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  • 21 Aug, 2019

  • By, Wikipedia

NGC 6781

NGC 6781, also known as the Snowglobe Nebula, is a planetary nebula located in the equatorial constellation of Aquila, about 2.5° east-northeast of the 5th magnitude star 19 Aquilae. It was discovered July 30, 1788 by the Anglo-German astronomer William Herschel. The nebula lies at a distance of 1,500 ly from the Sun. It has a visual magnitude of 11.4 and spans an angular size of 1.9 × 1.8 arcminutes.

The bipolar dust shell of this nebula is believed to be barrel-shaped and is being viewed from nearly pole-on. It has an outer angular radius of 61; equivalent to a physical radius of 0.44 ly (0.135 pc). The total mass of gas ejected as the central star passed through its last asymptotic giant branch (AGB) thermal pulse event is 0.41 M, while the estimated dust mass is 1.53 M.

The magnitude 16.88 central star of the planetary nebula is a white dwarf with a spectral type of DAO. It has an M-type co-moving companion at a projected separation of under 5,000 AU. The white dwarf progenitor star had an estimated initial mass of ~2.5 M. It left the AGB and entered the cooling stage around 9,400 years ago.