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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.