Also cataloged as NGC 6369, near the celestial equator, in the constellation of Ophiuchus, the Little Ghost Nebula was discovered by the 18th century astronomer William Herschel. Over 3,000 to 4,000 light-years away, it offers a glimpse of the fate of our Sun, which should produce its own pretty planetary nebula about 5 billion years from now. It has an apparent magnitude of 12.9.
Captured with the Wide Field Planetary Camera 2 (WFPC2) in 2002, the Hubble photograph of NGC 6369 reveals remarkable details of the ejection process that are not visible from ground-based telescopes because of the blurring produced by the Earth’s atmosphere. The remnant stellar core in the centre is now sending out a flood of ultraviolet (UV) light into the surrounding gas. The prominent blue-green ring, nearly a light-year in diameter, marks the location where the energetic UV light has stripped electrons off of atoms in the gas. This process is called ionisation. In the redder gas at larger distances from the star, where the UV light is less intense, the ionisation 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.
Prior to the collapse, the star might have been several hundred million miles across. Afterwards, it is less than ten thousand miles across, and as a result, has a density close to a million times the density of water. The central circular core of the nebula is a light year or two across, while the fainter outer regions cover about twice that distance. Both regions are expanding at about 15 miles per second, and will dissipate into interstellar space and fade from view over the next ten thousand or so years, partly because of their gradually decreasing density, and partly because the white dwarf is currently very hot, with a temperature of roughly 58,000 K, and emitting large amounts of ultraviolet radiation, which is energizing the nebula. As the white dwarf cools, its ultraviolet radiation will diminish, which would cause the nebula to fade from view even if it didn’t dissipate.
Credits: Courtney Seligman, ESA, Hubble, Wikipedia.