacement of the Spectral Lines in the companion of Sirius,” in Proceedings of the National Academy of Sciences, 11 (1925), 382–387; “The Past Twenty Years of Physical Astronomy,” in Publications of the Astronomical Society of the Pacific, 40 (1928), 213–228; “The Astronomer’s Measuring Rods,” ibid., 41 (1929), 195–211; “Absorption Bands in the Spectrum of Venus,” ibid., 44 (1932), 243–245, written with Theodore Dunham, Jr.; “The B-Band of Oxygen in the Spectrum of Mars,” in Astrophysical Journal, 79 (1934), 308–316, written with Theodore Dunham, Jr.; “The Planets and Their Atmospheres,” in Scientific Monthly, 39 (1934), 5–19; “The Sun’s Place Among the Stars,” in Annual Report of the Smithsonian Institution for 1935, pp. 139–151; “Sunspots and Stellar Distances,” in Cooperation in Solar Research, Carnegie Institution of Washington pub. no. 506 (1938), pp. 135–147; “George Ellery Hale,” in Biographical Memoirs of the National Academy of Sciences, 21 (1940), 181–241; “Newton’s Contributions to Observational Astronomy,” in The Royal Society Newton Tercentenary Celebrations (Cambridge, 1946), pp. 73–81; “Early Days at Mount Wilson,” in Publications of the Astronomical society of the Pacific, 59 (1947), 213–231, 285–304; the Henry Norris lecture of the American Astronomical Society (29 Dec. 1947), ibid., 60 (1948), 174–189; “Some Reminiscences of the Yerkes Observatory,” in Science, 106 , no. 2749 (5 Sept.1947), 196–200; “The History of the International Astronomical Union,” in Publications of the Astronomical Society of the Pacific, 61 (1949), 5–12; “The Founding of the Mount Wilson Observatory,” ibid., 66 (1954), 267–303; and “Early Solar Research at Mount Wilson,” in Arthur Beer, ed., Vistas in Astronomy, I (London, 1955), 619–623. See also the Annual Report of the Mount Wilson Observatory (1923–1945).

The bulk of Adams’ correspondence, original manuscripts, and other source materials are (as of 1968) in the Hale Solar Laboratory and in the director’s files of the Mount Wilson and Palomar observatories.

II. Secondary Literature. Biographical articles on Adams include Alfred H. Joy, “Walter S. Adams. a Biographical Memoir,” in Biographical Memoirs of the National Academy of Science, 31 (1958); Paul W. Merrill, “Walter S. Adams, Observer of Sun and Stars,” in Science, 124 (13 July 1956), 67; Harlow Shapley, “A Master of Stellar Spectra,” in Sky and Telescope, 15 (1956), 401; and F.J.M. Stratton, “Walter Sydney Adams (1876–1956),” in Biographical Memoirs of the Royal Society, 2 (Nov. 1956), 1–18.

Additional works that contribute to a picture of the development of astronomy in this period, and of Adams’ role in that development, are Charles G. Abbot, Adventures in the World of Science (Washington, D.C., 1958), esp. ch6; Giorgio Abetti, “Solar Physics,” in Handbuch der Astrophysik. IV (Berlin, 1920). 161–168, and VII (Berlin, 1936), 184; The History of Astronomy, Betty B. Abetti, trans. (New York, 1952). pp. 255, 258, 259, 274, 291–292, 298, 306, 327; and The Sun, J. B. Sidgwick, trans. (New York, 1957), pp. 103, 142, 143, 144, 145, 147, 148, 164–165, 206, 231, 232; Herbert Dingle, “The Message of Starlight,” in T.E.R. Phillips and W.H. Steavenson, eds., Splendour of the Heavens, II (London, 1924), 479–499; Sir Arthur Eddington. Stars and Atoms (Oxford, 1927), pp. 48–53; Edwin B. Frost. An Astronomer’s Life (Boston, 1933); George Ellery Hale, The Study of Stellar Evolution (Chicago, 1908); Caryl Haskins, The Search for Understanding (Washington, D.C., 1967), which includes a large part of “Early Days at Mount Wilson” (pp. 301–326) and other material relating to the observatory (pp. 234–277); Gerard P. Kuiper, ed., The Sun (Chicago, 1953), especially the excellent introduction by Leo Goldberg, pp. 7–22; Knut Lundmark, “Luminosities Colours, Diameters, Densities, Masses of the Stars,” in Handbuch der Astrophysik, V (Berlin, 1932), ch.4; A. Pannekoek, A History of Astronomy (New York, 1961); M. N. Saha. “Ionization in the Solar Chromosphere,” in Philosophical Magazine40 (1920), 479; Harlow Shapley, “Brief Historical Analysis on the Spectra of Stars and Nebulae,” in Source Book in Astronomy; 1900–1950 (Cambridge, Mass., 1960), pp. 159–161, 162–164 in which Shapley discusses Adams’ work on star distances and reprints two of his papers; Otto Struve, “The Story of an Observatory (The Fiftieth Anniversary of the Yerkes Observatory).” in Popular Astronomy, 55 , nos. 5–6 (May-June 1947); and “Fifty Years of Progress in Astrophysics,” in The Science Counselor (Mar, 1948). 4–6, esp. 6. 26–27; Otto Struve and Velta Zebergs, Astronomy of the 29th Century (New York, 1962); and Helen Wright Palomar the World’s Largest Telescope (New York, 1952), and Explorer of the Universe, a Biography of George Ellery Hale (New York, 1966).

Helen Wright