What is the name of the process in which heavier elements are created from lighter elements during nucleosynthesis?

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DOE Office of Science: Nucleosynthesis Contributions
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Nucleosynthesis is the creation of new atomic nuclei, the centers of atoms that are made up of protons and neutrons. Nucleosynthesis first occurred within a few minutes of the Big Bang. At that time, a quark-gluon plasma, a soup of particles known as quarks and gluons, condensed into protons and neutrons. After the universe cooled slightly, the neutrons fused with protons to make nuclei of deuterium, an isotope of hydrogen. Deuterium nuclei then combined to make helium. Further reactions between protons, neutrons, and different isotopes of helium produced lithium. The hydrogen and helium produced during this phase of the universe eventually created the universe’s first massive stars.

Since then, the nuclear reactions in the life and death of stars have formed most of the other nuclei in the universe. Stars can create nuclei through two processes: either by combining two smaller nuclei (called fusion) or breaking a larger nucleus into multiple nuclei (called fission). Both ways result in new atoms.

In the past, these processes also produced the elements on the Periodic Table that we know today. Stars of different types produce nuclei of different elements, leading over time to the range of natural elements. The universe’s first stars were massive, often more than 10 times the size of our Sun. They also had far shorter lives than stars that existed more recently. As they lived, they burned hydrogen and produced the elements up to iron in the Periodic Table. When they died, they ejected nuclei of these elements in a type of explosion called a core-collapse supernova. Supernovae can leave behind neutron stars. When neutron stars merge, they produce new nuclei, including elements that are heavier than iron. Other stars become white dwarfs as they die. These white dwarfs may also later merge and synthesize nuclei of elements.

DOE Office of Science: Nucleosynthesis Contributions

The Office of Nuclear Physics in the DOE Office of Science supports research in nuclear astrophysics—the physics needed to understand the reactions that produce the elements. Two university-based DOE Centers of Excellence, the Cyclotron Institute at Texas A&M University and the Triangle Universities Nuclear Laboratory, specialize in the study of nuclear astrophysics. DOE also funds the theory and modeling of the Big Bang, stars, supernovae, and neutron star mergers, all sources of elements. The DOE Office of Science’s Argonne Tandem Linac Accelerator System (ATLAS) user facility is home to the world's most powerful spectrometer for nuclear structure research. Moving forward, the Office of Nuclear Physics is now supporting the construction of the Facility for Rare Isotope Beams at Michigan State University. This accelerator will produce short-lived and never-before-seen neutron-rich nuclei that play a role in the production of the heaviest elements.

Nucleosynthesis Facts

We consist mostly of elements like oxygen, hydrogen, nitrogen, carbon, calcium, and phosphorus that are created through nucleosynthesis in stars that have since died, leading to cosmologist Carl Sagan's famous statement that we are made of "star-stuff."
Scientists believe the heaviest naturally occurring elements, including gold, platinum and uranium, are produced in violent neutron-rich environments such as the merger of two neutron stars or supernovae. Under these conditions, neutrons gain nuclei faster than they can decay.

Scientific terms can be confusing. DOE Explains offers straightforward explanations of key words and concepts in fundamental science. It also describes how these concepts apply to the work that the Department of Energy’s Office of Science conducts as it helps the United States excel in research across the scientific spectrum.

Stellar nucleosynthesis is the process by which elements are created within stars by combining the protons and neutrons together from the nuclei of lighter elements. All of the atoms in the universe began as hydrogen. Fusion inside stars transforms hydrogen into helium, heat, and radiation. Heavier elements are created in different types of stars as they die or explode.

The idea that stars fuse together the atoms of light elements was first proposed in the 1920s, by Einstein's strong supporter Arthur Eddington. However, the real credit for developing it into a coherent theory is given to Fred Hoyle's work in the aftermath of World War II. Hoyle's theory contained some significant differences from the current theory, most notably that he did not believe in the big bang theory but instead that hydrogen was continually being created within our universe. (This alternative theory was called a steady state theory and fell out of favor when the cosmic microwave background radiation was detected.)

The simplest type of atom in the universe is a hydrogen atom, which contains a single proton in the nucleus (possibly with some neutrons hanging out, as well) with electrons circling that nucleus. These protons are now believed to have formed when the incredibly high energy quark-gluon plasma of the very early universe lost enough energy that quarks began bonding together to form protons (and other hadrons, like neutrons). Hydrogen formed pretty much instantly and even helium (with nuclei containing 2 protons) formed in relatively short order (part of a process referred to as Big Bang nucleosynthesis).

As this hydrogen and helium began to form in the early universe, there were some areas where it was denser than in others. Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space. Once these clouds became large enough, they were drawn together by gravity with enough force to actually cause the atomic nuclei to fuse, in a process called nuclear fusion. The result of this fusion process is that the two one-proton atoms have now formed a single two-proton atom. In other words, two hydrogen atoms have begun one single helium atom. The energy released during this process is what causes the sun (or any other star, for that matter) to burn.

It takes nearly 10 million years to burn through the hydrogen and then things heat up and the helium begins fusing. Stellar nucleosynthesis continues to create heavier and heavier elements until you end up with iron.

