NGC 6751

In Aquila

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6751 NGC 6751
"Nearly round...indistinct evidence of a very irregular ring formation...rather faint" (Curtis). Indeed, NGC 6751 in Aquila (1.1 degrees due south of Lambda Aquilae) clearly shows irregularity in Curtis's composite drawing (left) along with a very prominent 14th magnitude (13.9) central star. The Hubble view on the right (rotate about 30 degrees to the left for a match) nicely supports Curtis's, and adds a lot more detail. We see the same brightening along the western edge, and now a whole set of subtle radial fingers rather reminiscent of those in IC 3568, though clearer. Moreover, there is wispy evidence of a surrounding outer shell that extends some 35 percent farther than the inner prominent ring.

Distance is always a problem. The statistical method (which uses average nebular parameters) gives 8000 light years, but that is most likely to be an upper limit. At that distance, the inner shell (21 seconds of arc wide) is 0.85 light years across, the nebula getting bigger at a speed of 40 kilometers per second.

A central star temperature of 76,000 Kelvin is consistent with a bit of doubly ionized helium, that and a luminosity of 9200 Suns (again an upper limit) revealing the star to be in its heating phase following the ejection of its giant-star envelope. The star has its own broad emission lines that tell of a stiff outbound wind of the sort often seen in stars still in the heating phase of their planetary-nebula lives, as mass is still being lost before they turn into real white dwarfs.

The chemical composition of the nebula is un-enhanced, showing no obvious effects of nuclear-processed materials in the predecessor star, which is now returning more or less pristine matter back into interstellar space from which it came.

Left: Quote and image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: NASA and the Hubble Heritage Team (STScI/AURA).