M13 Hercules Star Cluster

M13 star cluster

Great Globular Star Cluster in Hercules

M13 is a magnificent globular star cluster with perhaps as many as one million stars. M13 is rated at magnitude 5.8, and is located in the constellation Hercules. Hercules is relatively close to the Virgo cluster of galaxies. Consequently, a magnitude 11.4 spiral galaxy known as NGC 6207 is located so close to M13 from our line of sight (28 arc-minutes to the northeast) that it can often be seen in the same telescopic field of view. M13 is not only a splendid globular star cluster, but it also offers the bonus of a spiral galaxy nearby when viewed through a telescope. However, M13 and the galaxy only appear close together because they are both on the same line of sight viewed from Earth. M13 is about 25,000 light-years away. NGC 6207 is located millions of light-years away.

M13 was discovered by Edmond Halley in 1714. It was among the first objects added to the Messier List by Charles Messier. The object was first resolved into a cluster of stars by Willaim Herschel in May 1787. Welsh amateur astronomer Isaac Roberts is believed to have taken the first published photograph of M13 on May 22, 1887 (a sixty minute black and white photograph taken with a 20-inch reflector). The stars in M13 are extreme Population II stars that are poor in heavy metals, and were formed 12 billion years ago. The globular cluster is 150 light-years in diameter.

According to Burnham's Celestial Handbook, if you made a scale model of M13 where the stars are the size of a grain of sand, the grains of sand would be separated from each other nearly a mile, even in the closely packed center of the cluster. In such a scale model, the million grains of sand representing the stars of the cluster would be distributed in a spherical volume of space about 300 miles in diameter. Burnham's Celestial Handbook gives a description of what it would be like to live on a planet orbiting a star in the center of the cluster. The "night" sky would be filled with many bright, blazing stars much brighter than any stars in our night sky. Many thousands of stars ranging in brilliance between Venus and the full moon would be visible at all times, so their "night" sky would never be dark. It would be difficult for inhabitants of such a planet to see dim galaxies, even using a telescope.

This image was published in the June 2009 issue of Astronomy Magazine at page 51.

This is an RGB CCD image taken with an SBIG STL-11000M CCD and a RCOS 24-inch telescope at the Mount Lemmon SkyCenter. The image was one of the first images taken with a new telescope that is operated by Adam Block. Adam is conducting a nightly observing program that is open to the public. This image was processed by Adam Block.

M13 (NGC 6205)
Constellation: Hercules
RA: 16h 41m 37.7s Dec: +36d 28' 22"
April 17, 2008
Image by Sid Leach and Adam Block
Mt. Lemmon, Arizona

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