The Cosmic Origin of Elements
From primordial hydrogen to heavy elements forged in the stars
The Big Bang
13.8 billion years ago
In the first moments after the Big Bang, the universe was extremely hot and dense. During the first few minutes, primordial nucleosynthesis created the first atomic nuclei.
Elements formed (H → Be)
Hydrogen (H)
~75% of the ordinary matter in the universe. The first and simplest element.
Helium (He)
~25% of matter. Formed by fusion of protons in the primordial plasma.
Lithium (Li)
Tiny traces created during primordial nucleosynthesis.
Beryllium (Be) and Boron (B)
Minute quantities formed by cosmic ray spallation.
Key point: After 20 minutes, the universe had cooled enough for nucleosynthesis to stop. All elements heavier than beryllium had to wait for the stars.
Stellar Fusion
From carbon to iron
Stars are true cosmic forges. In their cores, extreme temperatures and pressures enable nuclear fusion, progressively creating heavier and heavier elements.
Fusion stages
1. Hydrogen fusion (H → He)
Temperature: ~15 million °C. This is the main process that powers most stars, including our Sun.
2. Helium fusion (He → C, O)
Temperature: ~100 million °C. The triple-alpha process creates carbon, the basis of life, and the oxygen we breathe.
3. Intermediate elements (Ne, Mg, Si, S, Ca...)
Temperature: 500 million – 3 billion °C. In massive stars, the inner layers successively burn different elements in an onion-shell structure.
4. Fusion up to iron (Fe)
Temperature: >3 billion °C. Iron is the end point: fusing it consumes energy instead of producing it.
Iron limit: Iron-56 is the most stable nucleus. No normal stellar fusion can create heavier elements in an energetically favorable way.
Supernovae
From iron to uranium and beyond
When a massive star exhausts its fuel, its iron core collapses in a fraction of a second, triggering a cataclysmic explosion: the supernova. It is in this spectacular event that the heaviest elements are born.
Explosive nucleosynthesis processes
r-process (rapid neutron capture)
In the explosion, an intense neutron flux bombards iron nuclei. Neutrons are captured so rapidly that nuclei don't have time to decay, creating very heavy elements.
Elements created:
s-process (slow neutron capture)
In the final phases of red giant stars, neutrons are captured more slowly, allowing beta decay between captures.
Elements created:
Neutron star mergers
Neutron star collisions (kilonovae) also create extreme conditions for forging the heaviest elements, particularly gold and platinum.
Cosmic dispersal: The supernova explosion ejects these newly formed elements into interstellar space, where they mix with gas clouds and will form new stars, planets... and us.
We are stardust
Every atom in our body was forged in the heart of a star or during a supernova explosion. The oxygen we breathe, the calcium in our bones, the iron in our blood, the carbon in our DNA... all were born in the stars billions of years ago.