Astronomy Picture of the Day

Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.

February 1, 1996

Lensing through Baade's Window
Credit: Photograph made from plates taken with the UK Schmidt Telescope.
Color photography by David Malin.
Copyright: Anglo-Australian Telescope Board

Explanation: What is the shape and composition of our Milky Way Galaxy? This question would be easier to answer if there wasn't so much obscuring dust! In the 1940s, however, astronomer Walter Baade identified a "window" near the center of our Galaxy where there is comparatively little opaque dust. Now called "Baade's Window", this sky region contains millions of stars and is used for many studies of the distant Milky Way. One clever use, devised by Bohdan Paczynski, is to monitor millions of stars in our Galactic Bulge - many through Baade's window - for sudden brightening due to gravitational lensing. Current observations by the OGLE and MACHO collaborations have now identified dozens of gravitational amplification events. This unexpectedly large number supports previous claims that our Galaxy has a "bar" of stars across the central nucleus, pointed nearly at the Sun.

Tomorrow's picture: A MACHO View of Galactic Dark Matter

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Authors & editors: Robert Nemiroff (GMU) & Jerry Bonnell (USRA).
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