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November 23, 2017

John Mitchell was one of the most insightful scientists of the 18th century. He made important contributions to the study of earthquakes, magnetism, and other topics, including astronomy.

250 years ago, for example, he studied the Pleiades, the star cluster that marks the shoulder of Taurus, the bull. Telescopes of the day revealed a few dozen tightly packed stars. Mitchell calculated that the odds of finding so many unrelated stars in such a tiny volume were just one in 500,000. From that, he reasoned that the stars were bound together by their mutual gravitational pull. That made him the first scientist to suggest that the stars in a cluster are all related.

Mitchell was right. The Pleiades contains more than a thousand stars. All of them were born more than a hundred million years ago, from a single cloud of gas and dust. And today, they move through the galaxy as a family, bound by gravity.

That won’t always be the case, though. The gravity of the rest of the galaxy is tugging at the Pleiades, pulling some of its stars away. In about 250 million years, the cluster will have fallen apart, with its member stars orbiting the center of the galaxy on their own.

For now, though, look for the Pleiades low in the east as darkness falls, above the star Aldebaran, the bull’s orange eye. The cluster’s brightest stars form a tiny dipper. It crosses high overhead around midnight — a big stellar family that won’t stick together forever.


Script by Damond Benningfield

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