NGC 6781 - Snowball Nebula
The well-known "Snowball Nebula" with the catalogue designation NGC 6781 is located at a distance of about 4,000 light years in the middle of
the star-rich Summer Milky Way in the constellation Aquila.
Although the apparent visual brightness is given as 11.8mag, NGC 6781 has a rather low surface brightness due to its large size. It is nevertheless bright
enough to be easily seen with smaller telescopes. The apparent diameter on the sky is 1.8' (arcminutes), which corresponds to a true diameter
of 2.1 light-years.
The central star has a mass of 0.6 solar masses and an effective temperature of 100,000 Kelvin, which is normal for a white dwarf inside a planetary
nebula. However, with an apparent brightness of 16.0mag, it is only visible in very large telescopes.
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The "snowball" appears at a magnification of 270x in combination with an UHC filter as a rather large, roundish brightening, which responds very
well to the use of nebula filters. A closer look reveals that the disk does not glow evenly - the inside is darker than the edges of the
PN's envelope. However, the darkest areas are not found exactly in the centre of the nebula, but appear slightly shifted to the north. This results
in the southern area of the PN being brighter than the northern area.
The central star was not visible even at a magnification of 270x.
