Nucleosynthesis means “the process of combining nucleons” which results in the formation of new nuclei. Nuclei change by either capturing or releasing particles, which increase, decrease, or alter the bound nucleons. Nucleons refer to protons and neutrons, the building blocks of each nucleus. Different types of nuclei are called nuclides. The number of protons in a nucleus determine it's atomic number and element symbol. This determines the charge of the nucleus, and thus most of the chemical properties. Nuclei with the same number of protons and different numbers of neutrons are called isotopes. Since the discovery of nuclear processes early in the twentieth century, scientists in multiple fields — physics, chemistry, geology, and astronomy — have collaborated to form a model for the creation of all the known elements and isotopes. The story of the origin of the elements depends on the reactions undergone in earlier stages of the universe. Only the lightest elements hydrogen, helium, and lithium formed in the early universe after the big bang. Models of burning inside stars and supernova explain the enrichment of nearly all the rest of the elements.
Overview of nucleosynthesis processes
The variety of synthesis processes that change nuclei split into two general types: capture and decay. Nuclear fusion refers to the addition of lighter nuclei into a heavier product, while nuclear fission refers to the creation of lighter nuclei from the splitting of a heavy nucleus. Similarly, capture events refer to a nucleus undergoing a reaction with a bombarding particle, while decay events refer to a nucleus emitting a particle spontaneously.
In a capture process, an energetic particle smashes into a nucleus and sparks a reaction. Common nuclear capture reactions add a neutron, proton, beta particle (electron), alpha particle (Helium nucleus) or a heavier nucleus to an existing one. Except for the electron, the captures are driven by the attraction of nucleons to one another due to the strong nuclear force and increase the weight of the nucleus. Electron capture receives impetus from the weak nuclear force. As the name suggests, this process is less likely to occur during a collision of sufficient energy and often determines the overall rate of a multi-step reaction.
If a nucleus reaches an unstable configuration, a spontaneous decay process will return the nucleus to a more stable state. The most common decays are alpha and beta. Losing an alpha particle lowers the weight and charge; emitting a beta particle leaves the weight nearly constant while interchanging a neutron and proton. When a neutron changes into a proton, the nucleus emits an electron and anti-neutrino; when a proton changes into a neutron, the nucleus emits an anti-electron (positron) and neutrino. The anti-particles quickly annihilate with a standard particle converting their energy into gamma particles (photons).
A third type of nuclear reaction occurs when a colliding particle gives so much energy to the bombarded nucleus, that it splits into multiple pieces. This type of collision is called spallation.
Nucleosynthesis in our Universe
From the first existence of elements in the big bang, through the evolution of stars and the final blast of a stellar explosion dispersing the elements across the galaxy, nuclei transmute from one nuclide to another. In 1957, Burbidge, Burbidge, Fowler, and Hoyle wrote the seminal paper on element formation. The focus of the paper on stars as the primary synthesis factories of elements proved to win out over other theories that all the nuclei formed early after the big bang. Further research has mostly refined the work presented then. Cameron presents an updated summary in his 1982 essay.
The nuclear products we know today all derive from the big bang. As the primordial fireball cooled, the only stable nuclei in such a hot, dense environment were the lightest three elements. Big bang nucleosynthesis uses two parameters to determine the resulting nuclei several minutes into the life of our universe. Primarily, the elements hydrogen and helium — as protons and alpha particles — formed in a ratio due to the temperature at which nucleons can form from the elementary particles (quarks). At the estimated temperature of 1 billion degrees, neutrons and protons form in a ratio of 1:6. Combining nearly all the neutrons into alpha particles gives a ratio of helium to hydrogen in our universe near 1:10, or 1:3 by mass. This observed ratio, along with the cosmic microwave background radiation provide the strongest evidence supporting the big bang theory. To this day, 99% of our universe is hydrogen and helium. Deuterium and lithium, the first “heavy element”, also formed in small quantities during the first moments after the big bang. These elements have a strong dependence on the density of the early universe, and since stars consume rather than produce these nuclides, nearly all is believed to have formed in the big bang.
Before stars formed, only three elements existed. On earth, ninety elements have been found to exist naturally. Soon after Rutherford discovered the ability to transmute the nucleus in the laboratory, physicist started proposing models in which stars released energy through nuclear fusion. By 1957, processes were proposed for manufacturing all of the elements in stars, starting with just hydrogen. An exceedingly short summary of the journal article is that the elements from helium to iron are formed by nuclear fusion and that the heavier elements are built up by neutron capture. The only caveat at the time was little explanation of an unknown process to create the unstable elements of lithium, beryllium, and boron.
For the majority of a star's life, hydrogen burns into helium in the hot core at temperatures of several million degrees. The proposed process for small to mid-size stars — like our own — is the proton-proton chain. In larger, hotter stars, another process becomes more efficient, the carbon-nitrogen-oxygen cycle. In either case, four hydrogen nuclei (protons) combine to form a helium nuclei (alpha particle). When stars age and no longer have enough hydrogen in their cores to continue hydrogen burning, the core starts to collapse.
This collapse heats the core until helium can start burning. The likely process for this is the triple alpha process which combines three helium nuclei to form a carbon nucleus. The cores of the largest, hottest stars may continue fusing heavier nuclei and burn carbon into silicon, and silicon into iron. Side reactions with remaining hydrogen and helium fill out the elements lighter than iron, all of which form exothermically (releasing energy). This still leaves more than half of the elements on earth unexplained.
These heaviest elements build up by capturing neutrons followed by beta decays. Inside of stars, nuclei have a long time to decay between each neutron capture. This slow process forms nuclides along a line of stability balancing the number of neutrons and protons in the nucleus. Supernovae create a neutron storm. During a supernova, nuclei undergo neutron capture too quickly to allow beta decay between every capture. This rapid process creates elements with neutron excess, which undergo beta decay(s) back to the valley of stability after ejection out of the supernova.
The final category of reactions — spallation — explains the existence of one isotope of lithium, along with beryllium and boron, which are all unstable in stars. A likely source of energetic particles are the cosmic rays. Burbidge et al proposed spallation as the source, but later discoveries on satellite observatories pinpointed this unknown process as a result of cosmic ray collisions.
Further Areas of Interest
While most of the observable universe remains hydrogen and helium, our surroundings here on earth display a tremendous variety of compounds. To this, we owe thanks to a previous generation of stars that cooked up heavy elements. For those interested in how these elements fractionated into the composition of earth we know, check out the books by Cowley and Cox. In addition, they each present details of the synthesis reactions above at a level accessible to those with background enough in science to finish this article. I hope you have gained insight into the history of each nucleus in your body and now understand the phrase “we are starstuff”.
Bibliography
BURBIDGE, E. M., BURBIDGE, G. R., FOWLER, W. A. and HOYLE, F. (1957) Synthesis of the Elements in Stars. Reviews of Modern Physics 29 547
CAMERON, A. G. W. (1982) Elemental and nuclidic abundances in the solar system. In: BARNES, C. A. et al. (eds) Essays in Nuclear Astrophysics. Cambridge: University Press
COWLEY, C. R. (1995) An Introduction to Cosmochemistry. Cambridge: University Press
COX, P. A. (1989) The Elements. Oxford: University Press
SILK, J. (2000) The Big Bang. New York: W. H. Freeman and Company