Nova stands inside a cutaway cross-section of a massive main-sequence star, surrounded by cascading diagrams of proton-proton collisions and glowing plasma streams, pointing at a central core diagram where hydrogen nuclei are colliding and fusing into helium while energy pulses outward in golden waves.
Explain how nuclear fusion in stellar cores converts hydrogen into helium and releases energy via mass-energy equivalence.
Identify the sequence of fusion reactions that produce progressively heavier elements across a star's life cycle.
Compare the nucleosynthesis capabilities of low-mass stars versus high-mass stars and core-collapse supernovae.
Predict which elements a star of a given mass can fuse, using the concept of Coulomb barrier and core temperature.
Describe how supernova explosions and neutron star mergers disperse heavy elements, linking stellar death to the chemical composition of planets and life.
Key terms
Nuclear fusion
The joining of light atomic nuclei into a heavier nucleus, releasing energy when the product is more tightly bound.
Coulomb barrier
The electromagnetic repulsion between positively charged nuclei that fusion must overcome before the strong force can bind them.
Binding energy per nucleon
The energy holding each nucleon in a nucleus; it peaks near iron-56 and nickel-62, ending energy-releasing fusion there.
r-process
Rapid neutron capture forging elements heavier than iron, occurring in neutron star mergers and possibly core-collapse supernovae.
From Hydrogen to Iron
Fusion releases energy only when it produces a more tightly bound nucleus. In the Sun the proton-proton chain fuses four hydrogen nuclei into one helium-4, whose mass is about 0.7% less than the protons that formed it; that mass deficit becomes energy via E = mc². In hotter, more massive cores the CNO cycle dominates hydrogen burning. As fuel runs low the core contracts and heats, igniting helium burning (triple-alpha to carbon, alpha capture to oxygen) and, in massive stars, successive carbon, neon, oxygen, and silicon burning that builds up to iron-peak elements.
Why Fusion Stops at Iron
Binding energy per nucleon rises from hydrogen up to a maximum near iron-56 and nickel-62, then declines for heavier nuclei. This means fusing nuclei lighter than iron yields a more tightly bound product and releases energy, but fusing iron-peak nuclei into something heavier produces a less-bound product and absorbs energy. A massive star with an iron core thus has no fusion energy left to resist gravity, and the core collapses in about 100 milliseconds, triggering a supernova whose shock and neutrinos blast the outer layers into space.
Worked examples
Explain why hydrogen fusion releases energy while iron fusion does not, using binding energy.
Fusion releases energy only when the product nucleus has higher binding energy per nucleon than the reactants.
Helium-4 is more tightly bound than four separate protons, so its formation releases energy as a mass deficit.
Iron-56 sits at the binding-energy peak, so any nucleus made from fusing it is less tightly bound and the reaction absorbs energy instead.
Answer: Hydrogen fusion releases energy because helium is more tightly bound, but iron is at the binding-energy maximum, so fusing it consumes energy.
Nova here — and before we begin, hold on to one central question: why does fusion stop at iron? Keep that in mind as we go.
Think about the iron in your blood, the calcium in your bones, and the oxygen you are breathing right now. Every one of those atoms was forged inside a star.
Here is the core idea: stars are gravitational pressure cookers. Gravity squeezes a star's core so tightly that temperatures reach 10 to 15 million Kelvin. At those extremes, protons move fast enough to overcome their mutual electromagnetic repulsion — the Coulomb barrier — and tunnel close enough for the strong nuclear force to bind them together. This is nuclear fusion.
In the Sun, the dominant pathway is the proton-proton (pp) chain. Four hydrogen-1 nuclei (protons) fuse in a sequence of steps to produce one helium-4 nucleus, two positrons, two neutrinos, and gamma-ray photons. The key physics: the helium-4 nucleus has slightly less mass than the four protons that formed it. That missing mass — about 0.7% — converts directly to energy via Einstein's E = mc². Because c² is enormous (~9 × 10¹⁶ m²/s²), even a tiny mass difference releases a tremendous amount of energy. This is why the Sun has shone for 4.6 billion years. In more massive and hotter cores, the CNO cycle — which uses carbon, nitrogen, and oxygen as catalysts — dominates hydrogen burning instead of the pp chain.
As a star exhausts hydrogen in its core, gravity wins temporarily and the core contracts, raising the temperature. Higher temperatures allow fusion of heavier nuclei. Helium fuses into carbon-12 via the triple-alpha process (three helium-4 nuclei colliding in sequence); a further alpha-capture reaction then converts some of that carbon into oxygen-16. Together, these reactions define the helium-burning phase. In massive stars (roughly 8+ solar masses), the core temperature climbs high enough to fuse carbon into neon and magnesium, neon into oxygen and magnesium, oxygen into silicon, and silicon into iron-peak elements. Each stage runs faster than the last — silicon burning lasts only days.
Iron and nickel mark the absolute end of fusion energy production. Nuclei near iron and nickel (especially Ni-62 and Fe-56) sit at or near the peak of the nuclear binding energy curve, meaning fusing them or splitting them both require energy rather than releasing it. A massive star with an iron-nickel core therefore has no energy source left to resist gravity.
The core collapses in a fraction of a second (roughly 100 milliseconds). The rebound shock wave, amplified by escaping neutrinos, tears the star apart in a core-collapse supernova. Crucially, elements heavier than iron — gold, platinum, uranium — are forged by rapid neutron capture (the r-process), where nuclei absorb neutrons faster than they can beta-decay. This occurs primarily in neutron star mergers (kilonovae) and possibly in core-collapse supernovae — astrophysicists confirmed neutron star mergers as a major r-process site when gravitational waves detected the GW170817 merger in 2017. The explosion and merger ejecta then seed the interstellar medium with all these elements.
Low-mass stars like the Sun never reach carbon fusion temperatures. They expel their outer layers as planetary nebulae and leave behind a white dwarf core, contributing hydrogen, helium, carbon, and oxygen to the galaxy but not the heavier metals. Massive stars are the factories; supernovae and neutron star mergers are the delivery mechanism.
So the next time you look at the night sky, remember: those points of light are not just burning — they are building the periodic table, one fusion reaction at a time.
Activity
Arrange the following nucleosynthesis stages in the correct order they occur inside an 8-solar-mass star, from earliest to latest.
Practice
Order the fusion stages inside a massive star from hydrogen burning to silicon burning, naming the approximate core temperature each requires.
Explain why elements heavier than iron, such as gold and uranium, are forged by neutron capture rather than ordinary stellar fusion.
Common mistakes to avoid
Stars fuse all elements including gold and uranium.Stellar fusion stops at the iron-nickel peak; heavier elements form by rapid neutron capture in neutron star mergers and supernovae.
The Sun never fuses carbon because it lacks carbon.The Sun's core never contracts hot enough to overcome carbon's larger Coulomb barrier, not because carbon fuel is absent.
Check your understanding
Why does nuclear fusion in a stellar core release energy when hydrogen fuses into helium, but NOT when iron fuses into heavier elements?
A student claims that stars produce all elements heavier than hydrogen by fusion inside their cores, including gold and uranium. What is the most accurate correction?
Which statement best explains why a low-mass star like the Sun will never fuse carbon in its core, while a star of 15 solar masses can?
Recap
Stars fuse hydrogen to helium and onward to iron-peak elements, releasing energy only because binding energy per nucleon peaks near iron; heavier elements like gold and uranium are forged by rapid neutron capture in supernovae and neutron star mergers, seeding the galaxy.
Reflect
How does knowing the iron in your blood was forged inside a star and scattered by its death change how you see the night sky?
Astronomy