Nucleosynthesis

Nucleosynthesis is the creation of new atomic nuclei from existing protons, neutrons, and other nuclei. The very first nuclei formed just minutes after the Big Bang, during a period known as Big Bang nucleosynthesis. Within about twenty minutes, the universe had expanded and cooled enough to halt the high-energy collisions between nucleons, allowing only the fastest, simplest reactions to finish. This left the universe composed mostly of hydrogen and helium, with small amounts of lithium and the hydrogen isotope deuterium. Later, nucleosynthesis inside stars and during stellar events like novas and supernovae produced the full range of elements and isotopes we see today, a process called cosmic chemical evolution. Even now, the total mass of elements heavier than hydrogen and helium—what astrophysicists call "metals"—remains only a few percent, so the universe’s overall composition has changed little.

Inside stars, nuclear fusion reactions convert light elements into heavier ones, releasing energy in a process known as stellar nucleosynthesis. These reactions create many lighter elements up to iron and nickel in the most massive stars. The products of stellar nucleosynthesis stay trapped in stellar cores or remnants unless they are ejected by stellar winds or explosions. Heavier elements, from iron upward, are produced through neutron capture reactions in two main processes: the r-process (rapid neutron capture) and the s-process (slow neutron capture).

Supernova nucleosynthesis, which occurs in exploding stars, is largely responsible for elements between oxygen and rubidium. This happens through three channels: the ejection of elements already produced during stellar nucleosynthesis, explosive nucleosynthesis during the supernova itself, and the r-process, which involves the absorption of many neutrons during the explosion.

Neutron star mergers have recently been identified as a major source of r-process elements. When two neutron stars collide, a large amount of neutron-rich matter is ejected, quickly forming heavy elements.

Cosmic ray spallation is a process where cosmic rays strike atomic nuclei and break them apart. It is a significant source of lighter nuclei that stellar nucleosynthesis does not produce, particularly helium-3, beryllium-9, and the boron isotopes boron-10 and boron-11. This spallation can happen in the interstellar medium, on asteroids and meteoroids, or on Earth in the atmosphere and ground, contributing to the presence of cosmogenic nuclides on our planet.

On Earth, new nuclei are also created through radiogenesis—the radioactive decay of long-lived, primordial radionuclides like uranium, thorium, and potassium-40.

The first protons and neutrons formed from the quark-gluon plasma about 13.8 billion years ago, as the universe cooled below roughly ten billion Kelvin (or one MeV). Over the next few minutes, these particles combined into hydrogen (deuterium) and helium nuclei, with trace amounts of lithium also appearing. Because radiation vastly outnumbered matter at that time, the high binding energies of heavier elements prevented their formation. About 380,000 years after the Big Bang, free electrons combined with these nuclei, decoupling matter from radiation and producing the Cosmic Microwave Background. This initial process—Big Bang nucleosynthesis—was the first type of nucleogenesis in the universe. Even today, about 74% of all matter is hydrogen from BBN, and another 24% is helium. The first stars, called Population III stars, formed almost entirely from hydrogen and helium, with only traces of lithium, beryllium, and boron. (Some lithium, beryllium, and boron came from BBN, but most was later made by cosmic ray spallation or stellar fusion.)

The synthesis of heavier elements—what astronomers call "metals"—requires the extreme temperatures and pressures inside stars and supernovae. These processes began when hydrogen and helium from BBN collapsed into the first Population III stars, a few hundred million years after the Big Bang. Star formation has continued in galaxies ever since, a period often called "Cosmic Dawn." Today, heavier elements are made through stellar nucleosynthesis (like carbon and nitrogen), supernova nucleosynthesis (like magnesium and aluminum), or exotic events such as neutron star collisions (like europium). Some elements form through multiple processes, and others, like argon-40, arise from radioactive decay.

Because different elements form at different stages in a galaxy’s life, astronomers can use their abundances to date the formation of a star or galactic structure. For instance, most magnesium in the interstellar medium was ejected by core-collapse supernovae, while much of the universe’s iron came from type Ia supernovae. Type Ia supernovae occur in binary systems: one star dies, leaving a white dwarf, which then accretes matter from its companion until its temperature and density trigger a violent explosion. The effects of type Ia supernovae on the interstellar medium typically appear about one billion years after star formation begins in a galaxy. In contrast, core-collapse supernovae are the deaths of massive stars that live only a few million years. As a general rule, stars that formed earlier than one billion years after the start of star formation in a region have a higher magnesium-to-iron ratio than later stars, because magnesium was incorporated into the interstellar medium sooner.