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.