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Open Clusters

Pleiades Star Cluster
Credit: HST/NASA

An open cluster is a group of up to a few thousand stars of about the same age that were formed from the same cloud of gas and dust, and are still loosely bound (or tied) to each other by gravity.

In contrast, globular clusters are very tightly bound (tied together) by gravity.

Open clusters are found only in spiral galaxies and irregular galaxies, in which star formation is taking place.

The clusters are usually less than a few hundred million years old (so very young as far as the universe is concerned) and are slowly drifting apart. Young open clusters may still be contained within the gas cloud from which they formed; radiation from the stars can illuminate it to create a glowing nebula such as the Horsehead Nebula in the constellation Orion.