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Disappearing Cluster

June 2, 2014

A star cluster that’s evaporating before our very eyes stands low in the west as night falls, not far to the upper right of the crescent Moon. Under dark skies, it’s visible to the unaided eye as a hazy patch of light. Binoculars reveal many of the cluster’s individual stars.

M44 — the Beehive — is one of the nearest star clusters, at a distance of about 600 light-years. It’s also one of the most heavily populated clusters of its kind, with perhaps a thousand stars.

Its most massive stars are concentrated in the cluster’s middle, while the lighter stars are on its edge. That’s a sure sign that the Beehive is “evaporating” — it’s losing member stars to the greater galaxy.

When a cluster is born, its stars are packed so tightly that they constantly give each other gravitational nudges. The heavier stars are harder to push around, so they pretty much stay put. But lighter stars get a stronger push, so they move away from the cluster’s center.

Over time, the least-massive stars get pushed farthest — so far that the cluster can’t hold onto them. They’re easily stripped away by the gravity of passing gas clouds or by “tides” generated by the galaxy’s other stars.

That process is well underway in the Beehive. The stars in the middle are relatively heavy, while those on the edges are lighter. And the least-massive stars have already been stripped away. So the Beehive is evaporating — a big cluster that’s losing its little stars.

Script by Damond Benningfield, Copyright 2014

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