Walter Baade

German astronomer

• Born: March 24, 1893
• Birthplace: Schröttinghausen, Westphalia, Germany
• Died: June 25, 1960
• Place of death: Bad Salzuflen, Westphalia, Germany

Baade was one of the most influential astronomers of the twentieth century. He made major discoveries that led to greater scientific understanding of stellar and galactic evolution, was instrumental in the discovery of the supernova phenomenon, and introduced the concept of stellar populations.

Early Life

Walter Baade (BAH-duh), christened Wilhelm Heinrich Walter Baade, was the oldest son of Konrad Baade and Charlotte Baade. Walter suffered a hip disorder from birth that plagued him his entire life. Konrad was a schoolmaster, and had hopes of his son studying theology and entering the clergy. From 1903 to 1912, the young Baade studied at Friedrichs Gymnasium (a German secondary school), where he showed a strong aptitude for science. He then spent a year at the University of Münster before transferring to the University of Göttingen in 1913.

Baade continued studies at Göttingen until 1919, when he received his Ph.D., with his doctoral thesis looking at the eclipsing binary star Beta Lyrae. His hip injury exempted Baade from military service in World War I, though he was pressed into service from 1916 to 1918 performing aerodynamics weapons research. The Göttingen Observatory was poorly equipped, and Baade did not get much observing experience. After receiving his Ph.D., Baade took a position at the Hamburg Observatory in Bergedorf, operated by the University of Hamburg, where he became an expert in observational astronomy. Baade married Johanna “Muschi” Bohlmann, a computational assistant at the University of Hamburg, in 1929. Asteroid 966 Muschi was named for her.

Life’s Work

Baade became a well-known astronomer because of his work at Bergedorf, where he discovered Comet Baade (C/1922 U1) and the asteroid 944 Hidalgo. For many years 944 Hidalgo was the most distant asteroid discovered; it had an orbit that ranged nearly as far as Saturn. Baade secured a Rockefeller Fellowship to travel to Harvard, Yerkes Observatory in Wisconsin, and Mount Wilson Observatory in Southern California for a year beginning in 1926. Then, in 1929, Baade traveled together with famed opticianBernhard Voldemar Schmidt to the Philippines to observe a solar eclipse. In 1931, Walter Adams of Mount Wilson Observatory, greatly impressed with Baade from his earlier work there, offered him a permanent staff position. Baade quickly accepted, and he and his wife moved to California, where he would remain for most of his life.

At Mount Wilson, Baade began work with Fritz Zwicky studying a certain type of nova that he had referred to as a hauptnova (chief nova) in a lecture that Baade had given at Hamburg a few years earlier. The two astronomers realized that novae of this type were far too luminous to be anything other than an exploding star. Together, they worked out the physics of stellar explosions and the phenomenon that is now called a “supernova,” a term Baade and Zwicky coined to replace Baade’s earlier hauptnova designation.

By the late 1930’s, the Nazis had secured much power in Germany. Baade worked very hard to help a Jewish friend and colleague, Rudolph Minkowski from the University of Hamburg, escape from Germany and the Nazis. Disgusted with what was happening in Germany, Baade reportedly initiated an application for U.S. citizenship, but the paperwork was lost. Scornful of the bureaucracy that had lost his paperwork, Baade never reapplied. Thus, when the United States entered World War II, Baade, still a German citizen living in California, was declared an enemy alien. However, his colleagues at Mount Wilson managed to keep him from being sent to an internment camp by ensuring the U.S. government that he would remain in the immediate vicinity of Mount Wilson for the duration of the war.

With most of the observatory’s scientists performing research elsewhere in support of the war effort, Baade had the place to himself. Adding to this ideal situation for Baade was that the greater Los Angeles area, which Mount Wilson overlooks, was under blackout conditions early in the war because of fears of a possible Japanese invasion along the West Coast. The night skies at Mount Wilson were the darkest they had been since the observatory was built. Under such dark skies, Baade began studies of the Andromeda galaxy . He found that there were two distinct types of stars in the galaxy. One type, which he called Population I, was found in the spiral arms of the galaxy and was bluer than the other type. The second type, which he called Population II, was found in the central regions of the galaxy, in its globular clusters, and in its elliptical satellite galaxies. Population II stars were much redder and dimmer on average than the Population I stars. More research showed that Population I stars are far richer in metals than are Population II stars. (Astronomers call anything other than hydrogen and helium a “metal.”) Theoretical studies later showed that this implies that Population I stars formed more recently and Population II stars formed earlier in the history of the galaxy. Researchers can make this implication because metals are produced in the fusion reactions in the cores of stars and are seeded into space when stars die, which allows them to become constituents of the next generation of stars.

