M23
NGC 6494
Properties
Position & Identifiers
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M23 · 16.8′ diameter · N up, E left
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About M23
Description
M23 is a large, rich open cluster in Sagittarius, about 2,150 light-years from Earth. It contains roughly 150 stars spread across about 15 light-years, with an age of about 300 million years. The cluster is well-detached from the Milky Way background and shows beautiful chains and curves of stars.
Observing Tips
Located about 5 degrees northwest of M20 in the Sagittarius Milky Way. Binoculars show a large, bright hazy patch. A telescope at 50-80x reveals a beautiful field of stars with curving chains and dark lanes threading through the cluster. The cluster spans about 27 arcminutes, so keep the magnification moderate for the best view. One of the more underrated Messier open clusters. Best observed from June through August.
History
Discovered by Charles Messier on June 20, 1764. He described it as 'a cluster of stars near the ecliptic.' Despite being a fine visual target, it remains relatively unknown compared to the splashier nebulae nearby.
Fun Facts
M23 is an excellent example of a 'middle-aged' open cluster. Its 300 million years is old enough that the most massive stars have already died, but young enough that the cluster is still well-defined and hasn't begun to disperse into the galactic background.