A Hubble Space Telescope (HST) image of NGC 6826. Image credit: HST/NASA/ESA.
NGC 6826 is a planetary nebula that we can find in the constellation of Cygnus. It is called "the eye that flickers" because it seems to oscillate. In its center is located a star of magnitude 10, looking alternately to that star and to the surroundings that effect of flickering occurs in the nebula. A really curious phenomenon, although we will need telescopes of big size (from 150 mm of aperture) to be able to begin to appreciate it.
NGC 6826 is a little closer than NGC 40, 2,000 light-years away, and its diameter is somewhat smaller. Like most planetary nebulae, it has a spheroidal shape. In the image it can be seen how NGC 6826 is composed of two superimposed layers, one larger rounded, and one smaller more elliptic. Studies of 1990 suggest that the elliptical form of the nebula is due to the presence of a companion star that was evaporated by the central star. Apparently this would explain its ovoid shape, while the outermost layer, invisible to the visual telescope, is perfectly round.
In addition, the nebula has two characteristically illuminated polar zones that are known as FLIERs, which can be found in other planetary nebulae. They appear to come from flashes of the central star and reach supersonic speeds.