What elements are formed in supernovas?

The iron in your blood, the calcium in your bones, and the silicon that forms the rocky core of our world—all of it was forged in the inferno of a dying star. The spectacular, violent end of a massive star, the supernova, is the universe’s most potent chemical factory, seeding the cosmos with the raw materials necessary for planets and, eventually, life. While the very lightest elements, hydrogen and helium, were born in the Big Bang, nearly every other naturally occurring element owes its existence to the extreme pressures and temperatures found deep within stars or in the final cataclysmic explosion itself.

The story of elemental creation is one of progression, starting slowly in a star's quiet life and ending in a furious, second-long frenzy.

# Stellar Furnace

For the majority of its active life, a star, even one far more massive than our own Sun, is engaged in relatively gentle, sustained thermonuclear fusion. This is often described as a series of burning stages, operating under hydrostatic equilibrium—a balance between the inward pull of gravity and the outward push of thermal pressure generated by fusion.

In stars like the Sun, the process starts by fusing four hydrogen atoms into one helium atom, which releases the energy that sustains the star for billions of years. When the core hydrogen is depleted, the star contracts, heats up, and begins to fuse the resultant helium into heavier elements, chiefly carbon ($\text{C}$) and oxygen ($\text{O}$).

For stars several times the mass of the Sun, this process continues in layered shells, each heavier element forming in the core of the previous stage. Once the core is hot enough, helium burning is followed by carbon burning, then neon burning, oxygen burning, and finally, silicon burning. These sequential stages build up the atomic nuclei in an alpha-process chain, where successive helium nuclei ($^4\text{He}$) are added to an existing nucleus, primarily synthesizing alpha nuclides (nuclei composed of integer multiples of helium-4 nuclei). This process continues steadily, creating elements like sulfur ($\text{S}$), argon ($\text{Ar}$), calcium ($\text{Ca}$), and up through elements near the middle of the periodic table, culminating in nickel-56 ($^{56}\text{Ni}$).

# Iron Ceiling

The sequence of fusion halts abruptly at iron ($\text{Fe}$, atomic number $Z=26$) and its neighbor, nickel ($\text{Ni}$, $Z=28$). This is not a coincidence of engineering or insufficient pressure; it is a fundamental law of nuclear physics. The iron-56 nucleus possesses the highest nuclear binding energy per nucleon of all known isotopes.

What this means practically is that fusing lighter elements releases energy—this is the power source that keeps the star from collapsing. However, any attempt to force two iron or nickel nuclei together into a heavier element consumes energy; these reactions are endothermic. When a massive star’s core becomes dominated by iron, it can no longer generate the thermal energy needed to counteract gravity. The pressure support vanishes, and the star begins a final, catastrophic implosion lasting mere seconds.

What elements form from a supernova shockwave?

# Shock Synthesis

The core collapse generates an unimaginable amount of energy—more than a hundred times the Sun's entire 10-billion-year output—which slams into the dense core and rebounds as a tremendous outward-moving shock wave. This shock wave briefly spikes the temperature in the overlying layers, initiating explosive nucleosynthesis that is crucial for determining the final composition of the ejected material.

The silicon-burning phase, which was heading toward $^{56}\text{Ni}$ in the core, sees its progress finalized by this shock. In the silicon-burning quasiequilibrium, $^{28}\text{Si}$ is effectively transformed into $^{56}\text{Ni}$. The explosive burning phase, though it lasts only seconds, is the major contributor to the abundances of elements between mass numbers 28 and 60.

Once synthesized, this $^{56}\text{Ni}$ is radioactive. It decays within days to cobalt-56 ($^{56}\text{Co}$), which in turn decays to the stable isotope iron-56 ($^{56}\text{Fe}$). It is the radioactive decay of this newly formed nickel that is responsible for energizing the light curve of a core-collapse supernova days or weeks after the initial explosion.

For Type Ia supernovae—the explosion of a white dwarf exceeding the Chandrasekhar limit—the story is slightly different but the result for iron is similar. This type of explosion fuses most of its material into $^{56}\text{Ni}$ in a runaway thermonuclear reaction, and the subsequent decay of this nickel is what makes Type Ia events so optically bright for weeks. This process alone is theorized to create more than half of all the iron found throughout the universe. While core-collapse events produce the bulk of the primary elements (those from $\text{H}$ to $\text{Fe}$), the sheer scale of the $^{56}\text{Ni}$ production in Type Ia events makes them the dominant iron source overall.

