Beautiful Death of a Star

Stars like the Sun can turn out to be extraordinarily photogenic at the end of their life. A good illustration is NGC 2392, which is situated approximately 4,200 light years from Earth. NGC 2392, dubbed the "Eskimo Nebula", is what astrophysicists call a planetary nebula. This title, though, is misleading since planetary nebulas in fact have nothing to do with planets. The term is just a historic relic as these objects looked like planetary disks to astrophysicists in earlier times watching through small optical telescopes. In its place, planetary nebulas came into being when a star uses up all of the hydrogen in its core -- a phenomena our Sun will go through in about five billion years.

When this takes place, the star starts to cool and swell, increasing its radius by tens to hundreds of times its original size. Ultimately, the outer films of the star are carried away by a thick 50,000 kilometer per hour wind, leaving behind a hot core. This hot core has a superficial temperature of about 50,000 degrees Celsius, and is discharging its outer layers in a much faster wind flowing six million kilometers per hour. The radiation from the hot star and the contact of its fast wind with the slower wind produces the complex and filamentary shell of a planetary nebula. Ultimately the leftover star will breakdown to form a white dwarf star.

Now days, astrophysicists using space-based telescopes are able to perceive planetary nebulas such as NGC 2392 in ways their scientific descendants possibly could never imagine. This compound image of NGC 2392 comprises X-ray data from NASA's Chandra X-ray Observatory in purple displaying the position of million-degree gas near the center of the planetary nebula. Data from the Hubble Space Telescope show – colored red, green, and blue – the complicated pattern of the outer layers of the star that have been expelled. The comet-shaped filaments form when the faster wind and radiation from the central star interact with cooler shells of dust and gas that were previously emitted by the star.

The study of NGC 2392 was portion of a study of three planetary nebulas with hot gas in their center. The Chandra data show that NGC 2392 has remarkably high levels of X-ray discharge related to the other two. This leads scientists to infer that there is a hidden companion to the hot central star in NGC 2392. The contact between a pair of binary stars could clarify the raised X-ray emission discovered there. Meanwhile, the fainter X-ray emission detected in the two other planetary nebulas in the sample – IC 418 and NGC 6826 – is possibly produced by shock fronts (like sonic booms) in the wind from the central star. A compound image of NGC 6826 was added to the gallery of planetary nebulas published in 2012. 
A paper defining these outcomes is accessible online and was published in the April 10th, 2013 issue of The Astrophysical Journal. 



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