The Cosmos

#ww-before-4bya

~13.4 — 12 BYA

First Large-Scale Structure

• Ignition of First stars (Pop III):
  • Population III stars remain theoretically unobserved despite strong indirect evidence.
• Reionization begins: the ultraviolet light from early stars ionizes hydrogen.
• First galaxies:  Form from dark matter halos.

~12 — 8 BYA

Galaxies

• Galaxies grow: Through mergers and star formation.
• First generations of stars:
  • Synthesis of heavy elements (carbon, oxygen, iron) via nuclear fusion and supernovae.

Heavy Elements

• Heavy elements: Seed future planetary systems.
• The Milky Way: Forms and evolves during this period.

~8 — 4.6 BYA

Galactic Stabilization

• Star formation rate: Peaks and then declines.
• Spiral galaxies (like the Milky Way): Develop stable structures.
• Multiple generations of stars:
  • Enriched interstellar space with metals.
  • Conditions for rocky planets become common.

Posted: Jun 21, 2026