Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                                2020 March 19

                 M13: The Great Globular Cluster in Hercules
              Image Credit & Copyright: Eric Coles and Mel Helm

   Explanation: In 1716, English astronomer Edmond Halley noted, "This is
   but a little Patch, but it shews itself to the naked Eye, when the Sky
   is serene and the Moon absent." Of course, M13 is now less modestly
   recognized as the Great Globular Cluster in Hercules, one of the
   brightest globular star clusters in the northern sky. Sharp telescopic
   views like this one reveal the spectacular cluster's hundreds of
   thousands of stars. At a distance of 25,000 light-years, the cluster
   stars crowd into a region 150 light-years in diameter. Approaching the
   cluster core upwards of 100 stars could be contained in a cube just 3
   light-years on a side. For comparison, the closest star to the Sun is
   over 4 light-years away. The remarkable range of brightness recorded in
   this image follows stars into the dense cluster core and reveals three
   subtle dark lanes forming the apparent shape of a propeller just below
   and slightly left of center. Distant background galaxies in the
   medium-wide field of view include NGC 6207 at the upper left.

                   Tomorrow's picture: when night/day = 1
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