Old Star, New
Trick
Pasadena,
CA—The Big Bang produced lots of hydrogen and
helium and a smidgen of lithium. All heavier elements found on the periodic
table have been produced by stars over the last 13.7 billion years. Astronomers
analyze starlight to determine the chemical makeup of stars, the origin of the
elements, the ages of stars, and the evolution of galaxies and the universe.
Now for the first time, astronomers have detected the presence of arsenic and
selenium, neighboring elements near the middle of the periodic table, in an
ancient star in the faint stellar halo that surrounds the Milky Way. Arsenic and
selenium are elements at the transition from light to heavy element production,
and have not been found in old stars until now.
Lead author of the
Astrophysical Journal paper, Fellow Ian Roederer of the Carnegie
Observatories explained: “Stars like our Sun can make elements up to oxygen on
the periodic table. Other more massive stars can synthesize heavier elements,
those with more protons in their nuclei, up to iron by nuclear fusion—the
process in which atomic nuclei fuse and release lots of energy. Most of the
elements heavier than iron are made by a process called neutron-capture
nucleosynthesis.
“Although neutrons have no charge, they can decay into
protons after they're in the nucleus, producing elements with larger atomic
numbers. One of the ways that this method can work is by exposure to a burst of
neutrons during the violent supernova death of a star. We call this process the
rapid process (r-process). It can produce elements at the middle and bottom of
the periodic table—from zinc to uranium—in the blink of an
eye.”
Roederer, with co-author James
Lawler, looked at an ultraviolet spectrum from the Hubble Space Telescope public
archives to find arsenic and selenium in a 12 billion year-old halo star dubbed
HD 160617. These elements were forged in an even older star, which has long
since disappeared, and then—like genes passed on from parent to infant—they were
born into the star we see today, HD 160617.”
The team also examined
data for this star from the public archives of several ground-based telescopes
and were able to detect 45 elements. In addition to arsenic and selenium, they
found rarely seen cadmium, tellurium, and platinum, all of which were produced
by the r-process. This is the first time these elements have been detected
together outside the Solar System. Astronomers cannot replicate the r-process in
any laboratory since the conditions are so extreme. The key to modeling the
r-process relies on astronomical observations.
“What I find exciting is
that arsenic and selenium can be found in other stars, even ones like HD 160617
that we've been studying for decades,” remarked Roederer. “Now that we know
where to look, we can go back and study these elements in other
stars. Understanding the r-process helps us know why we find certain elements
like barium on Earth, or understand why uranium is so
rare.”
__________________
The paper is published in the May 1, 2012 issue. Ian Roederer
is supported by the Carnegie Institution through the Carnegie Observatories
Fellowship. James Lawler is supported by NASA grant NNX10AN93G.
The Carnegie Institution for Science (CarnegieScience. Edu)
is a private, nonprofit organization headquartered in Washington, D.C., with six
research departments throughout the U.S. Since its founding in 1902, the
Carnegie Institution has been a pioneering force in basic scientific research.
Carnegie scientists are leaders in plant biology, developmental biology,
astronomy, materials science, global ecology, and Earth and planetary
science.
