Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
The Little Ghost Nebula (NGC 6369) is a relatively faint planetary nebula that lies between 2,000 and 5,000 light-years away from Earth in the constellation of Ophiuchus (the Serpent-bearer). Its doughnut-shaped blue-green ring is about a light-year across, while the fainter outer regions cover about twice that distance. The nebula is expanding at roughly 24 kilometers per second and it is approaching us at approximately 106 kilometers per second. It is nicknamed the “Little Ghost Nebula,” because it appears as a small, ghostly cloud surrounding the faint, dying central star.
Despite their name, planetary nebulae have nothing to do with planets. The name was coined by Sir William Herschel because when he first viewed a planetary nebula through a telescope, he could only identify a hazy smoky sphere, similar to gaseous planets such as Uranus. The name has stuck even though modern telescopes make it obvious that these objects are not planets at all, but the glowing gassy outer layers thrown off by a hot dying star.
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.
This image reveals remarkable details of the star’s last gasps, when it expels its outer layers into space, producing the faintly glowing nebula. Many of the details of ejection process are not visible from ground-based telescopes because of the blurring produced by the Earth’s atmosphere.
The central white dwarf star appears slightly off-center, and is on the side away from the brightest part of the nebula. It is now relatively cool, about 58,000 Kelvin, but it is still sending out a flood of ultraviolet light into the surrounding gas and powers the expanding nebula’s glow. The prominent blue-green ring marks the location where the energetic ultraviolet light has stripped electrons off of atoms in the gas. This process is called ionization.
In the redder gas at larger distances from the star, where the ultraviolet light is less intense, the ionization process is less advanced. Even farther outside the main body of the nebula, one can see fainter wisps of gas that were lost from the star at the beginning of the ejection process.
Over the next several thousand years, the Little Ghost Nebula will gradually disperse into space, and its central white dwarf will gradually cool off for billions of years, and eventually wink out. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.
This image is taken in February 2002 with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. It has been produced by combining pictures taken through filters that isolate light emitted by three different chemical elements with different degrees of ionization. The blue-green ring represents light from ionized oxygen atoms that have lost two electrons (blue) and from hydrogen atoms that have lost their single electrons (green). Red marks emission from nitrogen atoms that have lost only one electron.