Wilhelm Heinrich Walter Baade (March 24, 1893 -- June 25, 1960) was a German-born astronomer, one of the most important figures in astronomy of the twentieth century. His observational work laid the foundation for much of modern astrophysics, including his discovery of two populations of stars, and his correction of Hubble's Cepheid distance Scale. His other achievements include encouraging the growth of Radio astronomy in the United States and giving us a better understanding of supernovae.

Baade was born in the town of Schröttinghausen in Westphalia to parents Konrad and Charlotte, and was the eldest of four children. He was first educated in his home town, then at the Gymnasium at Herford. There he received a classical education including several languages (French, English, Hebrew, Greek, and Latin), mathematics, and science. He developed an interest in astronomy in his early teens, and entered university in 1912 to study it further. He began his studies at Münster, but then moved to Göttingen, a very prestigious university known for its astronomy program. As did many students of that time, he began his research on variable stars, and began his doctoral dissertation work in 1916 on the spectroscopic binary star β Lyrae (α 18h 50m 4.79s, δ +33 21 45.6). This was his first exposure to very precise observational work -- his task was to study the stellar spectra photographed by his advisor, Johannes Hartmann, several years before, and from this derived both the binary's orbit, and the spectral types of the stars. Although World War I was raging at the time, he was exempted from service because of a hip defect which caused him to walk with a limp for his entire life.

He finished his dissertation in the summer of 1919 and took a position at the observatory at Hamburg, a site unfortunately marred by both air and light pollution. Still, during the time he spent there, he conducted many observations, and was credited with the discovery of several minor planets and a comet -- Comet Baade (1922 II). He also discovered his wife-to-be -- Johanna Bohlmann (also known by her pet name "Muschi") -- who joined the staff of the Hamburg Observatory shortly after Baade. While at Hamburg, he also began his observations of the RR Lyrae stars, his first work in the relatively new field of using pulsating stars to measure distances in the universe. His time in Hamburg was also noteworthy because he served as an assistant to the German mathematician Felix Klein. And he also met his lifelong friend Wolfgang Pauli during this time.

In 1929, Baade received a fellowship from the Rockefeller Foundation to visit the Mt. Wilson Observatory outside of Los Angeles. He permanently moved from Hamburg to Mount Wilson in 1931, and worked at there until his official retirement in 1958. It was at Mt. Wilson where his most lasting work in astronomy was completed. Baade's overwhelming strength was in taking meticulous, high-quality observational data, and Mount Wilson was the perfect setting. It was the site of the 100-inch (2.5-meter) Hooker Telescope, built by George Ellery Hale and the Carnegie Institution in 1917. Baade immediately set to work observing stars, nebulae, and nearby galaxies, and in 1934 he began publishing a long series of papers, mainly concerning the distances to star clusters. He also published a theoretical paper that suggested cosmic rays originate in the explosions and remnants of supernovae, a hypothesis which later turned out to the correct. He developed this theory with his co-worker Rudolph Minkowski, a fellow German whom Baade had brought out of Germany after the Nazis purged academia of Jews and "non-aryans." These early works by Baade were the first of many. Later in the decade, he would continue his observations of star clusters and nebulae, and of supernovae. For example, his 1938 paper on the seventeen extragalactic supernovae observed the previous year presented his important hypothesis that there are two kinds of supernovae -- type I and type II -- and that they tend to appear most often in late-type spiral galaxies. He also suggested (correctly) that the crab nebula was a supernova remnant in our own galaxy.

During World War II, Baade continued working at the observatory, unlike many of his American colleagues who were siphoned away for the war effort. Baade took advantage of the extra telescope time, and the fact that Los Angeles was essentially under blackout for much of the war, to take some of the most important observations of his life. In 1942, Baade used the 100-inch telescope to image the bulge of the Andromeda galaxy, and was able to resolve its individual stars for the first time. This was no small feat -- Andromeda is over two million light years away, and its stars usually blend together. By looking at individual stars, he was able to detect differences between the stars within the bulge, and the stars within the spiral arms. From this, he hypothesized that there were two populations of stars within most galaxies: population I stars which are younger and more metal rich, and population II stars which are older and metal-poor. From this came much of what we now know about the evolution of stars, cosmic chemical abundances, and galaxies. This observation also led to another important revolution in astronomy.

