You are here

Hercules Cloud

May 24, 2012

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


Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.