Galaxies & elementshow stars built the periodic table

The elements of the periodic table were forged step by step in stars, and the deaths of stars scattered them into space. Three helium nuclei make carbon. Carbon burning starts to build up the oxygen that life needs. Silicon burning runs all the way to iron, where fusion ends. Heavier elements such as gold, platinum and uranium have to be forged by the r-process in neutron star mergers.

This era runs from 1 billion to 9 billion years. It is also the era when galaxies grew: they grew through collisions, and the Milky Way is a barred spiral galaxy whose stars form mostly in its spiral arms.

Seven events

• Birth of carbon (triple-alpha process): Three helium nuclei fuse into carbon.
• Birth of oxygen (carbon burning): The oxygen that life needs starts to build up.
• Iron peak (silicon burning): Fusion ends at iron. Burning by fusion alone stops here.
• r-process (neutron star mergers): Gold, platinum and uranium are forged.
• 1 billion years · Protogalaxies: Galaxies grow through collisions.
• 4 billion years · Milky Way forms: A barred spiral galaxy takes shape.
• 8 billion years · Spiral arms: Star formation concentrates in the spiral arms.

How the elements were cooked

For stars to make enough carbon, carbon-12 must have a special energy level. This was a conclusion predicted first and confirmed by experiment later. The paper deals with nuclear reactions in very hot stars and the synthesis of elements from carbon to nickel (Hoyle, The Astrophysical Journal Supplement, 1954). A few years later a hundred-page paper systematically explained how stars forge the elements of the periodic table step by step. The saying "we are all stardust" starts here (Burbidge, Burbidge, Fowler and Hoyle, Reviews of Modern Physics, 1957).

Where gold comes from

How were neutron stars discovered? Graduate student Jocelyn Bell noticed a radio signal that pulsed with split-second precision. It was first jokingly called "little green men", and later proved to be a neutron star (Hewish, Bell et al., Nature, 1968). When two neutron stars merged, humans for the first time both "heard" and "saw" the same cosmic event, which is called a kilonova (Abbott et al., Physical Review Letters, 2017). The matter thrown out by the merger contains large amounts of heavy elements, and the gold in your ring may well come from such a collision (Kasen et al., Nature, 2017).

Galaxies and dark matter

How do galaxies grow? If dark matter is "cold", meaning slow-moving, galaxies gather bottom-up step by step, which matches observation (Blumenthal, Faber, Primack and Rees, Nature, 1984). Computer simulations also showed that cold dark matter halos, from dwarf galaxies to galaxy clusters, follow the same density curve, which was later named after its three authors (Navarro, Frenk and White, The Astrophysical Journal, 1996).

Seeing galaxies clearly

The Hubble telescope stared at a small patch of "empty" sky for ten days of exposure and captured about 3,000 galaxies (Williams et al., The Astronomical Journal, 1996). The Gaia satellite measured the positions and motions of more than a billion stars and drew the most detailed 3D map of the Milky Way (Gaia Collaboration, Astronomy & Astrophysics, 2018). Even earlier galaxies have been found: the Webb telescope confirmed bright galaxies that existed only about 300 million years after the universe began, much earlier than expected. The paper is a spectroscopic confirmation of two luminous galaxies at redshift 14 (Carniani et al., Nature, 2024).

Want to read the originals? Search these titles

• Hoyle (1954): On nuclear reactions occurring in very hot stars. I. The synthesis of elements from carbon to nickel
• Burbidge et al. (1957): Synthesis of the elements in stars
• Hewish, Bell et al. (1968): Observation of a rapidly pulsating radio source
• Abbott et al. (2017): GW170817: Observation of gravitational waves from a binary neutron star inspiral
• Kasen et al. (2017): Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event
• Blumenthal et al. (1984): Formation of galaxies and large-scale structure with cold dark matter
• Navarro et al. (1996): The structure of cold dark matter halos
• Williams et al. (1996): The Hubble Deep Field: Observations, data reduction, and galaxy photometry
• Gaia Collaboration (2018): Gaia Data Release 2. Summary of the contents and survey properties
• Carniani et al. (2024): Spectroscopic confirmation of two luminous galaxies at a redshift of 14

How the game shows it

In the game, the time range of the Galaxies & elements era is "1 billion – 9 billion years" and its resource is heavy elements. The generators are Stellar fusion, Supernova explosions, Neutron star mergers and Interstellar medium recycling, named for the element assembly line described above.