FUSION AND NUCLEOSYNTHESIS
Fusion is a type of nuclear reaction in which two atomic nuclei combine to form a heavier nucleus. During this process large amounts of energy (such as visible light) are produced. Einsteins famous E = mc2
equation can be used to calculate the amount of energy released when a given mass is converted. The m in the equation represents the nuclear mass defect. This mass defect is the difference between the mass of the stable nucleus that was produced during the nuclear change, and the sum of the masses of its constituent protons and neutrons. This process releases energy during the formation of the lighter elements (helium up through iron and nickel) and is therefore exothermic.
Fusion is the nuclear process which occurs in hydrogen bombs. It is hoped that nuclear fusion will some day be used to generate cheap electrical energy for sustaining the massive needs of our world. As yet scientists have not been successful at using controlled fusion to produce electrical power for use by the public.
In all of the stars throughout the universe fusion is the process which creates the tremendous amounts of energy that is released. Astrophysicists refer to fusion when it occurs in stars as nucleosynthesis because, along with releasing energy, it creates (synthesizes) new elements. As difficult as it may be to believe, the majority of the elements around us were created at some time in the distant past by this process. Astrophysicists do believe that the majority of hydrogen and helium in the universe was created during the Big Bang.
On the Sun and other medium and smaller size stars the fuel for this nucleosynthesis is hydrogen. The sequence of reactions for these stars is called the proton-proton (PP) cycle. Each second on our Sun 675,000,000 tons of hydrogen fuses to form 653,000,000 tons of helium. Subtracting these two masses shows that 22,000,000 tons of the Suns mass is converted into energy each second. The sequence of atomic collisions in the proton-proton cycle is:
The following nuclear notation is used to represent each nuclide.
||X represents the symbol for the element
||Z represents the atomic number (number of protons or charge)
||A represents the mass number (the total number of protons and neutrons)
During a nuclear reaction both the charge and the total number of nucleons (protons and neutrons) are conserved. This means that the total charge must be the same on each side of the equation as well as the total number of nucleons.
Although the output of energy from the Sun is enormous (1034 joules per year), the Sun has such a large supply of hydrogen (4 X 1030 Kg), that it could survive for another five billion years at its current rate of use.
On larger more massive stars where internal temperatures exceed 15 million degrees, fusion occurs through a multiple step nucleosynthesis process referred to as the carbon-nitrogen-oxygen (CNO) cycle. The first and last step of this process are shown below.
The PP cycle and the CNO cycle compete with each other for the purpose of producing energy on stars. The two factors that determine which process will occur are the availability of the fuels, and the temperature. The CNO cycle cannot occur if those isotopes are not available on the star. Since only a very small amount of these isotopes is required, this condition is often fulfilled and temperature becomes the determining factor. Stars with masses five to ten times the mass of our Sun possess temperatures high enough for the CNO process to dominate. Most of the heavy elements from oxygen up through iron and nickel are thought to have come from fusion on stars of this type.
PRODUCING ELEMENTS LARGER THAN IRON
Both the PP cycle and the CNO cycle occur without the influx of any additional energy. The already high temperatures and pressures found on these stars is sufficient for the reactions to occur. To produce all of the heavier elements (the elements above iron and nickel on the periodic table), energy must be put in for the fusion to occur. For that reason these reactions are referred to as endothermic reactions. Temperatures are sufficient for this to occur in the explosion of a large star, a supernova, and most of the heavier elements are produced there. But, this is a relatively rare occurrence, there is a comparatively low percentage of the heavy elements found in the universe.
THE ROLE OF THE ACE MISSION
Scientists have long had questions about the origin and the evolution of the elements. As our understanding of the process of nucleosynthesis improves so does our ability to answer some of those questions. The ACE (Advanced Composition Explorer) spacecraft was launched in August of 1997 to make observations that are being used to test current theories on the creation and evolution of the galaxy. The purpose of the ACE spacecraft is to sample the matter that comes near the Earth from the Sun, the space between the planets, and the Milky Way galaxy beyond the solar system. ACE specializes in detecting these cosmic ray particles (atomic nuclei and electrons that possess exceedingly high energies). As scientists study data from ACE it is hoped that current theories of nucleosynthesis can be substantiated or revised.