Where did the chemical elements come from?

Light nuclei came first

An element is defined by the number of protons in its nucleus: one means hydrogen, two helium and eight oxygen. Changing the electrons around a nucleus does not turn hydrogen into oxygen. Nuclear transformations that change the nucleus itself are needed. DOE: elements and isotopes.

During the first minutes, the hot Universe expanded and cooled. Some protons and neutrons bound together into light nuclei, especially helium-4; deuterium, helium-3 and a very small amount of lithium also remained. Many protons did not join more complex nuclei: each one is already a hydrogen-1 nucleus. Hydrogen therefore did not first need to be forged in stars.

After this primordial nucleosynthesis, roughly three-quarters of the mass of ordinary matter was hydrogen and about one-quarter helium. This is a rounded description of the initial mixture, not shares of everything in the Universe or the exact composition of any present-day star. Rapid expansion and the properties of nuclear reactions prevented substantial amounts of carbon, oxygen or iron from accumulating then. Particle Data Group: primordial nuclei and abundances.

A nucleus and a neutral atom are different things. In the hot material, electrons were not yet stably bound to nuclei. Widespread formation of neutral hydrogen happened much later, approximately 380,000 years after expansion began. The element already existed; its electrical state changed. ESA: the nuclear era and later recombination.

Stars reprocess the initial mixture

High temperatures and densities inside stars allow nuclei to participate in reactions. Nuclear fusion joins lighter nuclei into heavier ones. A star such as the Sun, for example, turns hydrogen into helium. During later stellar stages, reactions involving helium can create carbon, and a carbon nucleus capturing a helium nucleus can make oxygen.

Not every star follows the same set of stages. Massive stars can reach conditions for further reactions that alter the amounts of neon, magnesium, silicon and nuclei near iron. A lower-mass star is not simply a slower copy that will inevitably make all the same things eventually. OpenStax: fusion in stars of different masses.

Why not keep building nuclei this way forever? Near iron and nickel, nuclei are very tightly bound. Further fusion does not provide the same energy gain that supports the preceding stellar stages. This does not forbid heavier elements from existing: other nuclear pathways become important for them. Neutron capture, in particular, is not simply an extension of two charged nuclei merging.

Making a nucleus and sending it into space are different jobs

Oxygen locked inside a star is not yet oxygen in future water on Earth. The material must reach the surroundings, mix with gas and eventually enter new objects. Stellar winds, streams of matter leaving stars, return some material to interstellar space. This does not always require a supernova.

A supernova explosion can both eject nuclei created during the star’s earlier life and make new ones in rapidly heated material. These are different contributions from one event. We should not describe all the ejected oxygen as a product of the instant of explosion.

Supernovae also come in different types. Explosions of massive stars involving core collapse and thermonuclear explosions of white dwarfs in binary systems have different histories. The latter, called Type Ia, are an important source of iron-group nuclei. Some products are initially radioactive and later transform into other nuclei. Even for iron, we should therefore ask about the process, the stellar type and subsequent decay, rather than look for one universal stellar furnace. Kobayashi, Karakas and Lugaro: different sources of chemical enrichment.

Neutrons open routes to many heavy nuclei

A nucleus can capture a neutron. At first this changes its isotope, a form of the element with a different neutron count. Some unstable nuclei subsequently undergo beta-minus decay: a neutron becomes a proton while other particles are emitted. The proton count changes, producing another element. Repeated captures and decays build new nucleosynthesis pathways.

The slow, or s-process, generally gives an unstable nucleus time to decay before the next capture. It occurs, among other places, in low- and intermediate-mass stars during the late giant phase called the asymptotic giant branch, or AGB. Such stars contribute to strontium and barium, for example. Mixing carries some products towards the outer layers, and the loss of those layers enriches the surroundings. Karakas and Lattanzio: AGB stars and slow neutron capture.

The rapid, or r-process, allows neutron capture to outpace the relevant beta decays. It requires special conditions with a large neutron flux. One established channel is material ejected in neutron-star mergers, collisions of extremely dense stellar remnants. This is a route to many heavy nuclei, including those that eventually produce gold. But “gold forms only in neutron-star mergers” would be a stronger claim than the evidence allows.

Compare the channels in the diagram. Strontium, Sr, appears in two entries: an element need not have just one birthplace. Contributions depend on both isotopes and the history of the environment.

Sometimes breaking apart matters more than building up

There is another mechanism: fast cosmic-ray nuclei collide with interstellar matter and break into lighter fragments. This is called spallation. It produces some lithium, beryllium and boron. Here cosmic rays are energetic particles, not just light. Measurements of secondary Li, Be and B nuclei help investigate these collisions. This does not mean all lithium has that origin. AMS: secondary cosmic-ray nuclei.

Nucleosynthesis is therefore broader than the fusion of light nuclei. A short list of channels does not replace the full reaction network: some rare isotopes need additional pathways. Even two isotopes of one element can have different histories.

Testing a history nobody watched in full

In 2017, astronomers observed the neutron-star merger GW170817 and its light-emitting aftermath. A 2019 study identified strontium in the spectra by comparing measured light with calculated signatures of elements. This links particular material to a particular event. It is not a separate spectroscopic detection of gold in the same outburst, nor does it mean that all strontium in the Universe formed this way. Watson and colleagues: strontium in merger ejecta.

For the broader picture, researchers compare nuclear reactions, the composition of stars and gas, meteorite isotopes and models of element accumulation. These let us estimate contributions from different sources. They do not let us name a particular ancient star for every atom in your glass.

Test your understanding: if a stellar wind carried barium away, must the barium have formed in the wind itself? No. The nucleus could have formed inside the star, while the wind performed a different job: carrying it outside. Likewise, the birth of an oxygen nucleus and the later formation of a water molecule are different events.

The Solar System also formed after material from many earlier sources had mixed. Water brought together hydrogen of largely primordial origin and oxygen created by stellar reactions. We inherited a mixture of cosmic histories, rather than one batch of material from one star. Next we will explore why stars have different colours and what their light can reveal about them.