Image Credit: B. Balick, J. Alexander (University of Washington), et al., NASA
The Blinking Eye Nebula (NGC 6826) is a planetary nebula with a size of about 0.22 by 0.20 light-year, located some 2,200 light-years away in the constellation of Cygnus. It is slowly moving toward us at 6.2 kilometers per second. This eye-shaped nebula lasts perhaps 10 thousand years (compared to a 10 billion year stellar life span).
Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.
When a star with a mass up to eight times that of the Sun runs out of fuel at the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star (collapsing from a red giant to a white dwarf) to radiate strongly, causing this outward-moving cocoon of gas to glow brightly as a planetary nebula.
Over the next several thousand years, the nebula will gradually disperse into space, and then the star will cool and fade away for billions of years as a white dwarf. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.
In the center of the Blinking Eye nebula is such a hot, very bright remnant star (white dwarf) that drives a fast wind into older material, forming a hot interior bubble which pushes the older gas ahead of it to form a bright rim. (The star is one of the brightest stars in any planetary nebula, and when viewed through a small telescope, its brightness overwhelms the eye, obscuring the surrounding nebula.)
The surrounding faint green is believed to be gas that made up almost half of the star’s mass for most of its life. The two mysterious bright red patches on either side are FLIERs (Fast Low-Ionization Emission Regions), which appear to be relatively young in comparison to their parent nebula.
Some characteristics suggest that they are like sparks flung outward from the central star at supersonic speeds about a thousand years ago. (If so, their “bow shocks” point in the wrong direction!) 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. In either case, the formation of FLIERs cannot be easily explained by any models of stellar evolution.
The nebula has gotten its nickname because when viewed through a small telescope the nebula appears to blink or disappear. This is because the nebula is faint compared to the inner 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.
This image was taken with the Wide Field and Planetary Camera 2 onboard the Hubble Space Telescope on Jan. 27, 1996.