Just as moons orbit most of the planets, satellite galaxies orbit most large galaxies. And the Milky Way is no exception. Our home galaxy is a giant -- it weighs roughly a trillion times more than the Sun. And two dozen lesser galaxies orbit it -- most of them discovered only during the past decade.
By far the brightest of these many satellites are the Magellanic Clouds. These two galactic puffs are visible to the unaided eye, but they’re so far south in the sky that they’re not visible from most of the United States.
The Large Magellanic Cloud is about 160,000 light-years from Earth, while the Small Magellanic Cloud is a bit farther -- about 200,000 light-years. Both galaxies abound with gas and dust, which are the raw materials for creating new stars; indeed, both of them glisten with the glow of many young stars.
That makes the Magellanic Clouds unique among the Milky Way’s satellites. All the other satellite galaxies have very little gas -- probably because the Milky Way stole it for itself. As a result, these galaxies can no longer give birth to new stars the way the Milky Way does.
In 1994, astronomers discovered a satellite on the other side of the galaxy that the Milky Way is tearing apart. It’s about half as far from Earth as the Large Magellanic Cloud is. Ultimately, the stars of this disintegrating galaxy will join our galaxy -- and the mighty Milky Way will grow even bigger.
More about satellite galaxies tomorrow.
Script by Ken Croswell, Copyright 2012