A colorful planetary nebula—the nebular remnant of a Sun-like star that died—is pictured in an image taken by the Hubble Space Telescope. Called NGC 6818, it can be found approximately 6,000 light-years from Earth toward the Sagittarius constellation.
Like all planetary nebulae, NGC 6818—more popularly known as the Little Gem Nebula—was forged from the ashes of a medium-mass star. Instead of going out in a fiery explosion like high-mass stars, a star around the same size as the Sun (perhaps a few times larger) dies a slower death, which generally kicks off around 10 billion years after the star begins nuclear fusion in its core. Once its hydrogen reservoir runs dry, it dips into its helium supply—leading it directly to its demise.
In one of the final stages of its life, the enlarged star shrinks back down to a fraction of its original size—leaving only a white-hot husk around the size of Earth, with the mass of the Sun behind—and sheds the remainder of its gaseous envelope. The remnant, known as a white dwarf, emits energizing ultraviolet radiation, which sets the gaseous filaments alight, and generates the planetary nebula.
While taken by Hubble, this image was reprocessed by Judy Schmidt (Geckzilla) in order to highlight ionized oxygen and hydrogen emission. It was subsequently entered into the Hubble Hidden Treasures image processing contest. (See a larger image here)
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