NGC 6997, Open Star Cluster
NGC 6997 is a rather large and bright open star cluster, which is located in the middle of the well-known "North America Nebula" NGC 7000 -
more precisely in the northeastern part of the nebula. But whereas the distance to NGC 7000 is given with 1,600 light-years, the star cluster is
about 2,700 light-years away from us according to the latest data of the GAIA mission. Therefore it is far behind the luminous nebular masses.
That nebula and cluster have nothing to do with each other also results from the age of NGC 6997, which is given to be around 580 million years.
After this time the surrounding nebula should have "dissolved" long ago.
The brightest stars reach an apparent magnitude of 10.2mag, which corresponds to a luminosity of 50 suns. Very bright stars are therefore missing
completely - another sign for the advanced age of NGC 6997. About 140 stars up to a brightness of 17mag are attributed to the cluster. From the
true distance the diameter of the cluster can be determined to 7 lightyears. The cluster is only loosely concentrated and is expected to dissolve
completely within the next few hundred million years.
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The cluster appears quite large and extended in a 20 inch f/3.2 Dobsonian even at a low magnification of 120x. About 50-60 individual stars
with very different magnitudes can be seen. The cluster appears roundish and the stellar density increases slightly towards the center. It also
stands out very well from the surrounding star field - but only at low magnification. If you magnify too high, the star cluster character is lost
in the eyepiece. The shape is irregular with areas of increased star density and star-poor areas. In the center there are also two nice double stars,
which can be separated very easily.
If you put a UHC filter in the eyepiece, you can observe some of the extensive gas masses of the North America Nebula, especially if you move
the telescope across the field. These nebulae are only indicated in the drawing, because I did not pay attention to the exact shape. All in all a great
object!
