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Moving Dipper

February 25, 2011

Most of the time, the stars that form a "connect-the-dots" pattern in the sky aren't related to each other -- they just happen to line up in the same direction in our sky.

One exception is the five stars in the middle of the Big Dipper -- Mizar, Alioth, Megrez, Phad, and Merak. They're all about the same distance from Earth -- about 80 light-years. And they're moving in the same direction and at about the same speed. What's more, a lot of other stars in that region of the sky are moving along with them.

All of these stars are members of the Ursa Major Moving Group -- Ursa Major because that's the constellation to which the Big Dipper belongs.

All of these stars were probably born about 500 million years ago, from a single vast cloud of gas and dust. They formed a large star cluster. As they orbited the center of the Milky Way galaxy, though, the cluster was slowly pulled apart. Stars closer to the center of the Milky Way moved a little faster than those farther away, stretching the cluster into a long streamer of stars.

Today, the stars are too widely spread to form a cluster -- their gravity no longer binds them to one another. But not enough time has passed to disperse them through the galaxy. So the stars still move through the galaxy in the same way -- forming a wide but rapidly spreading group.

The two stars at the ends of the Big Dipper aren't members of the group, so they go their own way.


Script by Damond Benningfield, Copyright 2010


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