05 Oct Discovering Mount Wilson Chapter 17: Walter Baade
Image caption: Walter Baade (1893-1960), astronomer (Photo Courtesy: The Huntington Library, San Marino, California)
We continue ‘Discovering Mount Wilson’ with Chapter 17, showcasing Astronomer Walter Baade (1893-1960), discoverer of stellar populations, with use of the Observatory’s 100-inch telescope.
Born and educated in Germany, astronomer Walter Baade worked at the Hamburg Observatory, in the Bergedorf borough of Hamburg before departing ‘in search of bigger telescopes.’ He arrived in the US in 1931 to join the team at Mount Wilson Observatory (MWO).
Ten years after Baade came to MWO, the United States was drawn into war with his home country after Pearl Harbor was attacked in 1941 and four days later Germany declared war on the US. Even before the US entered the war, MWO had begun participation in an in-house government supported weapons development program. As a precaution, Baade was kept from knowing what was happening with the program since he was still a German citizen.
With the United States now officially joining the allies against Germany, Baade was classified as an ‘enemy alien’ and was required to register with the US government. His movements were restricted to Los Angeles County, with strict adherence to the Government issued curfew between 8pm and 6am. Clearly not a workable restriction for an astronomer as this meant he couldn’t use the telescope, and was thus unable to observe during the prime night hours.
When astronomer Milton Humason heard that Baade could not continue his research due to the restrictive curfew, he stepped in to find a solution. The work Baade was doing was too important to be sidelined and something had to be done so he went to the Provost Marshall’s office for a conversation with the Colonel in charge of enemy aliens…
Now back at work making observations, Baade was able to define two distinct populations for stars. Population I stars are younger, show higher abundances of elements heavier than helium, and are found mostly in the spiral arms of spiral galaxies. Population II stars are older, show lower abundances for heavier elements, and are mostly found in the central bulge, halo, and globular clusters around spiral galaxies. Baade was able to see the population II stars, which had eluded Edwin Hubble, because they are much redder than population I stars. Baade’s photographic emulsions were more sensitive to red light than were Hubble’s photographic emulsions. So even though they both used the 100-inch telescope, and both looked at the same galaxies, Baade could see stars that Hubble had been unable to detect.
Baade also discovered two types of Cepheid variable stars, which are stars that pulsate radially, varying in both diameter and temperature. They produce changes in brightness with a well-defined stable period and amplitude. With this new discovery, Baade recalculated the size of the known universe made by Edwin Hubble in 1929. Both Hubble, in calculating the distances to galaxies, and Harlow Shapley, in his earlier determination of the size of the Milky Way, had assumed that there was a single period luminosity relationship, for all Cepheids. But in showing that there were two types of Cepheid, Baade also showed that there were two distinct period-luminosity relationships.
Baade’s recalculations doubled the previous size & age for the universe, in big bang cosmology. This is an important event, because at that time the calculations for the age of the universe derived from the big bang data made it younger than its oldest stars. Baade’s discovery & recalculation made the universe older than its oldest stars, and rescued big bang cosmology in the process.
After these great discoveries, Baade continued his research with the new 200-inch telescope at the Palomar Observatory. If Milton Humason hadn’t stepped in to keep Baade’s research going, Baade might not have become one of the most influential observational astronomers of the twentieth century. Baade continued his research and studies until he retired in 1958.