Several years before, Milton Humason and Edwin P. Hubble had detected the expansion of the universe from their measurements of galaxy redshift, combined with observations of Cepheid variables. However, they derived very high recession velocities for galaxies, and a very high Hubble constant of over 500 km/second/megaparsec. In the early 1950's, Baade continued his observations of Andromeda, now using the 200-inch telescope on Mount Palomar. He failed to find any RR Lyrae variables there, which suggested that the galaxy was farther away from us than previously thought. His reasoning was that they are probably there, but if the galaxy is too far away, they will be too faint to see. However, he did find the brighter Cepheid variables in Andromeda, and based upon his detections of two stellar populations in Andromeda Baade finally figured out that there are also two types of Cepheid variables. He found that the "true Cepheids" (those analogous to the class prototype δ Cephei) belonged to population I, while the metal-poor stars found in globular clusters belonged to population II. When this distinction was made, there were clearly two different period-luminosity relations, resulting in a revised Hubble constant of only 200 km/second/megaparsec -- the observable universe was over twice as large as Hubble thought it was. Baade summarized these observations in a speech he gave after receiving the Bruce Medal of the Astronomical Society of the Pacific (PASP v. 67, 5, 1956). (Allan Sandage would later go on to revise the Hubble constant downwards to less than 100 km/second/megaparsec, as some of Hubble's original data were spurious.)

Walter Baade's last great "discovery" -- more an observation -- was the presence of a window on the inner parts of the Milky Way. Our solar system lies in the plane of Galaxy's disk, about 25,000 light years from the center. The plane of the galaxy is filled with stars, but also with gas and dust. This dust serves to block our view of the inner parts of the galaxy, as well as our view outside the Galaxy. Baade's observations of Andromeda's bulge drove him to study the stars in our own Galactic center. Although our view toward the disk is almost completely obscured by dust, Baade found a small region toward the galactic center where he could detect the Popluation II stars he saw in Andromeda. This region came to be known as Baade's window.

Aside from his work in optical astronomy, Baade encouraged the growth of radio astronomy in the United States. He and Minkowski worked to identify the optical counterparts of bright radio sources in the sky. Many of these were found to be supernova remnants (like the crab nebula), but many others were associated with distant galaxies including the AGN galaxies Cygnus A and Centaurus A. He also took images of the crab nebula using polarizing filters, to see whether the nebular light was polarized. He found a huge degree of polarization in the nebulae, showing that much of the optical light in the nebulae is synchrotron radiation, rather than black body radiation or emission line radiation. This is important because the energetic, charged particles responsible for this radiation propagate through the galaxy as cosmic rays which we detect here on Earth.

Halton Arp notes in his obituary that Baade did not publish very much toward the end of his career, not because he wasn't working, but because he wanted to gather as much high-quality data as possible before publishing a result. Baade worked extensively with several other astronomers, particularly Henrietta Swope, who would later finish and analyze Baade's observations, and publish them graciously under Baade's name. Baade retired from Mount Wilson in 1958, and briefly served as visiting faculty at Harvard University and the Mount Stromlo Observatory in Australia, before returning to his native Germany for the last time.

Baade was a life-long German citizen, though he spent nearly all of the last thirty years of his life in the United States. In fact, he lost his US citizenship application just prior to World War II while moving to a new house, and as a result was tagged as an enemy alien during the war. He wasn't interned, but was required to stay in the immediate vicinity of Mount Wilson and Pasadena for the duration. The year before he died, he and his wife returned to Germany so that Baade might guest lecture at German universities. Unfortunately, his chronic hip ailment took a severe turn for the worse. He underwent corrective surgery but remained bedridden through the latter half of 1959 and early 1960. His death was probably caused by an embolism triggered by sitting up in bed for the first time in several weeks. He was buried in Bad Salzuflen, his grave marker a boulder engraved only with the name "Baade."

Walter Baade: a life in astrophysics, Donald Osterbrock, Princeton U. Press (2001)
Obituary by Halton Arp, Journal of the Royal Astronomical Society of Canada 55, 113 (1961)

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