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."
Sources:
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)
www.phys-astro.sonoma.edu/BruceMedalists/Baade/Baade.html
www.astro.caltech.edu/observatories/palomar/history/