Planetary Nebula NGC 6891
This planetary nebula was discovered on 22nd September 1884 by the Scottish astronomer Ralph Copeland using visual spectroscopy with a 6.1 inch refractor at Earl of Crawford's Observatory, Dun Echt, Aberdeen. 
NGC 6891 is a triple-shell planetary nebula: It shows a bright central nebula which is surrounded by an attached shell and a detached outer halo. The inner and intermediate shells are ellipsoids with similar major to minor axial ratios, but different spatial orientations. The kinematical age of the intermediate shell is 4800 years and of the halo is 28'000 years. An outflow is observed to protrude from the tips of the major axis of the inner nebula and impact on the outer edge of the intermediate shell. The mass of the central white dwarf star is estimated to 0.75 times the mass of our Sun. Its surface temperature is 50'000 K. The distance to Earth is 3.8 kpc. 
|Designations||PN G054.1-12.1: NGC 6891, PK 54-12.1, ARO 37, VV 253, VV' 529|
|Right Ascension (J2000.0)||20h 15m 09s|
|Declination (J2000.0)||+12° 42' 07"|
|Radial Velocity||+42.4 km/s ± 1.0 km/s|
|Expansion Velocity||7.0 km/s (O-III)|
|C-Star Designations||AG82 403, BD +12 4266, EM* CDS 1140, GCRV 12622, HD 192563|
|C-Star Magnitude||12.30 mag (B filter), 12.42 mag (V filter)|
|C-Star Spectral Type||Of (H)|
The planetary nebula NGC 6891 is located in the constellation Delphinus. The best observation time is May to October.
Description pending ...