History › The cosmos › First stars and galaxies
Forging of heavy elements in stars
since the first stars exploded, over 13 billion years ago· Event· Universe & Earth
Well documentedSolid records, physical evidence or scientific consensus.
Stars fuse light elements into heavier ones and scatter them when they die, seeding space with carbon, oxygen and iron.
See it on the History map →What happened
The Big Bang made almost nothing but hydrogen and helium. Inside stars, fusion builds carbon, oxygen and the rest up to iron. Many heavier elements, such as gold and uranium, form in violent events: exploding stars (supernovas) and collisions of dead neutron stars. Each generation of stars enriches the gas that forms the next. The Curious Science map covers how this works.
Why it matters
Nearly every atom in the human body except hydrogen was made inside a star. Rocky planets and life depend on these elements.
How we know
Light 'fingerprints' (spectra) show older stars hold fewer heavy elements. Burbidge, Burbidge, Fowler and Hoyle laid out the theory in 1957; a neutron-star collision seen in 2017 showed signs of forging heavy elements such as gold.
Still debated
How much of the gold and other heaviest elements comes from neutron-star collisions versus rare kinds of supernova is still debated.
Connections (3)
▸First stars and galaxies began the Forging of heavy elements in starsWell documented
The connection
The first stars, massive and short-lived, fused hydrogen and helium into carbon, oxygen and heavier elements in their cores. Within a few million years they exploded, scattering these new elements into space, where they mixed into the gas that formed the next generation of stars.
When and how
In the early universe, starting more than 13 billion years ago and continuing in every generation of stars since.
How we know
The oldest known stars contain tiny amounts of heavy elements, apparently inherited from first-generation stars that had already exploded.
Debates and open questions
No first-generation star has been seen directly, so how big they were and how they died is inferred, not observed.
▸Forging of heavy elements in stars supplied the raw material for Birth of the Sun and EarthWell documented
The connection
By 4.6 billion years ago, generations of stars had seeded our region's gas with iron, silicon, oxygen and other heavy elements. The cloud that formed the solar system held only about 2 percent of them, but that was enough to build rocky planets like Earth, with its iron core and stony crust.
When and how
In the Milky Way's disk, before and during the solar system's birth about 4.6 billion years ago.
How we know
Meteorites contain traces of short-lived radioactive atoms, such as aluminum-26, that must have come from a nearby star shortly before the solar system formed.
Debates and open questions
Whether a nearby exploding star actually triggered the cloud's collapse, or only enriched it, is debated.
▸Forging of heavy elements in stars provided the carbon and oxygen for First lifeWell documented
The connection
Life is built mostly from carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. All except hydrogen were forged in stars and reached Earth in the dust that formed it. Carbon's ability to form long chains and rings makes it the backbone of every living molecule.
When and how
Elements made in stars across the Milky Way over billions of years, then built into the early Earth about 4.5 billion years ago.
How we know
Living cells use the elements that stars make most abundantly; organic molecules have been found in meteorites and in gas clouds in space.
Debates and open questions
Whether some of life's building blocks, such as amino acids, arrived ready-made on meteorites or formed on Earth is debated.