NGC 6881

In Cygnus

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6881 NGC 6881
Curtis only hints at the structure and the confusion attendant on NGC 6881, a visually tiny planetary nebula in a crowded region of central Cygnus 3.5 degrees southwest of Sadr (Gamma Cygni). He says: "Probably has a central star" (indeed, see below). As to the nebula, "Just distinguishable from a star; a minute disk 5 [seconds of arc] in diameter, with very faint ansae." In the Hubble image at right (rotate about 60 degrees to the right for a match), the "ansae," more like extensive wings than the bullets of NGC 7009, grow to five times the dimension of Curtis's inner nebula looking more like those of the "Red Spider," NGC 6537. The distance, as for most such nebulae, is pretty-much unknown. A trio of estimates averages 8000 light years, but that should not be taken too seriously. Given that figure, the inner nebula is 0.2 light years across, the outer wings a full light year from one end to the other. Notice the reflective symmetry. How one side knows what the other is doing is not known, but possibly involves binary-star action at the center, which stirs the mass loss from the progenitor advanced giant star. Chemical abundances seem normal, though there is some indication of enriched helium.

There are three divergent values for the brightness of the central star: a visual magnitude of 16.7 (which, compared with the nebular radiation gives a core temperature of 77,000 Kelvin), another of 18.4 (100,000 K), and an estimate purely from the nebular spectrum of 20.5 (143,000 K), the last probably not realistic, the differences vividly showing the difficulty of detecting a faint star against a bright background. The brighter of the two remaining values and the poor distance suggests a stellar luminosity around 7000 Suns and a core mass a bit over 0.6 solar, the star still heating. That will be the mass of the final white dwarf once the nebula (the inner portion expanding at 17 kilometers per second) dissipates into the darkness of space.

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: Hubble image by J. Schulman and the ESA/ESO/NASA Photoshop FITS Liberator.