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Life-Building Carbon Rings Found in Space
By SPACE.com Staff
posted: 03:19 pm ET
25 January 2001
NASA Craft Finds Possible Tar in Stars
Star Physics Prove the Delicacy of Life
Astronomers Discover Origin of Light Elements
Thank the Stars for Carbon-Based Life
The structure of benzene -- six carbon atoms chained to form a ring, with six atoms of hydrogen, one per carbon
In this image of the stellar cocoon CRL 618, singly ionised sulphur is shown in red, green represents neutral hydrogen, the blue-green color comes from neutral oxygen and blue light is continuum light seen through a so-called Strömgren y filter. The full extent of the nebula is 12 arcseconds from tip to tip. The original Hubble observations were obtained in 1998 by Susan R. Trammell from University of North Carolina.
There's no shortage of hydrogen in space. And there's always a sprinkling of all the other star stuff. But now, a team of Spanish astronomers has made the first detection of interstellar rings of carbon, the type of molecules upon which Earth's life is based.
The team used the European Space Agency's Infrared Space Observatory (ISO) to find benzene, the ring molecule par excellence. They think benzene is produced by stars at a specific stage of evolution, and that it is an essential chemical step towards the synthesis of more complex organic molecules whose true nature is still unclear. In industry, benzene is obtained from petroleum and has many uses.
Benzene is made of six atoms of carbon chained together to form a ring, plus six atoms of hydrogen, one per carbon. This structure was discovered in 1865 by the German chemist August Kekule -- who would later say a dream inspired him.
Chemists know today that benzene-type molecules make a whole family of compounds, called aromatic hydrocarbons because of their smell (they are basic constituents, for instance, of perfumes and candles).
Astronomers expected to find these ringed molecules in space, where long strings of carbon atoms have been detected. Moreover, it had been postulated that certain compounds of yet unknown nature, that are known to be very abundant in space, are actually aromatic hydrocarbons.
These compounds have left their chemical fingerprints, called Unidentified Infrared Bands (UIBs), in many places in the universe.
The astronomers started by looking for ringed carbon molecules around carbon-rich old stars. When stars of intermediate mass -- up to about three solar masses -- become old and reach the "red giant" phase, they begin to release huge amounts of gas and dust into their environment; because carbon is produced by the nuclear reactions in the core of the star, many carbonaceous compounds are present in the dust expelled by the red giant star.
The team, led by José Cernicharo (Instituto de Estructura de la Materia, CSIC), chose a typical red giant star to start the search. But it did not work: the star did have carbon-based molecules, such as acetylene, but not ringed molecules. So the astronomers turned to an even older star, a protoplanetary nebula -- a star that is about to die via becoming a white dwarf star surrounded by a beautiful cloud of glowing dust and gas. They focused on the protoplanetary nebula CRL618 (See image at right taken by the ESA/NASA Hubble Space Telescope).
"We knew that in a protoplanetary nebula the dust ejected in the previous red giant stage is bathed in powerful ultraviolet radiation coming from the central star, which also emits high velocity winds. The radiation and the winds break up the carbonaceous compounds in the dust and trigger new reactions. We thought benzene could be formed in this way, a process that we could call polymerization of acetylene in evolved stars," said Cernicharo.
This time, the team was right. As published in the January 10 issue of The Astrophysical Journal, they found benzene in the surroundings of the CRL618 protoplanetary nebula. The authors think that there could be a few molecules of benzene per cubic centimeter, a value considered to be high, although the estimated density of molecules of all kinds in the area observed is 10 million per cubic centimeter.
A 'missing link'
The team thinks that this molecule is the "missing link" between simple carbon molecules observed in red giants, made of no more than eight carbon atoms, and complex molecules responsible for UIBs, known to be made of hundreds of carbon atoms.
This theory therefore implies that UIBs are indeed due to aromatic hydrocarbons, a possibility that, according to the authors, becomes stronger after the first confirmed detection of an aromatic molecule in space.
The missing-link idea is based on observations of objects at each stage of evolution. UIBs have been detected around stars that are already "dead" -- the planetary nebula -- but not in the previous evolutionary stage of protoplanetary nebula, such as CRL618, where benzene has been found.
The transformation from protoplanetary to planetary nebula lasts for no longer than a thousand years, a quick process in astronomical terms. As an example, the central star of CRL618 was still a red giant only a few centuries ago, and will become a fully formed planetary nebula in a few thousand years.
"The molecules causing the UIBs must form in the relatively short period from protoplanetary nebula to planetary nebula," Cernicharo said. "It seems that carbon-rich protoplanetary nebulae are the best organic chemistry factories in space."
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FUTURE SPACE
Coming Monday - SPACE.com's coverage of NEAR's attempted landing on Asteroid Eros
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