NGC 6818 - Little Gem Nebula
The planetary nebula NGC 6818 has the beautiful name "Little Gem Nebula". It is located in the constellation Sagittarius at a declination of -14°,
so it can rise 27° above the horizon in the southern part of Germany. Since it is quite bright with an apparent magnitude of 9.3mag, it was first
discovered by F.W. Herschel even in 1787.
The distance to NGC 6818 is given with about 5,500 +- 800 light years. From the known distance and the apparent size of 22 "x15" (arc seconds)
the true diameter can be determined to be 0.6x0.4 light years.
The planetary nebula is an ellipsoid with an elongation of 3:2, so it consists of an almost spherical envelope with a vase-shaped, slightly elliptical
inner structure embedded in it. In the northern and southern part there are two openings in the envelope caused by a strong stellar wind. The
expansion velocity is 35 kilometers per second. This suggests an age of 3,500 years, so the PN is quite young and will shine in the sky for
another many thousands of years. Temperatures in the shell drop from 15,000 kelvin near the center to 12,000 kelvin at the edge.
The central star (ZS) is named "HD 186282" and with an apparent magnitude of 17.1mag it is quite faint, so that it is not or only very difficult to
see visually even in large telescopes. By comparison with old photographic plates it could be found out that the central star has become about 2mag
fainter during the last 100 years. The star belongs to the group of white dwarfs and has a temperature of nearly 160,000 Kelvin at the surface.
Although the ZS is not very heavy with 0.6 solar masses, it shines about 1,000x as bright as our Sun. Remarkable is that it is a very close double star -
the white dwarf is orbited by a red dwarf star at an apparent distance of 0.09".
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At a magnification of 120x, the PN appears quite bright in my 20" f/3.2 Dobsonian. It is located in a small triangle of stars. Increasing the magnification
to 270x, the ring shape is clearly visible. Also, I had the impression that the longer sides shine a little brighter than the short ends of the ring. Outside the
ring, a faint halo is also visible, and the interior of the ring appears slightly brightened as well.
The central star is not visible, although its brightness is given as 17mag.
