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M22 (NGC 6656) is one of the nearer globular clusters to Earth at a distance of about 10,600 light-years away. It spans 32' on the sky which translates to a spatial diameter of 99 ± 9 light-years. It is projected in front of the galactic bulge and is therefore useful for its microlensing effect on the background stars in the bulge. Despite its relative proximity to us, this metal-poor cluster's light is limited by dust extinction, giving it an apparent magnitude of 5.5 making it the brightest globular cluster in the norther hemisphere.
M22 is very unusual in that it is one of only four globulars (the others being M15
, NGC 6441 and Palomar 6) that are known to contain a planetary nebula
(aka: PK 9-7.1; PNG 98-7.5) after after the initials of Gillett, Jacoby, Joyce and Cohen. It was discovered using the IRAS satellite by Fred Gillett et al. in 1986 as a pointlike source (IRAS 18333-2357) and subsequently identified as a PN in 1989 by Gillett et al.
June 26, 2009