Unlike most globulars, M22 is in the galactic plane, in fact, in the direction of the galactic center. So it's possible that 1) its much younger than the majority of globulars in the galactic halo or 2) its exposed to the dust and gas of the galactic plane, and so still has star formation going on. On the other hand, all globulars must over the ages pass through the plane, and so probably do experience a relatively brief period of star formation in their outer regions. Stop me before my adlibbing gets me into REAL trouble. John R. Peterson