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First Globular Cluster Outside the Milky Way
Messier 54, NGC 6715
Globular Star cluster
RA 18h 55m 3.42s Dec -30° 28' 42.01"
90,000 light years
3.56 x 3.49 arcminutes
North is 3.7° right of vertical
ESA/Hubble & NASA
November 7, 2011
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The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy. Discovered by the famous astronomer Charles Messier in 1778, Messier 54 belongs to a satellite of the Milky Way called the Sagittarius Dwarf Elliptical Galaxy.
Messier had no idea of the significance of his discovery at the time, and it wasn't until over two centuries later, in 1994, that astronomers found Messier 54 to be part of the miniature galaxy and not our own. Current estimates indicate that the Sagittarius dwarf, and hence the cluster, is situated almost 90, 000 light-years away - more than three times as far from the center of our galaxy than the Solar System.
Ironically, even though this globular cluster is now understood to lie outside the Milky Way, it will actually become part of it in the future. The strong gravitational pull of our galaxy is slowly engulfing the Sagittarius dwarf, which will eventually merge with the Milky Way creating one much larger galaxy.
picture is a composite created by combining images taken with the Wide
Field Channel of Hubble's Advanced Camera for Surveys. Light that passed
through a yellow-orange filter (F606W) was colored blue and light passing
through a near-infrared filter (F814W) was colored red. The total exposure
times were 3460 sec. and 3560 sec., respectively and the field of view
is approximately 3.4 by 3.4 arcminutes.