Galaxies behind the Milky Way; October 1998; Scientific American Magazine; by Kraan-Korteweg, Lahav; 8 Page(s)
On a dark night, far from city lights, we can clearly see the disk of our galaxy shimmering as a broad band across the sky. This diffuse glow is the direct light emitted by hundreds of billions of stars as well as the indirect starlight scattered by dust grains in interstellar space. We are located about 28,000 light-years from the center of the galaxy in the midst of this disk. But although the Milky Way may be a glorious sight, it is a constant source of frustration for astronomers who study the universe beyond our galaxy. The disk blocks light from a full 20 percent of the cosmos, and it seems to be a very exciting 20 percent.
Somewhere behind the disk, for example, are crucial parts of the two biggest structures in the nearby universe: the Perseus-Pisces supercluster of galaxies and the "Great Attractor," a gargantuan agglomeration of matter whose existence has been inferred from the motions of thousands of galaxies through space. Observations also show a tantalizing number of bright and nearby galaxies in the general direction of the disk, suggesting there are many others that go unseen. Without knowing what lies in our blind spot, researchers cannot fully map the matter in our corner of the cosmos. This in turn prevents them from settling some of the most important questions in cosmology: How large are cosmic structures? How did they form? What is the total density of matter in the universe?