NGC 6888, Crescent- Nebel
The well-known "Crescent Nebula" has the catalog number NGC 6888. It is located at a distance of 4,700 light-years in the constellation of
Swan and was discovered by F.W. Herschel in 1792.
In the center of the nebula we find with a brightness of 7.5mag a quite unimpressive star - WR136 (or HD 192163). But the appearance is
misleading, because in reality WR136 is not only one of the most luminous stars in the sky, but also belongs to the rare group of "Wolf-Rayet
stars". It is 5 times as big as the sun, 21 times as massive and shines 600,000 times as bright as our central star. The absolute brightness can be
calculated to -9.6mag. If you would put WR136 in the same distance as Sirius, it would shine with -12.7mag as bright as the full moon and
even in a distance of 350 lightyears still as bright as Venus. The temperature at its surface is nearly 71,000 Kelvin, which is extremely hot
for a star. It is also believed to be a binary star system in which a low-mass red dwarf star orbits the main component once every 5 days.
The age of WR136 is estimated to be just 4.7 million years and already after this short period of time it has almost reached its end of life.
250,000 to 300,000 years ago the star became a red giant and during this phase it released about 5 solar masses of matter into space, which is
still expanding at 80 kilometers per second. This exposed the hot inner layers. Over time, a strong stellar wind set in, now racing outward at a
speed of 1,700 kilometers per second. In the meantime, the stellar winds have caught up with the old outer shell of the star, which is now
stimulated to glow by the UV radiation, forming the "Crescent Nebula".
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All this can be followed wonderfully in the telescope - the more detailed the larger the aperture. In my 20" f/3.2 Dobsonian NGC 6888 is already
visible as a pale crescent without a nebula filter, but only with a UHC filter its true beauty is revealed. With magnifications of V=200x and more,
the widely scattered fragments of the nebula can be easily seen and tracked - especially clearly at the outer shock front, which appears brighter
than the rest of the Crescent Nebula. The interior of the nebula glows faintly and is not as rich in structure as the turbulent edge.
And in the middle of all these nebulae shines a whitish-yellow star which is responsible for the whole spectacle: WR 136!
