Most people associate the creation of elements heavier than iron with the violent collapse of massive stars in a supernova explosion. This rapid process creates extreme elements, such as gold and platinum. However, the majority of elements heavier than iron—including materials like barium, strontium, and lead—are forged in the gentle environment of dying, low-mass stars. This occurs over millions of years through a slow, steady nuclear process. The final stages of these stars seed the galaxy with the building blocks for future generations of stars and planets.
The Stellar Environment of Low-Mass Stars
A low-mass star has an initial mass between 0.8 and 8 times the mass of our Sun. After exhausting core hydrogen fuel and expanding into a Red Giant, these stars enter the Asymptotic Giant Branch (AGB) phase. The star is organized like an onion, featuring a dense, non-burning core of carbon and oxygen at the center. Surrounding the core are two actively burning shells: an inner shell fusing helium into carbon, and an outer shell fusing hydrogen into helium. This multi-shell arrangement is unstable, leading to periodic energy releases that drive heavy element production.
The Slow Neutron Capture Process
The mechanism by which low-mass stars build heavy elements is the slow neutron capture process, or s-process. This process relies on free neutrons being captured by existing atomic nuclei, primarily iron, which act as “seed” nuclei. A nucleus captures a single neutron, increasing its atomic mass but remaining the same element. The rate of neutron capture is slow compared to the rate at which an unstable nucleus undergoes beta decay.
If a neutron-rich isotope is unstable, it has time to decay into a new, heavier element before capturing another neutron. This decay involves a neutron transforming into a proton, increasing the atomic number and creating the next element on the periodic table. The s-process steadily creates elements like strontium, yttrium, zirconium, and barium. This mechanism contrasts with the rapid neutron capture process (r-process) in supernovae, which involves an instantaneous flux of neutrons that overwhelms the decay rate. The steady, lower-density neutron flux within AGB stars is perfectly suited for the s-process, generating about half of all elements heavier than iron in the universe.
The Engine Room: Neutron Sources and Thermal Pulses
Nucleosynthesis is driven by the periodic instability of the stellar shell structure, known as “thermal pulses.” These pulses occur when the helium-burning shell becomes unstable, leading to a runaway thermonuclear reaction called a helium shell flash. This energy burst causes the star to briefly swell and cool, temporarily shutting down the outer hydrogen-burning shell. Thermal pulses happen repeatedly over the star’s final million years, spaced out by 10,000 to 100,000 years.
These pulses activate the two primary nuclear reactions that supply free neutrons. The first and most dominant neutron source in lower-mass AGB stars is the C-13(alpha, n)O-16 reaction, which occurs during the quiescent period between pulses. The second source, Ne-22(alpha, n)Mg-25, requires higher temperatures and is briefly activated during the peak of the thermal pulse. The combined effect of these neutron releases over multiple cycles progressively builds up s-process elements, including the heaviest stable element, lead.
Distributing the Elements: Dredge-Up and Planetary Nebulae
For heavy elements to enrich the galaxy, they must escape the star’s interior via “dredge-up,” specifically the third dredge-up, which follows a thermal pulse. During this event, the star’s outer convective envelope deepens dramatically, extending into the helium-burning shell that hosted the s-process. This mixing transports the newly synthesized material—including carbon and s-process elements like barium and lead—up to the star’s surface.
From the surface, these elements are expelled by intense stellar winds characteristic of the AGB phase. The star sheds its entire outer envelope into space, creating a planetary nebula. This ejected material, rich in s-process elements, mixes with the interstellar medium, becoming raw material for subsequent generations of stars, planets, and life.