We are stardust
The universe began with almost nothing but hydrogen and helium. See where the carbon, oxygen, calcium and iron in your body were actually made, and how anyone could possibly check that.
THE STARTING KIT
The universe began almost empty of ingredients
In its first few minutes the universe was hot and dense enough to fuse nuclei, and then it expanded and cooled too far to carry on. What it left behind was roughly three quarters hydrogen and one quarter helium by mass, plus a trace of lithium. That is the entire starting kit. No carbon, no oxygen, no iron, no gold.
You met this number in lesson 02, when Cecilia Payne read the spectra of stars. Lesson 10 will show how the same proportions are measured and why they are strong evidence for the Big Bang. For now, take the shortage seriously: everything on the periodic table past helium had to be built later.
The assembly line
- Hydrogen to helium
Every ordinary star does this, for most of its life. It makes no new kind of atom that the universe did not already have plenty of.
- Helium to carbon and oxygen
In a red giant's squeezed core, three helium nuclei stick together to make carbon, and adding one more helium makes oxygen. Two of the most important elements for life, made in a dying star's middle.
- Up to iron, in a massive star
Only a heavy star can squeeze its core hot enough for the next steps: neon, magnesium, silicon, and finally iron. Its inside ends up layered like an onion, each shell fusing a different element.
- The explosion scatters it
A supernova throws all of those layers out into space at thousands of kilometres per second. Without the explosion the elements would stay locked in a dead core forever. Dying is the delivery.
- Heavier than iron
Gold, platinum and uranium cannot be made by ordinary fusion, because past iron fusion costs energy. They come from nuclei soaking up loose neutrons, which needs conditions as extreme as two neutron stars colliding.
Check yourself
Babak says: 'If stars make the elements, the oldest stars should be the richest in iron, since they have had the longest to work.' Where does his reasoning go wrong?
- Old stars do have the most iron; the reasoning is fine
- Stars destroy iron as they age
- A star keeps what it makes; only what it throws out at death enriches later stars
- Iron is made in the cold space between the stars, not inside them, so a star's age changes nothing
Show the answer
A star keeps what it makes; only what it throws out at death enriches later stars
Right. A star is born from whatever gas was around at the time. The oldest stars formed from nearly pure hydrogen and helium, and that is exactly what their spectra show. Later stars formed from gas that earlier stars had already enriched.
Metals (in astronomy)
Astronomers call every element heavier than helium a metal. Carbon is a metal. Oxygen is a metal. It is a strange habit, but it makes sense from their side: there is hydrogen, there is helium, and then there is the small remainder that stars had to build.
A star's metallicity is how much of that remainder it contains. The Sun's is a bit under 2% by mass. The oldest stars in our galaxy have thousandths of that, which is how we know they were born long before most of the building had been done.
| In you or around you | Where those atoms were built |
|---|---|
| Hydrogen, most of your water | The first minutes of the universe |
| Carbon in every cell | Red giants, from three helium nuclei |
| Oxygen you breathe | Massive stars, before they exploded |
| Iron in your blood | Exploding stars |
| Gold in a ring | Colliding neutron stars, and perhaps rare explosions |
Check yourself
Match each element to the furnace that built it
Show the answer
- Helium in a party balloon → Mostly the first minutes of the universe
- Carbon in a pencil → The core of a red giant
- Calcium in your bones → A massive star and its explosion
- Platinum in a catalytic converter → Nuclei soaking up neutrons in a collision
THE CYCLE
The Sun is not a first-generation star
A cloud collapses and makes stars. The heavy ones die within a few million years and blow enriched gas back into the cloud. The next generation forms from slightly richer material, and does it again. Our Sun contains a couple of percent of elements that only stars can make, so the gas it formed from had already been through at least one round, and probably many.
The Earth is the leftovers. Rock, water, the air, your body: all of it is material that earlier stars made and returned. A supernova somewhere in this galaxy is the reason there was any iron here to become a planet's core, or your blood.
Check yourself
The Sun is making the carbon and oxygen that make up most of your body right now, and sending it to Earth in its light.
Show the answer
False
False, twice over. The Sun is only fusing hydrogen into helium at the moment; carbon comes later, in its red giant stage. And even then it will keep those atoms until it dies. The carbon and oxygen in you were made by other stars that died before the Sun was born, and were part of the cloud the whole solar system formed from.
Check yourself
- Star A sits in the outer halo of our galaxy. Its spectrum shows hydrogen and helium and almost nothing else: less than a thousandth of the Sun's iron.
- Star B sits in the galaxy's disc, in a region where stars are still forming today. Its spectrum shows slightly more iron than the Sun.
What can you conclude about the two stars?
- Star B is older, because it has had more time to make iron
- Star A is much older: it formed from gas that few stars had enriched yet
- Star A is closer to us than star B
- Star A is hotter, which destroyed its iron
Show the answer
Star A is much older: it formed from gas that few stars had enriched yet
Yes. A star wears the composition of the gas it was born from. Almost no metals means almost nothing had died yet, so star A is ancient. Star B was born recently, from gas that generations of stars had already seasoned.
The chain, in one breath
- The Big Bang made hydrogen and helium and stopped there.
- Stars fused their way up to iron, each step needing a heavier star than the last.
- Dying stars returned the results: gently as a planetary nebula, or violently as a supernova.
- Elements heavier than iron needed something more extreme still, such as two neutron stars colliding.
- The Sun, the Earth and you are made of the returned material. Every atom in you except the hydrogen was built inside a star.
Check yourself
Leila accepts that stars build heavy elements, but asks why that helps us at all, since the elements are made deep inside a star. What is the missing piece?
- Stars slowly leak their heavy elements into space through the light they give off, a little each year
- The star's death returns the material to the gas that forms new stars and planets
- Planets form inside stars and are later pushed out
- Heavy elements travel from star to star through gravity
Show the answer
The star's death returns the material to the gas that forms new stars and planets
Right. Making the elements is only half the job. Without a planetary nebula or a supernova to scatter them, they would sit in a dead core for ever. The Earth exists because earlier stars both built and released.
Lesson recap
- The Big Bang produced only hydrogen, helium and a trace of lithium. Everything else was built afterwards.
- Red giants make carbon and oxygen; massive stars go on up to iron; nothing heavier can come from ordinary fusion.
- Gold and platinum need neutron capture in extreme events, and a neutron star merger was watched doing it in 2017.
- Stars must die to deliver: a planetary nebula or a supernova returns the new elements to the gas between the stars.
- A star's metal content tells you when it was born, and the Sun's tells us it formed from well-recycled material.