First starshow the universe's first light switched on

The universe's first light came from the first star, lit about 150 million years after the Big Bang: a giant of hundreds of solar masses. Before that, dark matter halos had already formed at around 100 million years, and their gravity wells began to gather gas. Once these giants lit up, their starlight reionized the neutral hydrogen, and when they died they scattered heavy elements into space for the first time.

This era runs from 150 million to 1 billion years, and it saw both the birth of stars and the appearance of black holes and quasars.

Six events

• 100 million years · Dark matter halos form: Gravity wells begin to gather gas. Think of an invisible giant bowl: the gas slides down its walls and piles up at the bottom.
• 150 million years · First star: A giant of hundreds of solar masses ignites.
• 200 million years · Cosmic reionization: Starlight ionizes the neutral hydrogen again.
• 200 million years · First supernova: Heavy elements are scattered into space for the first time. Stars do more than shine; they are the first "alchemy furnaces".
• 300 million years · First black hole: The core of a giant star collapses into a black hole.
• 700 million years · Quasars: Supermassive black holes shine as they swallow matter.

These steps depend on each other. Without gravity wells, gas cannot gather. Without stars, there are no supernovae and no later black holes. Without black holes, there are no quasars. The years are approximate; in reality these processes happened continuously and overlapped.

The unseen gravity wells: dark matter

The earliest clue came from galaxy clusters. Galaxies in the Coma Cluster move too fast, so someone proposed "dark matter", an idea that was neglected for decades (Zwicky, Helvetica Physica Acta, 1933). The evidence came back in 1970: the outskirts of the Andromeda galaxy rotate too fast, and a large amount of unseen matter must supply the gravity. Dark matter was no longer just a guess (Rubin and Ford, The Astrophysical Journal, 1970).

More direct evidence came when two galaxy clusters collided: the gas stayed in the middle while the center of gravity ended up on either side, so the dark matter was "flung" out. The paper's title calls it a "direct empirical proof" of the existence of dark matter (Clowe et al., The Astrophysical Journal Letters, 2006).

Why stars shine, and how they die

How can the Sun burn for billions of years? Stars shine by nuclear fusion, a puzzle worked out in a paper titled "Energy production in stars" (Bethe, Physical Review, 1939). A star's fate after death also has a limit: a white dwarf has a maximum mass of about 1.4 solar masses, and beyond it the star is bound to keep collapsing (Chandrasekhar, The Astrophysical Journal, 1931).

Black holes: from indirect evidence to a shadow

Tracking the orbits of stars near the Galactic center for more than ten years showed that a black hole of about 4 million solar masses hides there. The work won the 2020 Nobel Prize (Ghez et al., The Astrophysical Journal, 2008). Two black holes merged 1.3 billion years ago, and ripples in spacetime were finally heard by humans, confirming Einstein's prediction from a century earlier (Abbott et al., LIGO and Virgo, Physical Review Letters, 2016). Radio telescopes around the world formed an Earth-sized "virtual telescope" and imaged the shadow of the black hole at the center of the galaxy M87 (Event Horizon Telescope Collaboration, The Astrophysical Journal Letters, 2019).

A puzzle that is still unsolved

When the universe was less than 700 million years old, there was already a black hole of 800 million solar masses. How did it grow so fast? That is still a mystery. The paper's title also specifies two conditions: a redshift of 7.5 and a "significantly neutral universe" (Bañados et al., Nature, 2018).

Want to read the originals? Search these titles

• Zwicky (1933): Die Rotverschiebung von extragalaktischen Nebeln, roughly "The redshift of extragalactic nebulae"
• Rubin and Ford (1970): Rotation of the Andromeda Nebula from a spectroscopic survey of emission regions
• Clowe et al. (2006): A direct empirical proof of the existence of dark matter
• Chandrasekhar (1931): The maximum mass of ideal white dwarfs
• Bethe (1939): Energy production in stars
• Ghez et al. (2008): Measuring distance and properties of the Milky Way's central supermassive black hole with stellar orbits
• Abbott et al. (2016): Observation of gravitational waves from a binary black hole merger
• Event Horizon Telescope (2019): First M87 Event Horizon Telescope Results. I. The shadow of the supermassive black hole
• Bañados et al. (2018): An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5

How the game shows it

In the game, the time range of the First stars era is "150 million – 1 billion years" and its resource is stars. The generators are Dark matter halos, Gas collapse, Protostars and Population III giants, in roughly the order of the "from gravity well to star" process above.