---
title: "How Stars Build Elements up to Iron Interactive Lab | Sandbox Physics"
description: "Why does fusion release energy only until nuclei approach iron? Binding energy per nucleon peaks near iron."
canonical_url: "https://sandboxphysics.com/labs/stellar-fusion-chain/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/stellar-fusion-chain/"
last_modified: "2026-11-04"
---

Nuclear physics · stellar evolution

# Stellar Fusion: Hydrogen to Iron

**Why does fusion release energy only until nuclei approach iron?** Binding energy per nucleon peaks near iron. Fusing lighter nuclei moves toward that peak and releases energy; beyond it, fusion absorbs energy instead. The star works through ever hotter burning stages until its core is iron, and fusing iron cannot hold the core up.

Step through the proton-proton chain, the triple-alpha process, carbon, neon, oxygen and silicon burning, and finally nickel decay. Each step shows the temperature it needs and which way the energy goes.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

This is a simplified reaction network for teaching, not a particle-by-particle nuclear calculation. Sun-like stars never complete the chain. Only massive stars build up onion-like burning shells that reach toward the iron peak before the core collapses.

Physics tutorial

## Stellar nucleosynthesis: why evolution moves toward the iron peak

**Background** Binding energy per nucleon rises rapidly with mass number and peaks near iron. Fusing light nuclei into more tightly bound ones releases the energy of the mass deficit, but fusion beyond the peak takes energy in.

**Why it matters** This energy curve controls how bright a star is, how long it lives, how its shells are arranged and how it ends. The Sun burns only light fuels, while massive stars ignite a series of hotter, shorter stages.

Start with the essentials

- **Focus question**: Why can a star not fuse hydrogen directly into iron, instead passing through increasingly hot multi-stage burning?
- **One-sentence intuition**: Every step up in nuclear charge raises the Coulomb barrier. When one fuel runs out, the core contracts and heats until the next set of reactions can start. At the iron peak, fusion can no longer supply this support.

### Core mathematical model

#### Mass deficit releases energy

$$
Q=(m_{\rm initial}-m_{\rm final})c^2
$$

When final rest mass is lower, Q is positive and the difference becomes particle kinetic energy, photons, and neutrinos.

#### Coulomb barrier scale

$$
V_C(r)\approx\frac{Z_1Z_2e^2}{4\pi\varepsilon_0r}
$$

Larger nuclear charges repel more strongly, so carbon, oxygen, and silicon burning require successively hotter cores.

### Common difficulties

#### The sequence is not one proton at a time

**Typical misconception** A star synthesizes every element in periodic-table order.

**Better mental model** The real process is a branching reaction network. The Lab shows only the dominant burning stages and representative products, and leaves out many isotopes and side reactions.

#### Silicon does not fuse directly into iron once

**Typical misconception** Silicon burning is simply two silicon nuclei colliding into one iron nucleus.

**Better mental model** At these temperatures, photodisintegration, captures and nuclear statistical equilibrium first favor nickel-56, which later decays through cobalt toward iron-56.

### Run the experiment

1. 01

   #### See where a Sun-like star stops

   Choose Sun-like star and advance the chain.

   **What to observe:** The star’s mass and temperature are too low for carbon burning. Not every star makes iron.

2. 02

   #### Open the massive-star chain

   Choose Massive star and click through successive stages.

   **What to observe:** The star is massive enough, but carbon, neon, oxygen and silicon burning each still need a high enough temperature to ignite.

3. 03

   #### Read the binding-energy curve

   Advance toward the iron peak and follow the red marker.

   **What to observe:** Each stage that releases energy climbs the curve of binding energy per nucleon. Beyond iron, fusion no longer supplies energy.

Physical conclusion

A star evolves through a feedback cycle: the fuel runs out, the core contracts and heats, and the next fuel ignites. The iron peak ends this energy-releasing cycle.

## Related experiments and articles

Nuclei and particles · Reading path

### The nucleus and its reactions

Articles on this path that use this experiment, followed by the other experiments they suggest.

1. [Article **What makes a star a steady nuclear engine**](https://sandboxphysics.com/stories/nuclear-particle/bethe-stellar-nuclear-power/)
2. [Article **Making the elements one measured reaction at a time**](https://sandboxphysics.com/stories/nuclear-particle/fowler-element-making-rates/)

Inside a star: pressure, heat and time · Reading path

### Inside a star: pressure, heat and time

Articles on this path that use this experiment, followed by the other experiments they suggest.

1. [Article **Read a stellar track by following the fuel**](https://sandboxphysics.com/stories/stellar-evolution/following-fuel/)
2. [Lab **Stellar Structure and Evolution** Match each HR point to its interior](https://sandboxphysics.com/labs/stellar-hydrostatic-structure/)

Astronomy and gravity · Reading path

### Stars and new windows on the sky

Articles on this path that use this experiment, followed by the other experiments they suggest.

1. [Article **Why a white dwarf has a maximum mass**](https://sandboxphysics.com/stories/astronomy-gravity/chandrasekhar-stellar-support/)
2. [Lab **Stellar Structure and Evolution** Look at the pressure that supports a stellar shell](https://sandboxphysics.com/labs/stellar-hydrostatic-structure/)

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)