What is nucleosynthesis, and what did we start with?
Nucleosynthesis means making new atomic nuclei by joining smaller ones. It happened in two main places: the early universe and the inside of stars.
In the first few minutes after the Big Bang, the universe was hot enough to fuse protons and neutrons. It then cooled and fusion stopped. The result: about three quarters hydrogen and one quarter helium (by mass), with tiny amounts of lithium. Nothing heavier was made in useful amounts. So the first stars had no carbon, no oxygen, no iron.
How a star makes helium from hydrogen
Gravity squeezes a cloud of gas. The core of a young star becomes very hot (more than about 10 million degrees Celsius). Hydrogen nuclei are all positive, so they push each other away. Only at this great heat and pressure do they get close enough to fuse.
Net result: 4 hydrogen nuclei (protons) become 1 helium nucleus. The helium nucleus is a little lighter than the 4 protons together. That small missing mass turns into energy (E = mc²), which makes the star shine. A star like the Sun spends about 90% of its life doing only this.
Bigger stars build heavier atoms: carbon to iron
When the hydrogen in the core runs low, the core shrinks and heats up. If the star is heavy enough, new fusion starts, one fuel after another:
- Helium fuses to carbon and oxygen (about 100 million degrees).
- Carbon, neon, oxygen fuse to atoms such as magnesium, silicon and sulphur.
- Silicon fuses to iron and nickel (billions of degrees).
Each new fuel burns faster than the one before: hydrogen lasts millions of years, silicon only about a day. The star ends up like an onion with layers: hydrogen outside, then helium, carbon-oxygen, silicon, and an iron core.
A star like our Sun is too light for this. It stops at carbon and oxygen and ends as a white dwarf.
Why fusion stops at iron, and the explosion
Fusing small nuclei gives out energy. Iron-56 is one of the most tightly held nuclei there is, so fusing iron costs energy instead of giving it. The core can no longer hold up the star. It collapses in a fraction of a second and the outer layers bounce off it: a supernova.
In that blast, many neutrons are made and nuclei catch them quickly. This builds atoms heavier than iron. Colliding neutron stars also do this and are a main source of elements such as gold. The explosion throws all the new atoms into space, mixed with gas that will form new stars.
Solar system materials: we are star stuff
The Sun and planets formed about 4.6 billion years ago from a cloud that earlier stars had already seasoned with carbon, oxygen, silicon, iron and more. That is why the Sun is mostly hydrogen and helium but about 2% heavier elements, and why rocky Earth is mostly iron, oxygen, silicon and magnesium.
Hydrogen in water came from the Big Bang. Most of the carbon in your body, the oxygen you breathe and the iron in your blood came from dead stars. In this sense we are made of star dust.
Try it: trace one atom
Open the 3D and go to the last step. Pick H, then He, C, Fe and Au. For each, say aloud: where was it made, and what had to happen? Then pick one element in your body or home (for example the calcium in milk) and trace its story back. Which star step made it?
Key formulas and definitions
- 4 H → 1 He + energy (stars)
- 3 He → C (about 100 million degrees)
- Si → Fe, then the core collapses (supernova)
- E = mc² (mass lost becomes energy)
- Big Bang mix: about 75% H, 25% He (by mass)
Worked examples
1. Which two elements did the Big Bang make almost entirely?
Hydrogen (about 75%) and helium (about 25%), with a trace of lithium.
2. In the Sun's core, how many protons join to make one helium nucleus? How many protons does that helium have?
Four hydrogen nuclei (4 protons) join. Helium has 2 protons, so 2 of the 4 protons are changed into neutrons on the way.
3. A star has onion layers. Put these in order from the centre outward: helium, iron, hydrogen, silicon, carbon-oxygen.
Iron, silicon, carbon-oxygen, helium, hydrogen. The heaviest, hottest products are at the centre.
4. Four hydrogen nuclei have a mass of 4.0291 u. The helium nucleus formed has a mass of 4.0015 u. How much mass becomes energy?
4.0291 − 4.0015 = 0.0276 u. This lost mass is released as energy by E = mc².
5. Why can't the Sun make iron?
The Sun is too light. Its core never gets hot enough to fuse carbon and beyond. It will stop at carbon and oxygen and become a white dwarf.
6. Gold is heavier than iron. Why can't fusion inside a normal star make it?
Fusing past iron takes in energy instead of giving it, so stars cannot keep it going. Gold is made when nuclei catch many neutrons quickly, in supernovae and neutron-star collisions.
Common mistakes
- Thinking stars burn like a fire. They fuse nuclei; no oxygen is needed for it.
- Saying all elements are made in stars. Hydrogen and most helium came from the Big Bang.
- Believing the Sun will end as a supernova. It is too light and will fade as a white dwarf.
- Thinking iron is the heaviest element. Fusion stops at iron, but heavier atoms are made by other events.