A New Discovery About the Synthesis of Chemical Elements in the Stars!

All known chemical elements in nature are created through the fusion of lighter elements, such as the fusion of two hydrogen atoms leading to the synthesis of a helium atom. While the majority of elements form through nuclear reactions in the core of stars, models suggest that elements heavier than iron (Fe) can only be formed during cataclysmic cosmic events, like supernovae or neutron star collisions. However, a recent study indicates that some elements can also form within stars through the fission reactions of elements heavier than uranium (U).

The formation of chemical elements (such as hydrogen or carbon) that make up our bodies, planets, and stars is now relatively well understood. According to models, just after the Big Bang, the universe consisted of a soup of neutrons and protons. Due to the rapid decrease in temperatures a few minutes after the Big Bang, these elementary particles began to associate to form the two lightest chemical elements: hydrogen, composed of one proton and one neutron, and helium, composed of two protons and two neutrons.

Scientists refer to this process as primordial nucleosynthesis, during which the two most abundant elements in the universe were formed. It then takes the formation of the first stars to witness the appearance of heavier chemical elements, which, broadly speaking, contain more neutrons and protons.

Heavy Chemical Element Factories

The primordial nucleosynthesis fails to explain the existence of elements heavier than hydrogen and helium, commonly referred to by astronomers as “metals.” The synthesis of these “metals” requires conditions where matter is highly dense and temperatures are exceedingly high, such as in the cores of stars.

It is essential to note that temperature corresponds to an excitation of matter, causing it to move more rapidly, thereby increasing the frequency and strength of collisions between different particles. In the context of stars, the formation of elements heavier than hydrogen and helium occurs through thermonuclear fusion reactions. Hydrogen atoms (H) fuse to form helium atoms (He), which, in turn, fuse to create beryllium (Be), and this process continues until the formation of iron (Fe).

The entirety of these processes collectively constitutes what scientists term stellar nucleosynthesis. Not all stars possess the requisite heat to generate elements as heavy as iron. For instance, the fusion of two oxygen atoms (O), leading to the formation of a sulfur atom (S), demands temperatures on the order of a billion Kelvins and only occurs in the inner layers of “supergiant” stars, which are at least five times more massive than our Sun.

Although iron atoms are the most stable atoms in nature (and therefore the most resistant to thermonuclear fusion reactions), the periodic table of elements doesn’t stop there, and there are many heavier chemical elements in nature, such as gold (Au) and uranium (U).

These elements are formed by neutron capture processes. When bombarded by intense neutron fluxes and bathed in conditions of extreme density and temperature, heavy chemical elements can absorb neutrons to form new, even heavier, albeit more unstable (or radioactive; they decay over time into lighter elements).

The process of rapid neutron capture occurs, for instance, within supernovae, significant cosmic events during which a dying star explodes, releasing immense amounts of energy and creating conditions hotter and denser than within stars. These conditions are conducive to the formation of elements heavier than iron.

Such processes can also take place during the merger of two neutron stars (primarily composed of neutrons held together by gravitational forces), producing vast amounts of energy and neutron densities so high that very heavy atoms, such as uranium, can be formed. These phenomena, referred to as “r-processes” (“r” for rapid), partially describe what scientists term explosive nucleosynthesis.

While the synthesis of known chemical elements appears to be well understood, scientists are uncertain about whether and how atoms heavier than uranium (referred to as transuranic elements) can form in nature. However, a recent study published in the journal Science sheds new light on nucleosynthesis processes.

By analyzing the compositions and abundances of ancient stars enriched with very heavy elements likely formed during explosive nucleosynthesis processes, the scientific team demonstrated that certain high-atomic-mass chemical elements could have formed from fission reactions of elements heavier than uranium.

Elements Heavier Than Iron Formed in Stars?

Analyzing the compositions and abundances of chemical elements in 42 ancient stars in our Milky Way, the team of scientists highlighted the presence of elements heavier than iron, such as ruthenium (Ru) or silver (Ag). The abundances of these elements appeared to be correlated with those of even heavier elements, including some lanthanides (belonging to rare earth metals).

Considering that the temperatures within stars are not high enough for the fusion of elements leading to the formation of atoms heavier than iron, scientists had only one plausible option: these elements would have actually formed through the fission reactions of even heavier atoms. It appears that elements heavier than iron can thus be formed in the cores of stars, not through thermonuclear fusion reactions but through the fission reactions of even heavier elements.

Stars form as a result of the collapse of enormous clouds of gas in interstellar space; thus, the chemical composition of the star reflects the initial chemical composition of the gas cloud. If the latter contains heavy chemical elements previously formed during large-scale cosmic cataclysmic events (such as supernovae or mergers between two neutron stars), the star formed from this cloud will also contain them.

Therefore, the presence of chemical elements heavier than iron in observed stars, such as ruthenium or silver, tends to indicate that these stars had an initial chemical composition enriched with even heavier chemical elements formed during supernovae or neutron star mergers.

However, what astonishes scientists the most is the supposed atomic mass of these very heavy chemical elements: according to the compositions and abundances of chemical elements in observed stars, atoms with an atomic mass exceeding 260 (even heavier than uranium) should have been present during the star’s formation; subsequent fission reactions would have caused their disappearance. Yet, such heavy elements have never been observed in their natural state, whether in space or on Earth, even during nuclear tests.

Nevertheless, their findings suggest that such elements could indeed exist in space, albeit for a very brief period. According to scientists, supernovae or neutron star mergers create conditions of density and temperature so extreme that chemical elements with an atomic mass higher than 260 can form; however, they have not been observed due to their extremely short lifespan.