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The stars that mark the ends of the Big Dipper are sometimes described as “renegades.” While the dipper’s five other stars move through the galaxy in unison, the stars at the ends go their own ways. In 50,000 years or so, their motion will have stretched the dipper so much that it will no longer look like a dipper.
The renegades are Dubhe and Alkaid. Dubhe marks the lip of the dipper’s bowl, while Alkaid is at the end of its handle.
The other five stars are all members of the Ursa Major moving group. These stars move through the galaxy at the same speed and in the same direction. Their chemistry is alike, too. That suggests that the stars were born together. In fact, they may once have formed a tight cluster. Over time, though, the cluster was pulled apart. But even though the stars are on their own, they maintain their similar motion through space.
The stars of the moving group are all roughly 80 light-years from Earth. Alkaid is right at 100 light-years, while Dubhe is about 25 light-years farther. And they’re moving more or less in the opposite direction across the sky from the others. So over time, that will distort the dipper’s shape, then eventually destroy it — leaving our descendants without one of the sky’s most recognizable star patterns.
For now, though, look for the Dipper high in the northwest at nightfall, with Alkaid at the top and Dubhe at the bottom.
Tomorrow: outlining the black hole at the heart of the Milky Way.
Script by Damond Benningfield