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Baade’s Window

Published on Jul 7, 2006 at 4:29 pm. No Comments.
Filed under galaxies.

Walter Baade was one of the most important astrophysicists of the Twentieth Century. He is known for many things, including the first description of supernovae and the identification of stellar populations. Virtually every work that he published was significant. One of his lesser known discoveries we call Baade’s Window.

Our galaxy is shaped sort of like a giant pancake, with a bulge in the middle. We can’t see the whole thing. In my last post, I mentioned the interstellar medium. Besides the hydrogen and helium gasses, the interstellar medium contains interstellar dust. This dust is tiny, mostly only a few molecules at a time. However, there is a lot of it, and it tends to block visible light. Infrared light can see through it somewhat, but infrared astronomy is a fairly recent developement. The dimming of light due to the intestellar medium is called interstellar extinction. There is similar interstellar extinction in most directions within the plane of the galaxy, so we can see about the same distance in most directions. For this reason, astronomers for centuries assumed that the Sun was located somewhere near the center of the galaxy. In the early 20th Century that the astronomer Harlow Shapley found that the center of the distribution of the galaxies globular clusters was beyond what everyone had thought was the edge of the universe. This led him to realize that the Milky Way was vastly bigger than anyone had thought. Robert Trumpler later came up with the interstellar medium as an explanation for why we could only see so far in the galaxy.

However, the interstellar medium is not really uniform. By freak coincidence, it turns out that there happens that in the general direction of Sagittarius, there is less interstellar medium than normal. Walter Baade is credited with the discovery that along this narrow direction, you can see almost all the way to the center of the galaxy. We call this narrow opening Baade’s Window. Actually, Baade’s Window is a few degrees off of the the line directly to the center, so we can’t really see the exact center of the galaxy. However, we can see within about 1800 lightyears of the center, which is pretty close, given that we are almost 26,000 lightyears out from the center of the galaxy. Baade’s Window lets us see into the central bulge of our galaxy, and much of what we know about bulge stars comes from studies of stars through Baade’s Window.

When you are looking towards Sagittarius, you are looking towards the center of our galaxy. You’ll notice that the Milky Way looks a bit fatter in that direction. Looking just a bit towards the south of the center of the Milky Way, you are looking as far as you can see within our own galaxy using visual light. That is Baade’s Window.

-Astroprof