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The lovely 'ghost' of a dying star

As a white dwarf in the final throes of its life span, a dying star throws off a nebula known poetically -- if not entirely accurately -- as the 'Ghost of Jupiter'.

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ESA/XMM-Newton & Y.-H. Chu/R.A. Gruendl/M.A. Guerrero/N. Ruiz (X-ray); NASA/ESA Hubble Space Telescope & A. Hajian/B. Balick (optical)

In the earlier days of astronomy, a lot of things were spotted in space with telescopes that weren't quite strong enough to tell precisely what those things were. Hence you'll occasionally stumble across a name for something that isn't entirely accurate.

Take planetary nebulas. Yes, they are nebulas; but they have nothing to do with planets. When astronomer William Herschel discovered them in the 1780s, the round shapes of the nebulas looked to him like the shapes of planets, and the name he gave them has never been amended to reflect what they really are: the exhalation of a dying star.

Most small and medium-sized stars don't explode, or supernova, when they die. Instead, they gently shed material in a sort of "puff", a glowing shell of expanding, ionised gas blown outwards by stellar winds as the star collapses. The Egg Nebula, the Cat's Eye Nebula, the Ring Nebula and the Soap Bubble Nebula are all stunning examples of planetary nebulas.

Planetary nebula NGC 3242, located in the constellation of Hydra, some 3,000 light-years from Earth, was discovered by Herschel 230 years ago, on February 7, 1785. And, because it occupies a space in the sky roughly the same size as that of Jupiter, it has been given the nickname "Jupiter's Ghost".

As perspective might indicate, Jupiter's Ghost is actually quite a bit larger than Jupiter. The blue-green bubble of gas measures about two light-years from end to end, compared to Jupiter's 69,911-kilometre radius. Inside burns the final stage of a star's life span: the white dwarf.

White dwarfs are very small -- about the size of the Earth in volume -- and very, very dense, with a mass comparable to that of the sun. It releases strong stellar winds -- about 2,400 kilometres per second -- which blows gas outwards into the bubble shape.

The colours in the image represent several layers. Blue is X-ray emission, snapped by the XMM-Newton telescope, from gas heated to over two million degrees Celsius by shocks caused by the stellar wind. The green glow, snapped by the Hubble, is cooler gas seen in optical light -- the inner and outer shell of the nebula.

The two reddish smudges are what is known as Fast Low-Ionisation Emission Regions -- FLIERs. These are volumes of low-ionisation gas at or near the symmetry axis of a dying star, jetting from its poles at supersonic speeds significantly higher than those of the surrounding nebula.

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