The globular cluster Messier 15 (NGC 7078) in Pegasus, shown from a 30-second red-light exposure (through clouds) with a Tektronix 2048x2048 CCD at the prime focus of the 4-meter Mayall telescope of Kitt Peak National Observatory. North is at the top and east to the left, for direct comparison with a chart or eyepiece view. The image has been block-averaged to 512x512 for this presentation, which uses a logarithmic intensity transformation to preserve information across a wide dynamic range. The field is 7.1 arcminutes square, showing the inner quarter of the original area images (pixels were averaged 2x2 for this display). Even with this short exposure, the bright core of the cluster saturated the CCD strongly enough for some of the charge to bleed along columns of the chip; the brightness has been set to conveniently hide the problem.
The colors of giants versus main-sequence stars are shown in this BVR composite image from the SARA 1-meter telescope at Kitt Peak. Even with this display, the dense cluster core is saturated in order to show individual stars elsewhere.
M15 is the most popular candidate for seeing a process known as core collapse . Simulations of the behavior of rich gravitating systems (such as globular clusters) suggest that transfer of energy between cluster members as they pass close to one another should systematically drop a fractio of them into an unstable core which might then either collapse further to become a black hole or, in the real world, rebound upon the stars' gaining energy at the expense of binary stars. HST images of the core of M15 have been used to shed light on this process and whether it has occcurred in this cluster. M15 also contains a planetary nebula (a rarity among globular clusters), nicely shown in this Hubble Heritage image.
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