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A starry origin for sandy compounds.

A starry origin for sandy compounds

Silicon is the most abundant element in the surface of the earth; silicates are the most abundant compounds there. Silicate dust also pervades interstellar space, and silicates are common in such things as meteorites and comets. The origin of silicon and silicates, like that of other elements and many compounds, must lie somehow in the stars or in processes associated with stars. Now, for the first time, astronomers have found an exploding star--a nova--that produces silicates. This nova is a star of unusually large mass for a white dwarf and so is likely to cause some changes in the generally accepted theory of stellar evolution.

Most novas, which are periodically exploding white dwarf stars, produce carbon dust. According to infrared spectroscopy done by Robert Gehrz of the University of Minnesota in Minneapolis and Gary Grasdalen of the University of Wyoming in Laramie, this nova is rich in silicon, magnesium, neon and oxygen. In the constellation Vulpecula, it is designated nova Vulpecula 1984, number 2.

Novas are supposed to be located in close binary systems with more or less ordinary stars for their companions. The very strong gravity of the white dwarf draws material from the companion. As this material falls on the surface of the white dwarf, it builds up an unstable condition that eventually triggers an explosion. This cycle repeats after a period of centuries or longer.

The nova explosion spews material from the white dwarf into space. It generates a shock wave moving outward from the star. Gehrz believes that the silicate compounds are made in this shock wave, but he says the silicon itself has to come from inside the white dwarf. A white dwarf that produces silicon is unique. Most of them operate on a cycle that makes carbon, oxygen and nitrogen.

To produce silicon, it seems that this white dwarf has to be much more massive than white dwarfs are expected to be, and that poses a challenge to stellar evolution theory. According to theory, a white dwarf is one of the things a star should become at the end of its life, but that particular fate should come only to fairly small stars, those the size of the sun or less. A star more than two or three times the sun's mass should undergo a supernova explosion and become a neutron star.

This silicon-producing white dwarf has to be about 15 times the sun's mass. If a white dwarf that size can exist, it presents a serious challenge to the theory, or, in Gehrz's words, "severely constrains it."
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Title Annotation:nova that produces silicates
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Mar 8, 1986
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