NGC 637, Open Star Cluster
The star cluster NGC 637 is located at a distance of a little more than 7,000 light years in the constellation Cassiopeia, more precisely, about 1.3°
west of "epsilon Cas". Therefore it is easy to find.
With an apparent size of 3' (arcminutes) it is relatively small, its true diameter is 6-7 lightyears. The age of the cluster is given as 10 million years,
although a recent photometric analysis from 2008 comes to a very young age of only 4 million years.
In NGC 637 there are five variable stars of the type "ß Cephei". These stars oscillate sine-like with an amplitude of up to 0.2 magnitudes, are quite
heavy (8 to 18 solar masses) and hot with surface temperatures of 22,000 to 28,000 Kelvin. There do not exist very many stars of this type, so it is
a bit unusual to find five of them in one cluster.
----------------------------------------------------------------------------------------------------------------------------------------------
In my 20" f/3 Dobsonian NGC 637 looks really great. Because of the small apparent size the best view is at 270x. Then about 35 single stars with
very different magnitudes are visible in front of a deep black sky background. Five of them are quite bright. While the brightest stars are around
11mag, the faintest are only 15...16mag bright. Striking is a curved serpentine line running from north to south. Also otherwise the shape of the
OC is irregular with star-poor areas.
