Researchers reach to the skies
to reveal the secrets of the
stars
Continued ...
Determining the fingerprints of stars
The instrument is housed in an Argonne laboratory thickly forested with electronic hardware, wires and cables. Green and blue laser beams glowed brightly in the laboratory earlier this year during an experiment involving silicon and barium. Lewis begins the sample preparation by attaching many interstellar grains to a soft piece of gold. Then he maps the tiny grains with a scanning electron microscope so that his colleagues can find them again later.
 University of Chicago researchers isolated individual particles (shown in the test tube) of the Murchison meteorite (at right) that are unchanged since their condensation from material ejected by a star. The isotopic and elemental composition of such grains provides detailed knowledge of stellar physics and cosmochemistry. |
In Pellin's laboratory, another microscope focuses a laser on the sample, vaporizing a portion of it into a cloud of carbon, silicon, molybdenum and many other atoms. Pellin can tune a second laser to a wavelength that will electrically charge one specific type of atom in an experimental sample containing a trillion atoms. After Pellin fires the lasers, an electric field shoots those atoms through a mass spectrometer, which measures their mass and abundance.
"You can't just go to a manufacturer and say, 'I'd like you to build me one of these," Davis said. "There are no commercially available instruments of this sort. But at Argonne, they're really good at building instruments."
In the past several years, this instrument has fueled a series of pathbreaking papers. These papers announced that the team had, for the first time, precisely measured the isotopic compositions of heavy elements such as strontium, zirconium and barium found in interstellar dust grains.
The only such grains available for study come from meteorites. Most interstellar dust grains melted and became mixed up with other material during the solar system's formation. The grains the Argonne-Chicago team studies are among the select few that survived. "These grains are the last pieces of our solar system that didn't get mixed up," Pellin said.
Lewis pioneered the technique of separating the grains from meteorites more than a decade ago with Edward Anders, the university's Horace Horton Professor Emeritus in Chemistry. The process involves dissolving the meteorites in acid, leaving a residue of grains behind.
They know these grains come from interstellar space because of their strange isotopic composition. "It doesn't look anything like the solar system, therefore it has to have come from some other place, some other star," Lewis said.
 This micrograph shows a pair of grains from the Murchison meteorite. The grains are remnants from a star's explosion. |
The grains the team studies typically measure two or three microns in diameter. This is small enough for 15 or 20 of them to fit comfortably across the width of a human hair.
Since 1987, Chicago researchers have identified three types of interstellar grains. The most interesting ones are made of silicon carbide and graphite, Davis said, though the most plentiful ones are diamond. All three types are preserved in the Murchison meteorite, which landed in Australia in 1969.
Most interstellar grains come from red giants. Stars once like the sun, red giants grow huge and shed much of their mass when they reach the end of their lifetimes. Scientists also have traced the grains to supernovae - exploding stars.
Slow neutron-capture — called s-process — nucleosynthesis takes place deep inside red giants in episodes lasting 10,000 years or more. The more violent rapid neutron-capture — r-process — nucleosynthesis occurs in supernovae, possibly within a matter of seconds. This process accounts for approximately half of the 117 elements.
But the Argonne-Chicago team has found a pattern that matches neither the s- nor the r-process in their isotopic measurements of molybdenum, a silver-grey metal that helps make stronger steel.
"It may be that there's some kind of neutron burst of some sort, intermediate between r and s process, that might explain the pattern we're seeing," Davis said.
"What we saw was puzzling," Pellin noted. "There's no question it's different and surprising."
The Genesis connection
No one yet knows what different and surprising results will flow from NASA's Genesis mission. Launched in August 2001, the spacecraft will collect samples of the solar wind - single atoms and electrically charged particles from the sun - and return them to Earth in 2003. If successful, Genesis will become the first mission to return a sample of extraterrestrial material from beyond the moon. These samples will allow a precise measure of the elemental and isotopic composition of our most important star - the sun.
Pellin and his research group at Argonne will be among the scientists to analyze the samples in an effort to better understand how the planets formed and how the sun works. No element heavier than calcium, which is far lighter than iron, has ever been measured in the sun. Scientists believe that the heavier elements are there, but in the tiniest of amounts.
"At some level, half the periodic table is betting on our instrument," Pellin said.
But until the Genesis samples arrive at Argonne, the researchers will continue to coax more secrets of the universe from their meteorites, one grain at a time.
"We already have a sample-return mission to the stars represented by these grains from meteorites," Davis said. "Nature's doing it on the cheap for us. It can't give us everything we want. We don't know where the grains came from, but we've sampled a huge number of different stars."
For more information, please contact Steve Koppes (773/702-8366 or s-koppes@uchicago.edu) at the University of Chicago.
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