By Lori Price
Dr. Carl Brune, a nuclear astrophysicist at Ohio University, talks about “Cosmic Cooking–the Origin of the Elements” on A&S TV’s presentation of Science Café.
He begins by quizzing the audience to identify the basic definition of an element, choosing definitions as a basic unit of matter and as a fundamental building block of matter. Brune explains that some of his work was focused on identifying how and where in the universe atoms were formed and how they became as we know them today.
The universe was very hot and dense 13 billion years ago, when the Big Bang occurred and expansion and cooling began. Brune shows images from the Hubble Telescope of typical gas clouds as they began to condense and form stars. The most common elements present at this phase are hydrogen and helium.
The most abundant element on earth, however, is iron, along with significant amounts of silicon and oxygen at the crust level. Much of Brune’s work has focused on determining how these and other elements were created. In a stable, non-ionized form, each element has a nucleus and a cloud of electrons. The nucleus is composed of protons and neutrons. It is these nuclei in particular that he feels hold many clues to the formation of the elements.
One important consideration comes from the area of nuclear physics. “Nuclei with a higher probability of capturing extra neutrons are less abundant, while nuclei with a lesser probability of capturing extra neutrons are more abundant,” he says, adding that hydrogen and helium, both of which are very stable at the nuclear level, are examples of this concept since both are also very abundant in the universe.
Stars: Nuclear Fusion Reactor Sites at their Cores
After the Big Bang, Brune says, the resulting expansion and cooling began the process of star formation, many of which then provided nuclear fusion reactor sites within their cores, a process known as stellar nucleosynthesis. This process could have been the crucible in which additional elements were created. As the stars completed their life cycle and some reached the novae and supernovae phase, these newly created elements were then cast out into the universe.
Brune reiterates that the study of the creation of the elements requires coordinated research on many fronts and involves input across many disciplines, such as astronomy, astrophysics, geology and nuclear physics. By integrating information from these many different angles, a more complete picture of the formation of the elements begins to come into view.
Another clue to the origin of the elements comes from comparing the current levels of abundance of the elements and consideration of where they are occurring. “Look at the stars, many of them have abundances of elements similar to those found on earth. It appears that there are some common processes throughout the universe,” he says.
He goes on to discuss alchemy. In medieval times, because they were both very dense, alchemists struggled to discover a way to change lead into gold. As a more modern understanding of chemistry and understanding of the elements came about, scientists understood that elements were already in their simplest form and could not be broken down further or readily changed into a different element. However, Brune says that modern nuclear physicists may revisit the concept of alchemy as they delve more deeply into how one element might be converted into another.
“Hydrogen and helium came from the Big Bang, but the stars cooked those elements via fusion of the abundant elements and iron, oxygen and other novel elements resulted,” he says.
Dr. Carl Brune
Testing Theories with Experimental Nuclear Physics
However, for this to be more than just theory, the discipline of experimental nuclear physics is needed to identify the processes and measure the rates of nuclear change. These data can then be fed into a model to calculate more specifically what a star might do to create elements. This is the one type of work that is currently being done at the John E. Edwards Accelerator Laboratory at Ohio University. “We can accelerate nuclei to 10 percent of the speed of light, similar to what you would find in the core of a star. It is very hot, with nuclei moving around very fast. And we measure what happens.” Brune uses a tabletop apparatus with the Science Café audience to illustrate how changing the speed of a collision could actually displace nuclear particles.
This laboratory at Ohio University is also being used for research by the University of Oslo, the University of Michigan, Lawrence Livermore National Laboratory, and the State University of New York (SUNY) at Geneseo. Scientists using the lab are studying questions related to the application of nuclear physics to the study of nuclear structure, neutron imaging and potential use for airport scanning, medical applications that include the use of isotopes for positron emission tomography (PET) scans and radioactive tracers, and to improve national security through the increased ability to detect uranium.
Brune explains that due to the positive charge of the nucleus, and the fact that two positively charged nuclei would repel each other, we do not see the demonstrated particle displacement outside of the lab. “Lower temperatures, lesser particle speed and nuclear charge all prevent nuclear fusion from happening in daily life.” He points out that this is why cold fusion with deuterium was not successful – the positive charges just wouldn’t come together, especially at room temperature. “This can be done in a laboratory with plasma physics and intense lasers, but would need extreme heat and it won’t happen easily.” With most of the reactions created in the lab, the heavier nuclei capture a single neutron or proton. “So we collide nuclei together in the lab and see what elements they made. Then we extrapolate to determine what happens within a star.”
Brune shows a table of “Nucleosynthesis in the r-process” that gave a representation of how elements were organized relative to their number of protons and neutrons. “The rapid neutron capture process, or r-process, is how we think many of the elements heavier than iron were formed. The neutrons were captured to build up heavier elements such as uranium, but no one knows where this happened.” He points out the current theories are that it occurred in coalescing neutron stars or that these elements were created as part of the supernovae process.
Experimental physics and laboratories can provide some answers and many additional questions in response to a study of how the elements originated. According to Brune, it is in this area that the division between chemistry and physics becomes somewhat blurry since both disciplines’ study of isotopes could potentially provide additional critical information.
Brune says a new laboratory is being constructed in the United States, a Facility for Rare Isotope Beams (FRIB). This national laboratory for the study of nuclear physics will be located at Michigan State and will cost approximately half a billion dollars to build. The money comes from the University of Michigan and the U.S. Department of Energy. It will be perhaps another decade before this large scale and complicated facility will be fully functional and able to shed light on the key processes of element formation.
Although much remains unknown and there are many open questions, Brune believes that the study of early elemental abundance, nuclear fusion within the stars, the particle structure of different elements, comparison of existing levels of the elements throughout the universe and dedicated study in laboratory simulations may all come together to provide an accurate answer for the question of how the elements were formed.
Comments