NGC 6818

In Sagittarius

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6818 NGC 6818
"A rather irregular oval ring...fainter along and at the ends of the major axis...a fainter narrow ring...outside of the inner bright ring. Fairly bright..." [Curtis]. NGC 6818 (the "Little Gem Nebula") falls in busy, but not overcrowded, far northeastern Sagittarius, outside the visual confines of the Milky Way 8.5 degrees due west of the line connecting Alpha and Beta Capricorni. Compact and elliptically-shaped, the nebula measures 15" X 22" in diameter, which, given a very approximate distance of 6500 light years, translates into a physical size of 0.5 X 0.7 light years. Closely surrounding the bright inner ring is a more-circular outer ring, a common sight among such nebulae. Curtis's drawing captures the structure well, including some of the detail seen in the Hubble image.

NGC 6818 is dimmed by about 0.6 magnitudes by interstellar dust absorption. Chemical compositions are more or less normal, though there may be some carbon enrichment. The central star is anomalous. Curtis shows it prominently and gives it a visual magnitude of 14, while it is only barely visible in the Hubble image, various measures indicating around 17.0. It seems unlikely that the star would fade three magnitudes over a mere century. Perhaps Curtis saw a nebular clump instead, though there is no indication of such by Hubble. With a temperature as high as 155,000 Kelvin and a luminosity estimated between 1000 and 2000 Suns, the relatively low mass central star (about 0.6 Suns, very typical and consistent with normal chemical abundances) seems to be near its maximum temperature, after which it will cool and dim to become a common sort of white dwarf.

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: R. Rubin (NASA Ames Res. Cntr.), R. Dufour and M. Browning (Rice U.), P. Harrington (U. Maryland), and NASA.