Celestial sandpaper: grit from the stars.

Celestial sandpaper: Grit from the stars

Silicon carbide manufactured in an electric furnace by heating sand in the presence of carbon is a tough, hard material often used for making sandpaper. The first laboratory evidence that the same material may also be created within gases ejected by carbon-rich stars has been found in microscopic silicon carbide grains recently isolated from a primitive meteorite.

The discovery of these grains by Edward Anders of the University of Chicago and his colleagues marks the first time that scientists have been able to examine samples of stellar silicon carbide in the laboratory. Astronomers had previously detected the compound's spectral signature in dust surrounding distant stars.

"People had predicted silicon carbide would be present," says Anders. "But seeing it from afar as dust grains in a star spectrum isn't the same as analyzing it in the lab and seeing...what sort of story it tells." The researchers report their findings in two papers in the Dec. 24 NATURE.

Anders and his group isolated the silicon carbide grains, each one a micron or less across, by gradually dissolving away the rest of the meteorite sample. A similar technique applied earlier had resulted in the detection of tiny diamond crystals within the same meteorite (SN: 3/14/87, p.166).

The resistance of both diamond and silicon carbide to chemical attack means that the grains have survived largely intact over time periods longer than the age of the solar system. Their presence -- as messengers from a distant past -- suggests how solid materials may form within a gas cloud shed by a star.

The researchers found that the silicon carbide grains contain an unusual combination of carbon and silicon isotopes. They conclude that initially the carbon and silicon atoms were probably created separately by different stars at different stages in their life cycles. In other words, more than one star may have contributed to the formation of silicon carbide. At a later stage, a nova explosion may have driven the atoms together to create the grains present in a meteorite.

Exactly how the silicon carbide and diamond grains were produced in gas clouds is open to debate. "When one goes to an unusual environment that is not really common on earth," says Anders, "one has to consider factors that one normally ignores on earth." Adding to the puzzle, Anders and his group have recently found meteorite diamonds that are large enough to be visible in an optical microscope.