NGC 6802, Open Star Cluster
NGC 6802 is a star cluster in the constellation "Vulpecula" and best observable in the summer sky. It was found in July 1784 by F.W. Herschel.
The distance of the cluster is relatively large. According to the latest data of the second GAIA release, NGC 6802 is located at a distance of 8,500
light-years with an error of +-300 light-years. This is quite a lot for an open star cluster.
Therefore the two brightest stars are only 12 to 12.5mag bright. The vast majority is in the range of 14mag and below. So it needs quite some
aperture to resolve this cluster completely into single stars. One should not forget that the reddening by dust plays a role and in the case of NGC
6802 this is in the range of E(B-V)=0.71mag. Leaving this aside, the absolute luminosity of the brightest stars is M=+0.1mag, which is still 75
times the luminosity of the sun. But most stars of the cluster are only about 2-5 times brighter than the Sun. This means that very bright and
luminous stars seem to be missing almost completely and NGC 6802 has to be very old. And indeed the age is given with 800 million to 1 billion
years. That the cluster has not dissolved after such a long time is due to its compactness.
From the apparent diameter of about 4 arcminutes we can easily calculate the true diameter to 9-10 lightyears. According to GAIA Release there
are about 380 stars in this sphere down to a magnitude of 19 magnitudes. This seems to be enough to gravitationally bind the stars over such a
long period of time.
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With the telescope the star cluster is found in no time: just set the famous " coat hanger" in the viewfinder, slew a bit to the east and a milky spot
appears in the eyepiece at 120x. There are about 15-20 single stars in it, the background appears grainy.
This calls for a higher magnification and because this is possible with 20 inch aperture, the magnification is increased to 380x. In the cluster itself
there are now 50 to 60 faint stars with nearly the same brightness. The cluster appears with 3:1 clearly elongated, almost rectangular. But if you
look closer, you can see some very faint stars in the east. If you add them to the cluster, it looks almost round again. The western edge, however,
seems to be slightly indented in some places; some brighter stars can be found in the north of the cluster. The background still appears slightly
grainy even at this high magnification, as if there were more starlets cavorting in the background. This can only mean one thing: more aperture
is needed ;)
By the way, the cluster is framed by some brighter 9mag stars, which offer a great contrast to the faint stars of the cluster.
