Fwd: Article: Circumstellar space - Where chemistry happens for the very first
- --- In email@example.com, "Robert Karl Stonjek"
Circumstellar space: Where chemistry happens for the very first time
The nebula RCW49 is a nursery for newborn stars and exists in
circumstellar space, where chemistry is done for the very first time.
Credit: NASA/JPL-Caltech/E.Churchwell U. of Wisconsin
Picture a cool place, teeming with a multitude of hot bodies twirling
about in rapidly changing formations of singles and couples, partners
and groups, constantly dissolving and reforming. If you were thinking
of the dance floor in a modern nightclub, think again.
It's a description of the shells around dying stars, the place where
newly formed elements make compounds and life takes off, said
Katharina Lodders, Ph.D., research associate professor of earth and
planetary sciences in Arts & Sciences at Washington University in St.
Chemistry for the very first time
"The circumstellar environment is where chemistry happens for the very
first time," said Lodders. "It's the first place a newly synthesized
element can do chemistry. It's a supermarket of things from dust to
gas and dust grains to molecules and atoms. The circumstellar shells
enable a chemistry that produced grains older than our sun itself.
It's generated some popular interest, and this year marks the 20th
anniversary of the presolar grain discoveries."
After the discovery of presolar diamonds in a meteorite in 1987 - the
first stardust found in a meteorite - researchers at Washington
University in St. Louis have been prominent in finding and analyzing
pre-solar grains made of silicon carbide, diamonds, corundum, spinel,
and silicates. The latest discovery - a silicate grain that formed
around a foreign star and became incorporated into a comet in our
solar system - was captured and returned by the STARDUST space mission
Lodders said that nucleosynthesis - the creation of atoms - takes
place in a star's interior, made of a plasma far too hot for any
molecular chemistry to take place. The event that enables chemistry is
the death of a star, when elements are spewed out of the core,
creating a shell around the star. As this circumstellar shell cools,
the elements react to form gas molecules and solid compounds.
A star comes of age
Our sun and other dwarf stars of less than about ten solar masses burn
hydrogen into helium in their cores. As they come of age, they become
Red Giant stars and burn the helium to carbon and oxygen. But many
heavy elements such as strontium and barium, even heavier than iron,
are also produced, albeit in much smaller quantities than carbon. At
the same time, the star begins to eject its outer layers into the
interstellar medium by stellar winds, building up a circumstellar
shell. So eventually, most of a star's mass, including the newly
produced elements, is ejected into the interstellar medium through the
circumstellar shell. Most interstellar grains come from such stars.
Heavyweight stars go out more spectacularly, in violent supernovae
such as SN2006gy, first observed late last year, which has turned out
to be the most massive supernova ever witnessed. But no matter what,
all stars like the sun and heavier ones like SN2006gy empty their
elements into their circumstellar environments, where gaseous
compounds and grains can form. From there, the gas and grains enter
the interstellar medium and provide the material for new stars and
solar systems to be born.
Lodders presented a paper on circumstellar chemistry and presolar
grains at the 233rd American Chemical Society National Meeting, held
March 25-29 in Chicago, where a special symposium was held to track
the evolution of the elements across space and time. A book of
proceedings is being prepared for publication.
Lodders said that just one percent of all known presolar grains come
from supernovas. She said that several million stars have been
catalogued and several thousand individual presolar grains have now
been analyzed. "Back in the 1960s, astronomers didn't know that
presolar grains existed in meteorites," Lodders said. "They were
discovered when researchers were looking at meteorite samples and
studying noble gases. They asked what is the mineral carrier of the
By separating minerals from samples of meteorites, they eventually
found the carriers of the noble gases - presolar diamonds, graphite
and silicon carbide -, and thus started the study of presolar grains
20 years ago. "So the genuine, micron-size star dust survived despite
the potential chemical and physical processing in the interstellar
medium, during solar system formation, and in the meteorite's parent
asteroid," she said. "Since the star dust preserved in meteorites must
have been already present before the solar system and the meteorites
formed, researchers call this star dust presolar grains".
"Laboratory astronomy of stardust has revealed much about stellar
element and isotope production, and about gas and dust formation
conditions in giant stars and supernovae."
Source: Washington University in St. Louis
Robert Karl Stonjek
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