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Carbon-rich Stars
Carbon-rich Stars
12 Mar 2009
(Source: University of Maryland)

University of Maryland
March 12, 2009
Contacts: Lee Tune, 301 405 4679 or ltune@umd.edu

Carbon- and Oxygen-rich Stardust Sheds New Light on Origin of
Elements of Life

COLLEGE PARK , Md. -- Using NASA's Spitzer Telescope, an international
research team has found evidence that some stars in the center of the
Milky Way galaxy have both carbon and oxygen in the dust that surrounds
them, a rare combination in galactic dust.

"Scientists have long expected to find carbon-rich stars in our galaxy
because we know that significant quantities of carbon must be created in
many such stars, but carbon had not previously shown up in the clouds of
gas around these stars," said Matthew Bobrowsky, an astrophysicist in
the University of Maryland's department of physics and one of the
authors of a paper published in the February issue of the Journal Astronomy and Astrophysics

"Based on our findings, this is because medium-sized stars rich in
carbon sometimes keep that carbon hidden until very near the end of
their stellar lives, releasing it only with their final 'exhalations',"
explained Bobrowsky, who noted that previously scientists hadn't looked
for carbon around stars that were so near the end of their active lives.

As a star burns hotter and hotter, the hydrogen gas that originally made
up almost all of its mass is converted, through nuclear fusion, first to
helium, and then to progressively heavier elements.The hottest region in
the core fuses together the heaviest elements. And these can reach the
surface of the star only when its life is almost over.

"The Big Bang produced only hydrogen and helium," Bobrowsky said.
"Heavier elements like carbon and oxygen only come from getting 'cooked
up' in stars. Nuclear reactions in stars created the heavier elements
found in 'life as we know it'."

In the last 50,000 years of their 10 billion-year lives, sun-sized stars
expel carbon atoms along with hydrogen and helium to form a surrounding
cloud of gas that soon disperses into space, perhaps to eventually
become the stuff of new stars, solar systems, or perhaps even life on
some earth-like planet. Much larger stars expel their heavier matter in
massive explosions called supernovae.

"All the heavy elements [which astronomers call 'metals,' and include
all elements heavier than hydrogen and helium] on Earth were created by
nuclear fusion reactions in previous generations of stars," said
Bobrowsky. "Those earlier stars expelled those elements into space and
then our solar system formed out of that gas containing all the heavy
elements that we now find in Earth and in life on Earth."

The team of scientists used the Spitzer Space Telescope to view each
star and its surrounding clouds of dust and particles, called a
planetary nebulae. The researchers measured the light emitted by the
stars and the surrounding dust and were able to identify carbon
compounds based on the wavelengths of light emitted by the stars.
Looking in an area at the center of the Milky Way called the "Galactic
Bulge," the team observed 26 stars and their planetary nebulae and found
21 with carbon "signatures."

But the scientists did not just find carbon around these stars; they
also found oxygen in these 21 dust clouds, revealing a surprising
mixture of ingredients for space dust. They report in their paper that
this is likely due to a thermal pulse where a wave of high-pressure gas
mixes layers of elements like carbon and oxygen and spews them out into
the surrounding cloud.

The finding of carbon and oxygen in the dust clouds surrounding stars
suggests a recent change of chemistry in this population of stars,
according to the authors.

"Stars in the center of the Milky Way are old and 'metal-rich' with a
high abundance of heavy elements," Bobrowsky said. They are different
in chemical composition than those found in the disc, farther out from
the center.

Studying the chemistry of these stars helps scientists learn how the
matter that makes up our earth and other planets in our galaxy left its
stellar birthplaces long ago.

"If we want to understand how our galaxy, and the stars, planets and
life in it, came to be the way they are, we need to understand the
creation of the chemical elements of which they are composed," Bobrowsky
said.

The paper is The mixed chemistry phenomenon in Galactic Bulge PNe.

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