Explanation: Hourglass Nebula Defies Explanation:
“A shimmering hourglass 8,000 light-years away, the planetary nebula MyCn 18, is striking in its beauty and complexity. However, the only time this hourglass can keep is the death knell of a star, and it is so odd that astronomers say it is forcing them to rethink the physics that shapes it.
Not only does MyCn 18 display a second, smaller hourglass of gas inside the larger one, but neither hourglass is position symmetrically around the central star. The central star is a white dwarf, a very hot, compact, spent star that wouldn’t have enough gas remaining to generate such an extraordinary nebula.
“What we thought we understood of planetary nebulae we no longer do,” says Raghvendra Sahai, an astronomer at the Jet Propulsion Laboratory in Pasadena, California. “Something different and dramatic is going on.”
The asymmetrical hourglass architecture and revealing images obtained by the Hubble Space Telescope are spawning a renaissance, he says in the study of planetaries. They form when an aging ordinary-mass star, having exhausted most of its nuclear fuel, starts to expand into a red giant. Escaping gas and dust accumulate first as a belt around the equator of the star. As the volume of escaping gas increases, the belt constricts the star’s midsection and forces the increasingly fast moving gas into an hourglass shape. The escaping gas, heated and ionized by the dying star, shines. However, MnCy 18 may very well be a highly unusual planetary.
Sahai and others reported in the July issue of The Astronomical Journal that MyCn 18 must result from a binary-star system. They argue that the nebula needs a source of a lot of gas, one that a white dwarf alone would lack. Gas escaping what might be a nearby red giant (or alternatively a smaller, dim, Jupiter-sized star) could be sucked onto the dwarf where it could accumulate and eventually fuel a nova explosion, the gas of which would spread outward to form the hourglass.
The second hourglass is more difficult to explain. “It’s very hard to see how you get it,” says Sahai. “You just don’t have enough material.”
Astronomers say that perhaps the explosion itself was asymmetrical. Arizona State University astronomer Paul Scowen Theorizes that the interstellar medium around the star system itself may have been inhomogeneous, allowing the expanding nebula to form asymmetrically.
The intrigue continues. Astronomers used Hubble to capture two other rings at the neck of the hourglass-possibly a belt of dense gas escaping the unseen star and a second ring perpendicular to it. What forces are responsible for the rings? “We have no idea,” says Sahai. “We have to make a fresh start to understand the physics of planetary nebulae.”
“Nebula has caused a lot of speculation,” says Scowen,” and for now it is just speculation.” Meanwhile, he is fascinated by the intricate pattern of rills and ripples setup in the gas at the upper reaches of the nebula, created by turbulence. Planned computer simulations that test various theories might give a clearer explanation of just how the puzzling, stunning nebula was sculpted.”
The information contained within the quotes was acquired from: Astronomy, December 1999
issue, Asymmetry, Hourglass Nebula Defies Explanation; pages 30,32; by David Graham; published by Kalmbach Publishing Co. copyright 1999
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