Most of the stars that make up the Big Dipper are members of a sky-spanning stellar family — the Ursa Major Moving Group. The stars in this group all formed from the same giant cloud of gas and dust. They've since spread out across the sky, but they all move through the Milky Way at about the same speed and direction.
But the stars at the ends of the Big Dipper don't belong to this family — they move through space independently of the others.
The brighter of these orphans is Dubhe, the star that marks the lip of the bowl. It’s really a family of its own — four stars in a single system. They orbit the center of the Milky Way galaxy as a single unit, bound by their mutual gravitational pull.
One star dominates this quadruple system. Known as Dubhe A, it’s a stellar giant — it's about four times as massive as the Sun, about 30 times wider, and hundreds of times brighter.
It’s also later in life than any of the other stars in the Big Dipper. It's used up the hydrogen fuel in its core, triggering a series of changes that caused it to grow big and bright. Now it’s burning the helium ash left over from the fusion of its original hydrogen. Eventually, it'll use up the helium as well, leaving a core that can no longer produce energy. The star’s outer layers will then stream out into space, leaving a tiny but hot stellar corpse known as a white dwarf. Dubhe will then disappear from view — and the Big Dipper will be no more.
Script by Damond Benningfield, Copyright 2013
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