Emission nebula | |
---|---|
Planetary nebula | |
Observation data: J2000 epoch | |
Right ascension | 19h 18m 28.085s [1] |
Declination | +06° 32′ 19.29″ [1] |
Distance | 1,500 ly (460 [2] pc) |
Apparent magnitude (V) | 11.4 [3] |
Apparent dimensions (V) | 1′.9 × 1′.8 [3] |
Constellation | Aquila |
Physical characteristics | |
Radius | 0.44 [2] ly |
Designations | IRAS 19160+0626, NGC 6781 [4] |
NGC 6781, also known as the Snowglobe Nebula, [5] is a planetary nebula located in the equatorial constellation of Aquila, about 2.5° east-northeast of the 5th magnitude star 19 Aquilae. [3] It was discovered July 30, 1788 by the Anglo-German astronomer William Herschel. [6] The nebula lies at a distance of 1,500 ly from the Sun. [2] It has a visual magnitude of 11.4 and spans an angular size of 1.9 × 1.8 arcminutes. [3]
The bipolar dust shell of this nebula is believed to be barrel-shaped and is being viewed from nearly pole-on. [7] 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☉. [2]
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. [8] 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. [2]