Baade next turned his attention to Cepheid variable stars. Cepheid variables are pulsating stars whose pulsation period is related to their luminosity. The brighter Cepheids pulsate slower than the dimmer ones. This relationship can be used to determine distances to star clusters and nearby galaxies. Baade’s studies showed that Cepheids fall into two categories, based upon their stellar population. By placing them into their separate types, Baade was able to improve the period-luminosity relationship and thus also provide for improved distance determinations. He found that nearly all distances to galaxies had been underestimated by a factor of at least two. The universe was, therefore, far larger, and far older, than had been thought. This was great news for geologists, for they had been coming up with ages for the earth that were billions of years older than cosmologists were computing for the universe. After Baade’s correction, the two ages were closer in agreement. This also provided an age for the universe more consistent with that needed for George Gamow’s big bangcosmology.

Baade finally retired in 1958. However, retirement did not stop him from working. In 1959, Baade taught a graduate class at Harvard on stellar and galactic evolution and how those topics can be better understood using stellar populations. Later, Cecilia Payne-Gaposchkin edited transcripts of his lectures and they were published as Evolution of Star and Galaxies (1963). Baade then flew to Australia, where he traveled and repeated his Harvard lectures.

Finally, in late 1959, Baade returned to his beloved boyhood home of the Westphalia region of Germany. He had plans to write some much delayed papers and to teach at the University of Göttingen. He also was considering doing more work at Hamburg’s Bergedorf Observatory. However, his congenital hip disorder had become far worse, and he was in constant pain. By early 1960, the condition had progressed to where he was beginning to suffer paralysis of his legs. He underwent major surgery to correct some of the problems. He was instructed to remain in bed for five months after the surgery, a “confinement,” as he called it, that was very difficult for him. Finally, he was allowed to sit up in a wheelchair, but he died days later, on June 25, in Bad Salzuflen. It has been speculated that perhaps a blood clot may have formed as a complication of his surgery and the five-month convalescence. Sitting up may have dislodged the clot.

Significance

Baade published less than many other astronomers. However, nearly every paper that he did publish had a major impact on astronomy and astrophysics, helping to shape twentieth century astronomy. Often regarded by some as “just” an observational astronomer, Baade’s grasp of astrophysics was extraordinary.

Baade’s discovery of Population I and Population II stars was one of the most important discoveries in stellar astronomy. It set the stage for astrophysicists to understand stellar and galactic evolution. Modern astronomical thought no longer groups stars into only two distinct populations. Rather, stars are now thought to lie along a continuum of metallicities, that is, the constitution of a star varies in its amount of matter beyond the chemical elements helium and hydrogen. Nonetheless, the terms “Population I” and “Population II” are still used by astronomers, and the metallicity of a star is considered an important measurement.

Baade’s, and Zwicky’s, research on supernovae was a major step forward in understanding the evolution of high-mass stars, and his work with distance scales also was instrumental in supporting the big bang cosmology and putting the steady state model to rest.

Bibliography

Baade, Walter. Evolution of Stars and Galaxies. Cambridge, Mass.: Harvard University Press, 1963. A transcript of a series of lectures given by Baade, edited by Cecilia Payne-Gaposchkin, that clearly explain Baade’s work in stellar evolution and galactic astronomy.

Osterbrock, Donald E. Walter Baade: A Life in Astrophysics. Princeton, N.J.: Princeton University Press, 2001. A well-researched, recommended biography of Baade by a historian of astronomy. Includes a bibliography.

‗‗‗‗‗‗‗. “Walter Baade, Master Observer.” Mercury 31, no. 4 (July/August, 2002): 32-41. A brief biography of Baade, focusing mostly on the highlights of his life.