NGC 6772, Planetary Nebulae
NGC 6772 is a planetary nebula in the constellation "Aquila", located at a distance of 4,140 light years. With an apparent brightness of 12.7mag it is
not particularly bright, but was nevertheless found even in 1784 by F.W Herschel with his 18.7" telescope.
The apparent size of the PN is about 1.5 arcminutes - from the known distance one can conclude a true diameter of 1.4 light years. From the expansion velocity,
the age of NGC 6772 can be determined to be about 11,000 years, so the PN is already slowly fading and will probably become invisible within the next 10,000 years.
The elliptical envelope with a mass of 0.17 solar masses shows a deformation along the NE-SW axis, suggesting interaction with the interstellar medium (ISM).
The central star is quite faint with a brightness of 18mag and remains invisible even in larger amateur telescopes. It has a mass of 0.64 solar masses
and an immensely high surface temperature of 135,000 Kelvin.
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In a 20 inch Dobsonian, NGC 6772 actually appears quite large at a magnification of 270x and rather pale in direct comparison with other planetary
nebulae in this constellation. The oval, slightly angular shape is striking, but also the darker centre, which is surrounded by an unevenly bright ring.
The ring seems to be brightest in the eastern part, while it is faintest in the south.
