The globular cluster Messier 12 (NGC 6218) in Ophiuchus, shown from a 2-minute red-light exposure (through clouds) with a Tektronix 2048x2048 CCD at the 2.1-meter telescope of Kitt Peak National Observatory by Bill Keel and Lisa Frattare. 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 by a factor of two for this presentation (and another factor of two as shown above), which uses a logarithmic intensity transformation to preserve information across a wide dynamic range. The field is 9.1 arcminutes square.
In this direction, galactic dust extinction is significant. In the course of a survey for eclipsing variable stars, Kaspar von Braun and colleagues have found that the difference in reddening from place to place iwthin the cluster is measurable, and its correction is important in deriving the actual Hertzspring-Russell diagram of this cluster.
Last changes: 7/2003 © 2003