NGC 6210

In Hercules

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6210 NGC 6210
"Central star...surrounded by bright matter in the form of an oblong 8" [seconds of arc] in length. Outside of this is an oblong of somewhat fainter matter 20" X 13", from the ends of which extend two fainter ansae...From the north and south sides of the central oblong extend two longer ansae...making the nebula 43" long..." says Curtis, who describes both his composite drawing (left) of this bizarre nebula AND the Hubble image (right) quite well indeed. Many planetary nebulae, NGC 6543, NGC 7009 among them, exhibit opposing bipolar flows or jets that emerge from the central source, perhaps as a result of stellar rotation or duplicity. Here we see not one set, but quite distinctly TWO of them that are so bright that Curtis saw both. (Something similar is seen in IC 4593.) No one knows what makes the bizarre form, which has been described as "a turtle swallowing a seashell" [Hubble].

In Hercules, four degrees northeast of Beta Herculis, NGC 6210's third dimension, its distance, is as usual problematic, with 6500 light years (estimated from common properties) seeming as good as any. The diameter of the inner oblong region is then around 0.4 light years, while the horizontal extension is 1.5 times larger, the north-south one 3.5 times as big.

The still-heating 13th magnitude (12.8) magnitude star has a temperature of 58,000 Kelvin, right at the edge at which it helium can become doubly-ionized. A luminosity of 3000 Suns is very approximate because of the distance uncertainty. As the star evolves toward higher temperatures, the nebula is expanding in its inner region by 21 kilometers per second, the outer part at 36 km/s. There is no chemical enrichment, suggesting a relatively low mass to the original star, which will someday heat to its maximum of somewhat over 100,000 Kelvin and then begin to cool and dim as a nascent white dwarf.

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