NGC 6565

In Sagittarius

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6565 NGC 6565
Curtis, from a century ago: "No central star can be seen. A minute oval ring...Fairly bright." Rotate the Hubble view on the right by about 35 degrees clockwise to align it with Curtis's picture. Embraced by the great Sagittarius star clouds, NGC 6565, just nine seconds of arc across, is at the northern apex of an equilateral triangle formed with Gamma and Delta Sagittarii, roughly 2.5 degrees from each. A typical fainter outer halo appears on the Hubble image. Though the distance is not really known, there seems to be some consensus that the nebula is far away, perhaps as much as 15,000 light years (giving it a highly uncertain diameter of 0.65 light years). Only six degrees from the direction to the Galactic center, it's roughly halfway to the Galaxy's actual central core (where we find the supermassive black hole of four million solar masses). The nebula is widely thought to be a part of the Galaxy's central bulge. Yet visual dimming by interstellar dust is fairly low, less than a magnitude, revealing a relatively clear tunnel through the haze (or that it is a lot closer than we think). One measure suggests that helium may be enriched in the nebula by a dubious 40 percent, while the nitrogen content appears more or less normal.

It's no wonder that Curtis could not see the central star, as it's so faint, eighteenth magnitude. The nebular background makes it even more difficult, though NGC 6565's center is "relatively vacant." A measured visual magnitude of 17.8 compared with nebular radiation (which responds to the stellar ultraviolet) gives a stellar temperature of 105,000 Kelvin and a luminosity that (depending strongly on distance) is somewhere in the hundreds of Suns. The star, with a relatively low mass of somewhat over half a Sun, appears to be near its turnaround point where it stops heating at a constant temperature and begins to cool to become a real white dwarf. Expanding at 15 to 20 kilometers per second, the planetary nebula will at the same time fade into the cosmic gloom, carrying with it another load of matter with which to build new stars and perhaps a bit of extra helium.

Left: Image and quote by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: Hubble Legacy Archives, A. Hajian et al.