- M 22 (NGC 6656)
M 22 (NGC 6656) is a globular cluster located in the constellation Sagittarius. At visual magnitude 5.1, it is the third brightest globular cluster behind Omega Centauri and 47 Tucanae. It can be easily seen with the naked eye under good sky conditions. At a distance of 10,400 LY, M 22 is one of the closest globulars, which in part accounts for its brightness. The other contributor to its brightness is the vast number of stars packed into it. With a physical diameter of around 100 LY, it contains the equivalent of half a million solar masses.
M22 was discovered by Abraham Ihle on 26 August 1665 while the German astronomer was observing the planet Saturn. Later observers, including Halley, de Cheseaux, Messier, le Gentil, and Lacaille all described the object as being "without star" indicating that they were not able to resolve it into individual stars with their telescopes. William Herschel was the first to recognize its true form.
In my image above, M 22 is suspended among a horde of tiny golden Milky Way stars. Note that you don't see many tiny bluish stars in the background. Blue light experiences significantly greater absorption and scattering than red light. Large amounts of interstellar dust along the plane of the Summer Milky Way (where we're generally looking toward the galactic center) can have the effect of significantly reddening the light from distant Milky Way stars by removing the blue light and leaving the remaining yellow and red light to color them. The light from M 22 itself experiences significant extinction by dust along the plane of the Milky Way.
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