NGC 6811 - Open Star Cluster
NGC 6811 is a relatively large open star cluster that is located around 3,600 light-years from Earth in the constellation Cygnus. It was discovered by John Herschel
on a warm summer night in August 1829. The cluster has an apparent diameter of 15' and a brightness of 6.8mag, so it can be easily observed even in smaller telescopes.
Because of its shape respectively its starless center, the cluster is also called “The hole in the Cluster”.
The cluster measures around 14...20 light-years in diameter and contains up to 1,000 stars, 380 of which have been confirmed so far. More are sure to follow.
The luminosity of all the stars adds up to 2,100 times the luminosity of our sun. It is assumed that NGC 6811 had many more stars at the time of its birth,
namely 6,000. These included 8 O-stars and 125 B-stars. Due to the cluster's age of around 1 billion years, these luminous stars of course do not exist anymore.
So the brightest stars have an apparent brightness of 10.3mag, which corresponds to an absolute brightness of M= +0.1mag or the luminosity of 80 suns.
In the meantime, 2 planets have been confirmed in NGC 6811. These are Kepler 66b with 0.31 Jupiter masses and 2.8 Earth radii and Kepler 67b with the same
mass and a slightly larger size of 2.9 Earth radii. This discovery confirms that planets can also form in denser star clusters.
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I was able to observe the irregularly shaped star cluster in my 20” Dobsonian on a dark summer night.
The stars are rather faint and loosely distributed. The center appears starless at first glance, but then there are other areas where the stars are more concentrated
and closer together. The 50-60 individual stars have more or less the same brightness. At a magnification of 210x, a few faint stars in the center emerge from the
dark background - however, NGC 6811 appears most beautiful at a low magnification of 120x.
Overall, a great object for smaller and medium-sized telescopes. :)