The burning of helium to produce heavier elements then continues for about 1 million years. Largely, it is fused into carbon via the triple-alpha process in which three helium-4 nuclei (alpha particles) are transformed. The alpha process then combines helium with carbon to produce heavier elements, but only those with an even number of protons. The combinations go in this order:

Carbon plus helium produces oxygen.
Oxygen plus helium produces neon.
Neon plus helium produces magnesium.
Magnesium plus helium produces silicon.
Silicon plus helium produces sulfur.
Sulfur plus helium produces argon.
Argon plus helium produces calcium.
Calcium plus helium produces titanium.
Titanium plus helium produces chromium.
Chromium plus helium produces iron.

Other fusion pathways create the elements with odd numbers of protons. Iron has such a tightly bound nucleus that there isn't further fusion once that point is reached. Without the heat of fusion, the star collapses and explodes in a shockwave.

Physicist Lawrence Krauss notes that it takes 100,000 years for the carbon to burn into oxygen, 10,000 years for the oxygen to burn into silicon, and one day for the silicon to burn into iron and herald the collapse of the star.

Astronomer Carl Sagan in the TV series "Cosmos" noted, "We are made of star-stuff." Krauss agreed, stating that "every atom in your body was once inside a star that exploded...The atoms in your left hand probably came from a different star than in your right hand, because 200 million stars have exploded to make up the atoms in your body."

As mentioned above, energy can be released by either nuclear fusion or fission reactions and there will be a tendency for material to be gradually converted into elements with maximum binding energy. As observations suggest that hydrogen and helium are much more abundant than other elements, and there is an abundance peak near iron, it is generally supposed that heavy elements have been built up from light elements. In addition, some sites in which element transmutations can occur are known; for example, the interiors of stars tend to get hotter as they evolve, and a succession of nuclear reactions provides the energy that they radiate. Whether or not stars are the site of major nucleosynthesis, some nucleosynthesis certainly occurs there.

Atomic nuclei interact through two strong forces. Because they have positive electric charges, they repel one another, but there is also a very short-range strong nuclear interaction that is attractive. This may cause fusion reactions to occur if the nuclei ever approach close enough for it to be operative. To overcome the electrical repulsion, the particles must be moving rapidly, as they will be if the material is at a high temperature. The overcoming of the electrical repulsion leads to what are known as thermonuclear reactions. Heavy nuclei have higher electric charges than light nuclei, and a higher temperature is required for reactions between them. The rate of thermonuclear reactions depends on density as well as temperature, but the temperature dependence is much more critical.

If one imagines a mixture of light elements gradually heated up, a succession of nuclear reactions occurs that is described below.

Hydrogen is converted into helium by a succession of nuclear reactions that change four protons into a helium nucleus, two positrons, and two neutrinos. (A positron is a particle like an electron but with a positive charge; a neutrino is a particle with no charge and negligible mass.) Two different reaction chains exist. In the proton–proton chain the helium nucleus is built up directly from protons. In another series of reactions that involve carbon and nitrogen, called the carbon–nitrogen cycle, the nuclei of carbon and nitrogen are used as catalysts to transform hydrogen into helium; protons are successively added to carbon or nitrogen until a helium nucleus can be emitted by them and the original carbon or nitrogen nucleus reproduced. Both of these reactions occur at temperatures of about 10,000,000 to 20,000,000 K (10,000,000 K is approximately 18,000,000° F).

At temperatures of about 100,000,000 to 200,000,000 K (1 to 2 × 108 K), three helium nuclei can fuse to form carbon. This reaction takes place in the following way: two helium nuclei combine to form an unstable isotope of beryllium, which has an extremely short life; rarely, a third helium nucleus can be added to form carbon before the beryllium decays. Subsequently, a fourth helium nucleus may combine with carbon to give oxygen. The relative amounts of carbon and oxygen produced depend on the temperature and density at which helium is burned.

At temperatures between 5 × 108 K and 109 K, pairs of carbon and oxygen nuclei can fuse to produce such elements as magnesium, sodium, silicon, and sulfur.

Further heating of the material leads to a complicated set of nuclear reactions whereby the elements produced in carbon and oxygen burning are gradually converted into the elements of maximum fractional binding energy; e.g., chromium, manganese, iron, cobalt, and nickel. These reactions have collectively been given the name silicon burning because an important part of the process is the breaking down of silicon nuclei into helium nuclei, which are added in turn to other silicon nuclei to produce the elements noted above.

Finally, at temperatures around 4 × 109 K, an approximation to nuclear statistical equilibrium may be reached. At this stage, although nuclear reactions continue to occur, each nuclear reaction and its inverse occur equally rapidly, and there is no further overall change of chemical composition. Thus, the gradual production of heavy elements by nuclear fusion reactions is balanced by disintegrations, and the buildup process effectively ceases once the material is predominantly in the form of iron and its neighbouring elements of the periodic table. Indeed, if further heating occurs, a conversion of heavy nuclei to light nuclei follows in much the same way as occurs in the ionization of atoms when they are heated up. The elements heavier than iron cannot be produced by fusion reactions between light elements; an input of energy is required to produce them.