Moon and Pleiades

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Moon and Pleiades
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The Sun’s birthplace has long since fallen apart. The Sun almost certainly was born in a cluster, with anywhere from dozens to thousands of siblings. Over a few hundred million years, though, the cluster fell apart, with all of the stars going their own way.

We can see that process at work in the best-known star cluster of all: the Pleiades. The cluster is close to the Moon tonight. Its brightest stars form a tiny dipper. It may be a bit hard to pick out through the moonlight, but binoculars will help.

The cluster is about 445 light-years away. It’s about a hundred million years old. And it consists of more than a thousand stars.

But the cluster is already starting to break up. It’s held together by the gravity of its stars. But that “stickiness” is overpowered by the gravity of the rest of the galaxy. It pulls at the Pleiades, causing the stars to spread out. Stars at the edge of the cluster are pulled out into their own orbits. That reduces the cluster’s overall gravity, making it easier to strip away even more stars.

Most estimates say the Pleiades is likely to completely fall apart over the next 250 million years. By then, several of its brighter stars will have expired. Those stars are especially heavy, so they “burn” through their nuclear fuel in a hurry. By the time the Pleiades disintegrates, only their dead cores will remain — remnants of a once-mighty cluster.

Script by Damond Benningfield

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