July 13, 2013
Life Of Eskimo Nebula Coming To A Beautiful End
April Flowers for redOrbit.com - Your Universe Online
At the end of their lives, stars like our Sun become remarkably photogenic. For example, NGC 2392, located approximately 4,200 light years from Earth, is giving astronomers a beautiful display as it nears the end of its existence.
NGC 2392, referred to as a "planetary nebula," has been nicknamed the Eskimo Nebula; however "planetary nebula" is misleading because nebulae have nothing to do with planets. Rather, the term is an historical relic as the celestial objects looked like planetary disks to astronomers looking through small optical telescopes in earlier times.
Planetary nebulae form when a star uses up all of the hydrogen in its core, which scientists expect our Sun to experience in about five billion years. As this happens, the star begins to cool and expand, increasing its radius by tens to hundreds of times its original size. The outer layers of the star are eventually carried away by a 30,000-mile-per-hour wind. This leaves behind a hot core with a surface temperature of about 90,000 degrees Fahrenheit, which is ejecting its outer layers in a much faster wind traveling 3.6 million miles per hour. The interaction of the fast wind with the slower wind, along with the radiation from the hot star, creates the complex and filamentary shell of a planetary nebula, which will eventually collapse to form a white dwarf star.
Modern astronomers, using space-based telescopes, are able to observe planetary nebulae like NGC 2392 in ways their scientific predecessors could never have imagined. A composite image of the Eskimo Nebula shows NASA's Chandra X-ray Observatory x-ray data in purple, revealing the location of million-degree gas near the center of the nebula. Hubble Space Telescope data -- colored in red, green and blue -- shows the intricate pattern of the outer layers of the star that have been ejected.
When the faster wind and radiation from the central star interact with cooler shells of dust and gas that were already ejected by the star, the comet-shaped filaments form. The observations of NGC 2392 were part of a larger study, published in The Astrophysical Journal, of three planetary nebulae with hot gas in their center.
Compared to the other two, the Chandra data shows NGC 2392 has unusually high levels of X-ray emission, leading researchers to deduce that there is an unseen companion to the hot central star in NGC 2392. The elevated X-ray emission could possibly be explained by the interaction between a pair of binary stars. The other two nebulae -- IC 418 and NGC 6826 -- weaker X-ray emission is likely produced by shock fronts, similar to sonic booms, in the wind from a central star. The composite image of NGC 6826 was released in 2012 as part of a gallery of planetary nebulae.
The international group of scientists involved in this study include Nieves Ruiz of the Instituto de AstrofÃsica de AndalucÃa (IAA-CSIC) in Granada, Spain; You-Hua Chu, and Robert Gruendl from the University of Illinois, Urbana; MartÃn Guerrero from the Instituto de AstrofÃsica de AndalucÃa (IAA-CSIC) in Granada, Spain; and Ralf Jacob,Detlef SchÃ¶nberner and Matthias Steffen from the Leibniz-Institut FÃ¼r Astrophysik in Potsdam (AIP), Germany.