- NGC 6633
NGC 6633 is a large, bright open cluster lying along the Summer Milky Way in the constellation Ophiuchus. It was discovered by Philippe Loys de Cheseax in 1745 or 1746. It's size (0.8 degree, almost as large as the full moon) and visual magnitude (4.6) make it visible to the naked eye under good conditions and an excellent target for binocular observations. The brighter stars of this cluster number around 30 and are stretched out along an axis running roughly northeast-southwest. The bright star located below the cluster is the magnitude 5.7 blue-white HD 170200— it is a foreground star located around 709 LY away. NGC 6633 itself is located around 1000 LY from us and has a physical diameter of around 5.8 LY. It is estimated to be 660 million years old. This relatively old age (for an open cluster) explains why it contains a number of more evolved yellow and orange stars.
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