Baade's window is a small patch of sky toward the center of our Milky Way Galaxy that is relatively free of obscuration by dust. We are able to see deep inside the Galactic Bulge through Baade's Window, and are thus able to study the stellar population of the Bulge. It is named for the astronomer Walter Baade, who studied the globular cluster NGC 6522 in this region during the 1940s.

The plane of the Milky Way is filled with dust, as are most spiral galaxies. Our solar system sits near the middle of this plane, and about 8,500 parsecs from the dynamical center of our galaxy. While we have a relatively unobscured look at stars in the solar neighborhood, we cannot see the stars in the Bulge of our galaxy in visible light, because dust within the plane blocks their light from reaching us.

However, by chance, there is a small region of the galactic interior which has relatively few patches of dust between us and the center, letting us study the Galactic Bulge's stellar population in detail. This is fortunate, because galaxies are made up of many different populations of stars, which originated at different times in each galaxy's evolution. Stars in the disk and the spiral arms tend to be very rich in elements heavier than hydrogen and helium, and are also the youngest stars in the galaxy. Stars in the galactic halo and in globular clusters tend to be metal-poor, and probably formed very early in our galaxy's life, out of the primordial gas. Most stars in the Bulge are also old but richer in metals, because the primordial gas in the center of the galaxy was rapidly enriched by supernova explosions of short-lived, massive stars. Studying the multiple populations of stars within our Galaxy is a bit like archaeology -- we can observe the stars as they are now to try and understand how and when our Galaxy formed. Without Baade's Window on the Galactic Bulge, it would be difficult to study this important population of stars.

Walter Baade first discussed his photographic observations of NGC 6522 in a paper presented at the 1946 meeting of the Astronomical Society of the Pacific (Publications of the ASP vol 56, 249). In it, he describes the detection of 152 variable stars, many of which were "cluster type" RR Lyrae stars. He calculated that the number of variable stars found in this region was twelve times higher than in other regions of the sky nearby. This suggested that this region was very rich in older stars. Furthermore, he used the brightness of these stars to determine their distances, and found that they lie very near the center of our galaxy. Thus, Baade helped to define an entirely new population of stars within our galaxy, and to determine that our own Milky Way is much like other spiral galaxies observed in the universe. In fact, he compared our own galaxy to the nearby Sb-type spiral M31 in the same paper.

The center of Baade's Window is located at α 18h 03m, δ -28°. It is best viewed from the southern hemisphere during the months of May through August, but is visible from latitudes below about 50° N. It is about 5 degrees south of the Trifid Nebula, in the constellation Sagittarius.

Sources:
A facsimile of Baade's paper can be found at
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1946PASP...58..249B
I also used http://simbad.harvard.edu for coordinates and references.

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