The Big Bang made almost nothing but hydrogen and helium. The carbon, oxygen and iron in your body were made in stars.
When the early universe cooled, it held hydrogen, helium and a trace of lithium. It held no carbon, no oxygen and no iron. Without them there could be no rock, no planets and no life. Stars built these elements by fusing light nuclei into heavier ones. This page shows which elements form where, and how a star’s mass and age decide what it makes.
Core Claims
- The Big Bang. It made hydrogen, helium and a trace of lithium.
- Inside stars. Fusion builds elements from helium up to iron (stellar nucleosynthesis). A star’s mass sets how far it goes.
- Explosions and collisions. Elements heavier than iron form in a supernova and when neutron stars collide.
- Recycling. Dying stars scatter new elements into space. New stars and planets form from that gas.
Where the elements form
Retrieval Checklist
- Name where hydrogen and helium formed.
- Say how a star’s mass changes which elements it makes.
- Say where gold and other elements heavier than iron come from.
- Say how new elements reach the gas that forms new stars.
Three kinds of events made all the elements.
The Big Bang made hydrogen, helium and a trace of lithium. Nothing heavier formed.
Inside stars, fusion joins light nuclei into heavier ones. This makes every element up to and including iron. The process is stellar nucleosynthesis.
Elements heavier than iron form in two violent events. A massive star can explode in a supernova. Two neutron stars can also collide. Both events scatter the new elements across space.
| Elements | Examples | Where they formed |
|---|---|---|
| Hydrogen and helium | H, He | The Big Bang |
| Lighter than iron, and iron | C, O, Si, Fe | Inside stars |
| Heavier than iron | Au, Pt, U | Supernovae and colliding neutron stars |
Thinking stars can fuse their way to every element, even gold. Fusion inside stars stops at iron. Gold needs a supernova or a collision of neutron stars.
A star’s mass and age decide which elements it makes.
A star makes elements by fusing nuclei in its core. How far it goes depends on its mass, because mass sets how hot the core gets.
A star like the Sun
The Sun fuses hydrogen into helium for about 10 billion years. Only when the hydrogen runs out does its core fuse helium into carbon and oxygen. It never gets hot enough to build anything heavier.
A massive star
A star with more than 8 times the Sun’s mass is squeezed harder. Its core gets hotter. When one fuel runs out, the core fuses the ash of the last reaction. Late in its life, the star has built elements up to iron.
Age matters too. A massive star begins with hydrogen only. It makes heavier elements one at a time as it ages. It lives fast and dies young.
Thinking every star makes iron. Only massive stars get hot enough. A star like the Sun stops long before iron.
Explosions and collisions make the heaviest elements and scatter them.
When a massive star runs out of fuel, its core collapses. The star then explodes in a supernova. The blast makes elements such as gold, platinum and uranium. Two colliding neutron stars make them too.
These events also throw every new element into space. The gas drifts together and forms new stars and planets. The iron in your blood was made in a star that died before the Sun formed.
Thinking an exploding star only destroys things. A supernova is how new elements reach space. Without one, there would be no gold and no rocky planets.
Shells, the iron peak and neutron capture.
EnrichmentHow a massive star builds iron, and how heavier elements form
Extra depth. The rest of the lesson works without it.
Shell-by-shell burning
In a star with more than 8 times the Sun’s mass, each fuel burns in turn. When a fuel runs out, the core shrinks and heats up. The ash of the last reaction then becomes the next fuel. The star ends up with burning shells like the layers of an onion.
What happens in each shell of a massive star?
The explanation appears after you predict.
- Hydrogen, helium and carbon shells, from outside in (blue, green, orange)
- Neon and oxygen shell
- Silicon shell and iron core, in the center
- The thick outline marks the shell you picked
- Compare the first and the last fuel. Load the hydrogen shell and press Record. Then load the silicon shell and press Record. Look for: hydrogen burns at 40 MK for 7 million years. Silicon burns at 3.5 GK for about 1 day.
- Look at the core. Load the iron core and press Record. Look for: no fusion, and the energy line reads “Takes in energy”. The star can no longer hold itself up.
Use your evidence
The iron peak
Why can the star not fuse iron too? The answer is in the binding energy per nucleon (Figure 5.5). Joining nuclei lighter than iron releases energy. Iron-56 sits at the peak of the curve, at 8.79 MeV per nucleon. Fusing iron takes in energy instead of releasing it.
