The Blinking Nebula
NGC6826 (C15) is a planetary nebula, approximately 2200 light years away, in the constellation Cygnus. As with all planetary nebulae, it's the remnant of a red giant star which ejected its outer gaseous shell leaving behind a hot, dense, white-dwarf star. It's been estimated that 95% of all stars, including our Sun, will end their lives in this way. The central white-dwarf star in this nebula is one of the brightest central stars in a planetary nebula.
The above image is a color composite consisting of individual exposures through white (minus IR), cyan, magenta and yellow filters (WCMY). The white (luminance) component is from 84, 15-second subexposures obtained on 6 and 8 October 1999. Cyan (49x10sec), magenta (30x10sec), and yellow (27x10sec) exposures were obtained on 6 August 2000.
A feature visible in this image are two bright patches in the ring on opposite sides of the central star, known as FLIERs, or Fast Low-Ionization Emission Regions. These regions are moving at supersonic speeds and are apparently relatively young in comparison to their parent nebula. According to Bruce Balick (University of Washington), "some of their observed characteristics suggest that they are like sparks flung outward from the central star late in the very recent past (a thousand years ago). Yet their shapes ... seem to suggest that they are stationary, and that material ejected from the star flows past them, scraping gas from their surfaces. Future Hubble observations will monitor any changes in the positions of FLIERs to resolve this issue. In either case, the formation of FLIERs cannot be easily explained by any models of stellar evolution."
Another feature visible in this image is a part of the faint halo that extends beyond the brighter ring. The below, over-processed monochrome image shows the edges of this outer halo. This halo consists of the initial matter ejected from the dying star.
The Blinking Nebula (NGC6826) is so named because when viewed through a small telescope the nebula appears to blink or disappear as the observer scans the eyepiece. This is because the nebula is faint compared to the inner relatively bright white dwarf star. When viewed directly the star is easily visible through the cones of the eyes, while the faint outer nebula is seen with peripheral vision using the more sensitive rods of the eye.
The HST imaged this nebula in January, 1996 and when the research team published their image, the press noted that the nebula appeared as a "Bloodshot Eye in Deepspace". This nebula was also highlighted at NASA's Astronomy Picture of the Day site in 1997. To learn more about these objects and view images of such nebulae from the Hubble Space Telescope (HST) click here.
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