IC 4997

In Sagitta

From Jim Kaler's STARS; Return to Planetary Nebulae

IC 4997 IC 4997
"Indistinguishable from a star" (Curtis). IC 4997, in extreme southeastern Sagitta (between the classical figures of Sagitta and Delphinus), is one of several "stellar" planetary nebulae that to the eye are physically so small or so far away (or both) that they just look like stars. In Curtis's photo the nebula appears as no more than a 10th magnitude dot. The category includes seven others with NGC or IC numbers. The only way to tell them from stars is through their spectra. Of the set, IC 4997, a most unusual object, is at the top of the list of significance.

Small as it is, the great resolving power of the Hubble (see the image at right) reveals an intricate bi-lobed structure seemingly overwhelmed by a relatively brilliant 14th magnitude central star. (The spikes on the Hubble image are artifacts of the telescope.) The object has been resolved at radio frequencies as well. The angular "diameter" depends on how you measure it and with what instruments. An outer shell about 2.5 seconds of arc in size surrounds a much more compact central region. The distance is not known. At a statistical guess of 8000 light years, even the outer nebula would be but a tenth of a light year wide.

The small size goes along with other extreme properties that include a very high near-record-setting nebular temperature of around 20,000 Kelvin (about double that of most nebulae) and a very high density variously measured in the tens of thousands of atoms per cubic centimeter up to 100,000 or more. Expanding at a rate of 20 kilometers per second (the outer region less), the dusty nebula appears to be very young and recently formed, one estimate suggesting less than 700 years ago. Consistently the star, still heating, is yet relatively cool, with a temperature of just 49,000 Kelvin. Even that is a bit odd. IC 418, for example, another young nebula, is larger and apparently farther along its evolutionary path, yet has a cooler central star, making IC 4997's star appear too hot for its age.

The most remarkable property of IC 4997 is its variability, which is unique among bright nebulae. A sudden change in the spectrum was noted in the 1960s that is usually ascribed to a drop in nebular temperature resulting from youthful expansion. But that is just a guess; variations in the central star may play a role too. Radio observations have also noted changes in structure over periods of not much more than a year. IC 4997, small as it is, is clearly a nebula to keep our eye on, as it is telling us something about how these marvelous objects are born and develop in their youth.

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: Howard Bond (STScI), NASA.