When the core holds about 1.4 solar masses of iron, fusion stops. Iron absorbs heat instead of releasing it. The core cools, and its outward pressure vanishes. In less than a quarter of a second, gravity pulls the star inward at 70,000 kilometers a second, 23% of the speed of light. The collapse ends in a supernova.
What happens to a star’s core in its last second?
The explanation appears after you predict.
- Gravity pulling in (stage 1)
- The core pushing out (stage 1)
- The core falling inward (stage 2)
- The shock wave moving out (stage 3)
- Compare the balanced core with the falling core. Load stage 1 and press Record. Load stage 2 and press Record. Look for: the core shrinks from 5,000 km to 80 km, and its speed goes from 0 to 70,000 km/s.
- Watch the rebound. Load stage 3 and press Record. Look for: a core only 12 km wide, and a shock wave that moves outward at 30,000 km/s.
Use your evidence
Neutron capture
Fusion past iron is impossible, so how did gold and uranium form? Neutrons have no electric charge. They can enter a nucleus without being repelled. Inside the nucleus, a neutron can change into a proton. This moves the element up the periodic table.
The s-process: slow capture
It happens in old, pulsing giant stars over thousands of years. An iron nucleus catches a stray neutron every few hundred years. It has time to change before the next one arrives. This builds strontium, barium and lead.
The r-process: rapid capture
It happens in a supernova and when neutron stars collide. Trillions of neutrons fill each cubic centimeter. A nucleus catches dozens of them in a microsecond. The heavy nuclei then change into gold, platinum and uranium.
Explain it
Use the binding energy curve to explain why a massive star cannot keep fusing once its core is iron.
Below iron, joining nuclei raises the binding energy per nucleon, so fusion releases energy. Iron-56 has the highest value. Fusing iron lowers it, so fusion takes in energy.
The core then loses its outward pressure. Gravity wins, and the core collapses.
Gas around a young star cluster contains carbon, oxygen, magnesium, iron and gold.
(a) Use the binding energy curve. Explain why massive stars make elements up to iron but cannot make gold by steady fusion. [2 marks]
(b) Name the process that makes the gold, and the places where it happens. [1 mark]
- Up to iron (1 mark): You say fusing nuclei lighter than iron releases energy, which supports the star.
- Past iron (1 mark): You say fusing iron takes in energy, so the core loses support and collapses.
- Gold (1 mark): You name rapid neutron capture in a supernova or a neutron star collision.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. The Big Bang made hydrogen, helium and a trace of .
2. Fusion inside stars builds elements up to .
3. A star needs a large to get hot enough to make heavier elements.
4. Elements heavier than iron form in a or when neutron stars collide.
5. The blast scatters new elements into space, where new and planets form.
A student says: “The Sun has fused for 4.6 billion years. Given enough time, any star makes any element, even gold.”
Explain what is wrong with this idea.
Think about what sets which elements a star can make. Write your answer first, then compare it with the model answer.
The student is wrong. Time does not heat a core. The Sun’s mass sets its core temperature, and that temperature is too low to build iron or gold.
Only massive stars get hot enough to build elements up to iron. Gold is heavier than iron, so no star makes it by fusion. It forms in a supernova or when neutron stars collide.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Astronomers study the gas around a young star cluster. The gas contains carbon, oxygen, iron and gold. A student says: “The Sun made all of these elements in the past 4.6 billion years.”
(a) Say whether the Sun made these elements. Give the reason, using the Sun’s mass and core temperature. [1 mark]
(b) Say where the iron and the gold formed. [2 marks]
(c) Say how these elements reached the gas around the cluster. [1 mark]
- The Sun (1 mark): You say the Sun’s core is too cool to make them.
- Iron (1 mark): You say iron formed by fusion inside massive stars.
- Gold (1 mark): You say gold formed in a supernova or a collision of neutron stars.
- Scattering (1 mark): You say the blasts threw the new elements into space, where the gas collected.
In August 2017, the LIGO and Virgo detectors sensed ripples in space from two neutron stars colliding 130 million light-years away. Telescopes watched the glowing debris. Its light showed that the collision made huge amounts of heavy elements, including gold and platinum. Wedding rings and electronics hold atoms made in events like it.
Adaptive Competency Practice: HS-ESS1-3
Test your understanding of how stars make elements one question at a time. The adaptive engine covers fusion inside stars, the iron limit, supernovae and neutron capture.