NGC 6537


In Sagittarius

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6537 NGC 6537
The comparison between Curtis's composite visual view (left) of NGC 6537 and the Hubble image (right) is among the most severe of any of the planetary nebulae presented here. Curtis sees no more than "a minute disk 5" in diameter, just distinguishable from a star." Hubble shows the minute disk as a bright core centered on a small halo. The disk in turn is centered on a huge hourglass-shaped structure that extends outward to a maximum diameter of 2.5 minutes of arc and that gives the object its popular name, "The Red Spider," the red color coming from hydrogen and ionized nitrogen emissions. Curtis did, however, see the dual structure of the inner core. Rotate the Hubble image 40 degrees to the left to place north at the top so as to align the pair of pictues.

NGC 6537 is located in northwestern Sagittarius just 10 degrees to the north-northeast of the center of the Galaxy (and 2.4 degrees northwest of Mu Sagittarii), and as such suffers from considerable dimming by interstellar dust (interstellar and circumstellar combining to make the object some 40 times dimmer than it would be with no dust present).

One distance measure has it at 3000 light years, while a standard statistical estimate places it at close to 8000 light years. If we adopt 5000 as not unreasonable, then the inner core is just a tenth of a light year across, while the outer assembly extends 3.5 light years from one extreme edge to the other. The origin of this strange structure is unknown, but might be related to a magnetic field discovered within the interior of the nebula that shaped the wind from the progenitor giant star. A binary companion might have done it too, but there is no evidence for one.

Buried within the object, the central star is so dim that it is virtually inaccessible. Curtis could not see one at all, and no wonder, as it is estimated from nebular properties to shine at twentieth visual magnitude (19.7) with a luminosity of more than 5000 Suns. The star is so hot, at 250,000 Kelvin one of the hottest known (more than for NGC 2440 or 7027), that it radiates most of its light in the far ultraviolet, which with dust absorption renders the visual magnitude quite faint. The star must be near its maximum temperature before it enters its final cooling phase to become a hot white dwarf. The interior expands relatively leisurely, at 18 kilometers per second, that and possible rapid evolution of the star rendering the interior rather small. For now, though, the Red Spider remains enigmatic.

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: Bruce Balick (U. Washington), Vincent Icke (Lieden U., Netherlands), Garrelt Mellema, Stockholm U.), and NASA.