Walter Baade

When the United States entered World War II, many of the country’s astronomers joined the war effort. But a German astronomer working in California took advantage of the wartime conditions to make some important discoveries.
Walter Baade was born 125 years ago today. He earned his PhD in 1919, then joined the Hamburg Observatory. He came to the United States in 1931 to work at the Mount Wilson Observatory, near Los Angeles — home of the world’s biggest telescope.
Baade never sought American citizenship, so when the war broke out, he was classified as an enemy alien. For a while, he was even barred from working at night. But with many of the astronomers working on other projects, and with the lights of Los Angeles blacked out, Baade had Mount Wilson’s 100-inch telescope all to himself.
Using that telescope, he classified stars into two populations. One is old and feeble, while the other is younger and more vigorous. It’s a classification that astronomers still use today.
After the war,