Ancient Flecks From Doomed Stars

Geochemists have dissolved bits of two meteorites to extract mineral grains spawned by stars before our solar system was born. The ancient stardust includes just the second known oxygen-rich grain forged in the nuclear furnace of a supernova explosion, according to a report in tomorrow's Science. The researchers hope to reconstruct the chemical history of our corner of the galaxy by analyzing the dust.

Galaxies are dusty places. Vast clouds billow out from supernovae, the explosive deaths of giant stars that have consumed all their nuclear fuel. Ordinary stars also blow dust into space when they swell into red giants near the end of their lives. This dust cycles back into new stars and planetary systems. Just as factory workers stamp serial numbers on the parts they manufacture, a star imprints its dust grains with unique chemical signatures. But pristine dust is rare today, mostly because heat and other processes in our solar system bake it over time.

A new detection technique allowed cosmochemist Byeon-Gak Choi and his colleagues from the California Institute of Technology in Pasadena to isolate 14 grains from two meteorites thought to be 4.5 billion years old. Dust from our own solar system has a characteristic ratio of heavy to light oxygen isotopes. However, Choi's 14 grains have unusual ratios, a sign that they predate our solar system. Most of the grains probably formed just above the churning atmospheres of red giant stars. One, however, contains a unique blend of magnesium, calcium, and titanium that could only have come from the titanic explosion of a star that was perhaps 15 times more massive than our sun. "These are grains from previous generations of stars," says coauthor Gerald Wasserburg, a geochemist and Crafoord laureate at Caltech. Finding more grains, he says, may reveal whether dusty gusts from neighboring red giants or a supernova gave the gaseous predecessor of our solar system the kick-start it needed to collapse.

The oxygen-rich supernova dust grain has a different composition than one reported earlier this year in Nature by cosmochemist Larry Nittler of the Carnegie Institution in Washington, D.C. "That's exciting," says Nittler. "But we can't tell whether the grains came from separate layers of a single supernova or from two different ones."