NGC 6369


In Ophiuchus

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6369 NGC 6369
"A regular, nearly round ring...quite faint" says Curtis, as seen in the left hand composite drawing. NGC 6369's faintness is no surprise, as the planetary nebula, in far southeastern Ophiuchus (two degrees northeast of Theta Oph) is only 7 degrees west of the center of the Galaxy (and of the Summer Solstice in Sagittarius), and is thus very much dimmed by intervening interstellar dust. Were it not there, the nebula and its 16th magnitude (15.6) star would be some 30 or more times brighter!

The distance, as usual, is uncertain. The best estimate seems to be about 2000 light years, but it could be twice as far. With a rather large angular diameter of 28 seconds of arc (which contributes to low surface brightness), at that distance the nebula would be 0.3 light years across.

With no (or at best marginal) twice-ionized helium, the central star must be relatively cool, the measured value 58,000 Kelvin. Its luminosity is also relatively low, about 1000 times that of the Sun (though if you double the distance, it becomes four times brighter). Given these values, the star (the old nuclear-burning core of a one-time advanced giant) has a modest mass of a bit over half solar (consistent with non-elevated abundances). If so, the progenitor star may have been rather like the Sun. While still heating a bit, the star will shortly turn around and begin to cool as a nascent white dwarf, as the nebula, expanding rather rapidly at 41 kilometers per second, fades off into interstellar space.

Curtis's view is a mere shadow of the spectacular Hubble image on the right (which for a match should be rotated 40 degree to the right). The high resolution reveals knotty detail and a huge, symmetric, curling outer halo about twice as big as the inner ring, which was already known from ground-based images. While the beautiful outer structure was too faint for Curtis to see, he did capture the inner's ring's variations in brightness.

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).