Messier 37, Open Star Cluster
Messier 37 is the most starry and with an apparent brightness of +6.2mag also the brightest star cluster in the constellation "Auriga". It is already
visible as a nebulous spot in small binoculars. The distance is about 4,400 lightyears.
The apparent diameter in the sky is given with 24' arcminutes, which corresponds to a true diameter of 20 to 25 lightyears. This is a really large
value for an open star cluster.
More than 500 individual stars are attributed to the cluster according to latest data, of which 150 stars are brighter than 12.5mag and even 200 stars
brighter than 13mag. The brightest star is named "HD 39183" and belongs to the spectral class F8. With a surface temperature of 6,200 Kelvin it is
several hundred degrees hotter than our Sun. Its apparent brightness is 9.2mag, which corresponds to a true brightness of -1.5mag or 340 solar
luminosities. Therefore "HD 39183" belongs to the yellow giant stars. If it would stand in the same distance as Sirius, it would shine as bright as
Venus in the winter sky. In addition, 15 red giant stars and some variable stars have been found in Messier 37. Two dozen white dwarfs also
populate the cluster.
The hottest main sequence star belongs to spectral class B9 with a surface temperature of about 10,000 Kelvin. From this its age can be determined
to 350 to 550 million years. The fact that the cluster is still so densely packed after such a long time and shows no signs of disintegration is due to
the fact that Messier 37 combines more than 1,500 solar masses. The so called "tidal radius" is about 50 light years. In this area the stars are
gravitationally bound to the cluster. Once in 220 million years the cluster completes an orbit around the galactic center and it is expected that it will
survive many more such orbits unharmed.
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The sight in my 20" f/3 Dobsonian is simply breathtaking:
Already in the finder a bright, small cloud shows up. The view at 120x is really breathtaking. The whole eyepiece is filled with a dense swarm of
stars. Almost directly in the center of the cluster is a bright, whitish-yellow shining star, around which more than 100 fainter stars are clustered.
Therefore the cluster reminds me a little bit of Messier 11. The shape is roundish to slightly oval. Very noticeable is a star-poor spot, which bores
into the cluster coming from the south and almost reaches the center. The individual stars have more or less the same brightness.
If you increase the magnification to 210x in the 20" Dobsonian, the view is really breathtaking. There are now many more stars visible against a
pitch black background. The whole field of view is an ocean of stars, in the center of which the white-yellow giant star emits its warm light like
a lighthouse. There are surely 200-250 stars to be seen.
