NGC 7026

In Cygnus

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 7026 NGC 7026
From Curtis: "Very irregular; there are two bright lobes...A bridge of nebulosity connects these and involves the central star so that it resembles the letter H...Irregular wisps extend from each end...Fairly bright." NGC 7026, not to be confused with better- known and nearby NGC 7027, lies within the heart of the Milky Way in northern Cygnus five degrees northeast of Deneb, just a third of a degree north- northwest of 63 Cygni (which makes a good guide star), and five and a half degrees almost due north of brighter NGC 7027. As a result of location and distance, NGC 7026 is dimmed visually by about a magnitude and a half by interstellar dust. The Hubble image at right (which should be rotated slightly clockwise to align the two) matches nicely with Curtis's century-old composite drawing, but reveals a more complex structure with an interior bipolar structure surrounded by a four-lobed, even conical, appearance. Explaining such forms is a daunting task, and might be related to binarity (though there is no evidence for it), magnetic fields, rotation, and who knows what else?

The distance as usual is uncertain. An average of various estimates with a measure from the amount of intervening dust suggests around 5900 light years. As seen in both pictures, no single angular diameter can describe the nebula with its great elongation and delicate outer wisps. Curtis's dimensions of 10 X 25 seconds of arc gives 0.3 X 0.7 light years. The nebula is expanding at a rate of 40 to 50 kilometers per second. There is no evidence for any particular abundance enhancements. The helium content relative to hydrogen is normal. While raised above the solar ratios, there if nothing unusual about the carbon/oxygen and nitrogen/oxygen ratios, both of which are well less than 1.

Curtis estimated the central star's magnitude at 14. Modern measures give visual magnitudes of 14.2 to 15.1. Adopting 14.2 combined with the brightness of the nebular hydrogen and helium emission lines gives a central star temperature of 80,000 Kelvin, a luminosity of 2100 times that of the Sun, a core mass of 0.56 solar, and shows the core (the nascent white dwarf) to be in a heating phase at constant luminosity. The parent star should then have had a mass of just over 1.1 times that of the Sun, which is consistent with the un-enriched nebular chemical composition, enrichment falling in the domain of higher mass stars.

Left: Image and quote by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: Hubble Legacy Archive, A. Hajian et al.