NGC 6830, Open Star Cluster
NGC 6830 is an open star cluster in the constellation "Vulpecula" and best seen in the summer sky. It was discovered in July 1784 by F.W. Herschel.
The distance is about 5,400 light-years, but there are also deviating values. With an apparent visual diameter of 6 arcminutes the true diameter can be
determined to 9.4 lightyears. The cluster contains only a few stars and with an assumed age of 125 million years it is neither very young nor very old.
Nevertheless NGC 6830 shows already signs of disintegration due to its very loose structure and should not exist anymore in some tens to hundreds
of millions of years.
The brightest stars reach a brightness of 9.9mag, which is 250 times the luminosity of the sun. It is also interesting to note that the cluster includes 3
"Be stars", two near the center and one about 25' (arc minutes) southeast of it. "Be stars" are characterized by a very fast rotation and are therefore
significantly flattened. Some stars even rotate so fast that they are about to be torn apart by centrifugal force. The rotation causes gas to detach at the
equator and to collect in a ring or disk around the star. This allows emission lines to be observed in the spectrum - quite unlike normal stars, whose
spectra contain absorption lines.
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In my 20" f/3 Dobson the cluster shows up quite bright and there are about 35 to 40 individual stars visible at 210x, which are of different brightness.
The shape is irregular and somewhat comparable to the letter "X" as four chains of stars lead outward from the center. A star-poor gap at the northern
edge is especially eye-catching. NGC 6830 is only loosely concentrated and therefore does not stand out very well due to its starry surroundings.
Nevertheless, it is easy to identify. The cluster appears to be completely resolved as there is no grainy/milky background.