# Neutron Showers

Elements heavier than iron cannot be formed via standard fusion because it requires energy input rather than releasing it. Creating these heavier nuclei requires overcoming the strong electrostatic repulsion between nuclei while bombarding them with neutrons. This necessitates an environment utterly foreign to stable stellar cores.

The key mechanism for building these elements during a core-collapse supernova is the r-process, or rapid neutron capture. In the dense, neutron-rich environment near the collapsing core, nuclei are bombarded by such an intense flux of neutrons (estimated at $10^{22}$ to $10^{24}$ neutrons per cubic centimeter) that they rapidly capture many neutrons before they have time to undergo radioactive beta decay. This process pushes nuclei far up the periodic table, often creating highly unstable isotopes. Only after the explosion ejects this material and the neutron density drops do these unstable isotopes decay via beta emission, settling into stable, heavier elements. The r-process is responsible for roughly half of the elements heavier than iron.

However, the $r$-process isn't the only game in town for heavy elements:

• The s-process (slow neutron capture) contributes to some heavy elements, occurring in the helium-burning and carbon-burning shells of massive stars before the final collapse.
• The rp-process (rapid proton capture) also plays a role in synthesizing some heavy elements.
• The $\gamma$-process (gamma process) utilizes high-energy photons in the explosion to rearrange existing heavier isotopes into the lightest, most neutron-poor isotopes of elements above iron.

The elements synthesized in the shock wave of a core-collapse event include elements like zinc ($\text{Zn}$), silver ($\text{Ag}$), tin ($\text{Sn}$), gold ($\text{Au}$), mercury ($\text{Hg}$), lead ($\text{Pb}$), and even uranium ($\text{U}$).

What elements are being fused in a supernova?

# Cosmic Alchemy Sites

For a long time, the core-collapse supernova was considered the primary, if not sole, source for all elements heavier than iron. However, recent observations have necessitated a nuanced view, splitting the authorship of the heaviest elements between traditional supernovae and a different, equally dramatic event: the merger of two neutron stars, a phenomenon culminating in a kilonova.

When two neutron stars spiral inward and collide, the event produces a tremendous burst of gravitational waves, a short gamma-ray burst, and an observable optical counterpart called a kilonova. The extreme tidal forces and resulting neutron-rich ejecta—estimated to be about 1% of the stars' mass—provide the perfect crucible for the $r$-process.

Evidence from the 2017 gravitational wave detection ($\text{GW}170817$) strongly confirmed that these mergers generate massive quantities of $r$-process material. Specifically, the observed spectra indicated the presence of lighter $r$-process elements ($A < 140$) and heavier, actinide-rich elements like uranium and thorium being violently expelled. Because the conditions seem exceptionally well-suited for creating the sheer volume of neutron-rich matter required, it is now argued that the bulk of the $\text{r-process}$ elements in the Milky Way—including precious metals like gold and platinum—may originate primarily from these binary neutron star mergers, rather than from every single core-collapse supernova. This highlights a significant evolution in astrophysics; while Type II supernovae are massive element factories for the lighter components, the absolute heaviest materials require the precise, high-density environment of a merger.

This constant input of synthesized material fundamentally alters the galaxy. The interstellar medium (ISM) is enriched over time, changing the composition of the gas clouds that form subsequent stellar generations.

If we consider the sheer amount of time involved in this cycle, the origin of the elements in our bodies becomes even more profound. A supernova that creates an atom of gold explodes, and its ejecta disperses into the vast interstellar medium. This material may take millions or even billions of years to cool, condense, and eventually become incorporated into a new star system, like our own. This means the gold atom inside a piece of jewelry on Earth today may have been created in a violent event that occurred long before the Sun itself ignited, having traveled across vast stretches of the galaxy for eons before settling into our solar neighborhood. The elements we interact with daily are the cumulative, well-mixed residue of countless stellar deaths spanning cosmic history.

# Cosmic Recycling

The supernova event, regardless of the specific type, concludes by sending its newly manufactured wares into space, often creating an expanding shell of hot gas called a supernova remnant. While the initial explosion drives a shock wave that compresses the ISM—potentially triggering the collapse of gas clouds to form new stars—the ejected material itself eventually mixes into the general ISM over hundreds of thousands of years.

This mixture is the cosmic inheritance. The elements born in the core of one star, or flung out during the death throes of another, become the building blocks for the next. Without this constant, forceful dispersal of heavier nuclei—from the common silicon ($\text{Z}=14$) up to the rarest actinides—the formation of rocky planets, liquid water, and complex biology would have been impossible. We are, quite literally, the recycled remnants of explosions that happened long before the Earth existed.