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As a boat glides along the surface of a lake, it creates a wake — small waves that ripple outward. The waves can push debris away from the boat’s path, concentrating the debris in a fairly small area.
The same thing can happen with the stars. In fact, the motion of a giant bar of stars in the middle of the Milky Way galaxy seems to have forced millions of stars to congregate in a giant cloud. It’s known as the Hercules Cloud because many of the stars appear within the borders of the constellation Hercules.
The cloud was discovered more than a decade ago as an unusually tight grouping of stars centered about 10,000 light-years away. As astronomers studied the region in detail, they discovered that the cloud contains millions or even hundreds of millions of stars, and it’s thousands of light-years long. It’s well above the plane of the Milky Way’s broad, flat disk, in a region that contains mainly older stars.
The motions of the stars suggest that the cloud formed in the wake of the Milky Way’s central bar — a brick-shaped region of billions of stars that rotates like a big propeller. The gravity of the passing bar collected and concentrated the stars above it — forming the Hercules Cloud in its wake.
The stars in the Cloud are too faint to see without a telescope, but Hercules itself is in the east as night falls. It’s marked by a lopsided square of moderately bright stars known as the Keystone. More about that tomorrow.
Script by Damond Benningfield, Copyright 2012