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Messier 21 is a very young open cluster (just 4.6 million years old), located 3,900 light years away in the constellation Sagittarius. Although it is not bright enough to have naked eye visibility, M21 is a fine object to observe through a telescope (or binoculars). Having only 57 stars, M21 hosts many blue giants. It is believed that its stars belong to the Sagittarius OB1 Association and eight of its ten brightest stars are spectroscopic binaries with periods shorter than six days.
Additional Information
Object
Name(s): Messier 21. M21. NGC 6531
Type: Open Cluster
RA: 18h 04m 12s
Dec: -22º 28’ 54”
Constellation: Sagittarius
Size (arcmin): 14×15 arcmin
Magnitude: +6.5
Distance: 3,900 ly
Image
Date: 2021-06-07 to 2021-07-11
Location: iTelescope.net, SSO near Coonabarabran, NSW Australia
Size (arcmin): 41×41 arcmin
Telescope: 17” f/6.8 Reflector (unguided)
Camera: FLI PL16803 (4096x4096pix)
Total exposure: 3 hours (RGB, 1 hour each)
Processing: CCDStack, Photoshop CC 2021