Berkeley 18, Open Star Cluster
Berkeley 18 is a massive, open star cluster in the constellation Auriga, which is over 20,000 light-years away from Earth and can best be observed
in the winter months. Due to the very low surface brightness, however, this requires a lot of aperture and a dark sky.
The apparent size of the inner region (derived from DSS images) is around 7' ( arcminutes), which corresponds to a true size of 42 light-years. This
is a very large value for an open star cluster. Most of the stars are very faint and lie in the 15-17mag range, with a concentration around 16mag. The
brightest stars associated with the cluster are 12mag bright, which corresponds to an absolute magnitude of M= -2.0mag or 540 solar luminosities
at this distance. The stars with an apparent brightness of 16mag, on the other hand, only have a luminosity of around 14 suns.
The age of Berkeley 18 is given as 3.16 billion years.
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In my 20" f/3 telescope at a magnification of 120x, only a very faint glimmer of light was visible at first. With increasing magnification, this glow
became weaker and weaker and disintegrated into a patchwork of bright and dark areas. At 270x, extremely faint stars flash out everywhere, especially
in moments of particularly good seeing. The background still appears milky, but not really grainy. More aperture is probably necessary for this.
Despite the very low brightness, this is still an exciting object. :)
