The largest known globular cluster is Omega Centauri (NGC 5139),
which contains millions of stars packed into a ball 620 light years
across. The ball is not quite spherical, but appears slightly
squashed. Omega Centauri is also the brightest globular cluster in
the sky, with a total magnitude of 3.6. It is 16,500 light years away.
Its name is of a kind normally given to single stars and was
adopted long ago by naked-eye observers who did not realize its
nature.
Globular clusters are known to contain some of the oldest stars in
the Galaxy. Omega Centauri is one of the oldest clusters of all. A
high proportion of its stars have evolved into red giants.
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