The Andromeda Galaxy, like all spirals, shows a population of older, redder
stars near its center (lower right) and younger, bluer stars in its
surrounding spiral arms. Walter Baade discovered the different populations
through observations made during World War II. NOAO/AURA/NSF
If you've been watching Ken Burns' World War II documentary,
The War, this week, you've seen the key role science played during that global conflagration. But there was more science involved in the effort than simply building better warplanes, tanks, ships, and rockets.
For example, the Battle of Britain turned, to a great extent, on the invention of radar, which allowed the Brits to see incoming German planes even at night and in bad weather. And, of course, we all know about the Manhattan Project and the development of the atom bomb that hastened the Pacific War's end.
Yet not all science of that era went into the war effort. German-born American astronomer Walter Baade took advantage of wartime restrictions to discover the stellar structure of galaxies. Baade, who was born in 1893, missed out on serving during World War I thanks to a congenital hip defect. In 1931, after more than a decade working at Hamburg University, he emigrated to the United States and California's Mount Wilson Observatory.
During World War II, Baade was classified as an "enemy alien," so he wasn't allowed to work on wartime projects. And he couldn't even leave Los Angeles County. Fortunately, the 100-inch Hooker Telescope at Mount Wilson wasn't going anywhere either. The wartime blackouts of Los Angeles coupled with the world's largest scope gave Baade the best views of the universe anyone had ever experienced.
He used the opportunity to explore the Milky Way's big neighbor, the Andromeda Galaxy. Photographing the galaxy through both red and blue filters, he found its central bulge chock full of bright red stars, while the galaxy's spiral arms and disk cornered the market on bright blue-white stars. Although Edwin Hubble previously saw the blue ones, Baade was the first to spot the inner region's red denizens.
Baade called the stars of the disk "Population I" and those of the central bulge — which matched those seen in the Milky Way's globular clusters and halo — "Population II." Only later did astronomers deduce that the bluish stars were young and possessed relatively large amounts of heavy elements, while the reddish ones were ancient stars that formed early in cosmic history.
Baade went on to make other discoveries at Mount Wilson and later at Palomar Observatory. Most importantly, he refined the scale of the universe and showed that galaxies lie much farther away than previously thought. This made astronomers realize the Milky Way is far smaller than they had thought, and not the universe's dominant galaxy. Still, his wartime discovery of Population I and II stars remains his greatest legacy — a "war story" with a happy ending.