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July 2003

NGC 6791 - An old star cluster

This rich open star cluster, with more than 500 stars lies between the constellations Lyra and Cygnus, high in the night sky late in July evenings. It may be glimpsed as a faint smudge of light in binoculars on a dark, moonless night. Light from these has taken over 16,000 years to reach the Earth.

This cluster is of unusual interest to astronomers who study the evolution of stars. It is one of the oldest such clusters in the sky, with an age of nearly 8 billion years. At that age, it should have formed from gas that had relatively little pollution: elements such as carbon, oxygen, iron, and others produced by earlier generations of stars. Yet these elements are present in the stars of NGC 6791 at levels similar to our own Sun, which was born a mere 4.5 billion years ago.

Note the red diagonal streak in the lower right - here, a man-made Earth satellite passed through the field of view during the red-filtered exposure.