NGC 6886

In Sagitta

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6886 NGC 6886
Located in a dense part of the Milky Way in far eastern Sagitta 3 1/3 degrees west of Gamma Sge (and more or less between the Dumbbell Nebula and IC 4997), the compact planetary nebula NGC 6886, though "quite bright" [Curtis], remains something of a mystery thanks to a very uncertain distance and a hidden central star. Curtis saw "a round disk 6 [seconds of arc] in diameter, of nearly equal brightness throughout...two wings bring the total length to about 9 [seconds]." The Hubble image on the right simply (though gloriously) expands on Curtis's picture, revealing complex detail in the "wings" that clearly surround the central structure. Distance estimates run from 6000 light years (from various individual methods) to as much as 17,000 l-y. Using a middle "statistical" value of 10,000 l-y gives dimensions of 0.3 X 0.45 l-y, not unreasonable at all. Given NGC 6886's location and distance, one might expect greater dimming of light by interstellar dust than the approximate 1.4 magnitudes.

While the central star is not seen thanks to the glare of the central structure, analysis of the nebular spectrum indicates that it should have a visual magnitude of 19.0, faint indeed, most of its radiation in the invisible ultraviolet. No wonder Curtis claimed that "no central star could be made out." Along with magnitude, we also find a substantial temperature of 168,000 Kelvin. Luminosity and condition then depend strongly on the uncertain distance. At the middle value, a 0.58 solar mass star with a luminosity of 1250 Suns (and what was once the core of a much more massive giant) has reached its maximum temperature and will shortly begin to cool and dim. (At the shorter distance, the cooling has already commenced.) Lack of chemical enrichment argues against a much higher mass. As the star cools and the nebula expands (now between 20 and 25 kilometers per second) and also dims, the star will emerge from its cocoon and then die as a modest white dwarf.

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: ESA/Hubble and NASA.