Science Made Simple: What Is Nucleosynthesis?

An artist’s depiction of a neutron star merger. These mergers produce heavy elements like gold and platinum in abundance.
Credit: Sonoma State University/A. Simonnet

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.

Nucleosynthesis Facts

  • Scientists believe the heaviest naturally occurring elements, including 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.
  • We consist mostly of matter created through nucleosynthesis in stars that have since died, leading to cosmologist Carl Sagan’s famous statement that we are made of “star-stuff.”

DOE Office of Science: Nucleosynthesis Contributions

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