Thursday, June 30, 2011

Globular Cluster Photo Series (Part 4): M22

Today's globular cluster is Messier 22, a nearby bright cluster in Sagittarius. Similar to M4, M22 is relatively close to us at a mere 10,600 light years, give or take a thousand. It is larger than M4 at about 100 light years across, but smaller than Omega Centauri or M13, though appearing about as bright due to its closer distance (in fact, it is the third brightest globular cluster in the sky after Omega Centauri and 47 Tucanae, just edging out M13 for the distinction). It has an apparent diameter of 32 arcminutes, making it almost the same size as the Moon, and about in the middle of the clusters shown so far. It has a somewhat smaller number of stars, only about 70,000.

Messier 22 in Sagittarius. Click for larger image.
It also has the distinction of being quite possibly the first globular cluster discovered, as far back as 1665 by an amateur astronomer by the name of Johann Ihle while he was observing Saturn in Sagittarius (M22 lies very near the ecliptic, the line the planets and Sun seem to traverse on the sky due to our perspective). Although Omega Centauri and 47 Tucanae were known well before this time (Omega Centauri as far back as the time of Ptolemy), they were thought to be simply faint stars, which is where their names come from.

M22 is also somewhat unique in that it is one of only four of the Milky Way's numerous globular clusters known to contain a planetary nebula, the wispy gaseous cocoon of a Sun-like star as it nears the end of its time on the main sequence and starts becoming a white dwarf. It's much too small and faint to be visible in my picture, though. I have some images I took of planetary nebulae within the Milky Way that I'll post when I run out of my backlog of globular cluster images. Also when I'm done with the current run of images I plan to make a little graphic showing the relative sizes of the various globular clusters we've seen.

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