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The first stars
1978· Cosmic event
Well supportedStrong evidence or math, but not fully tested.
The first generation of stars, lit about 100 to 200 million years after the start and made purely of hydrogen and helium.
See it on the Science map →How it works
Astronomers call them Population III stars. With no heavier elements to help gas shed heat, clouds had to gather more mass before collapsing, so these stars were probably huge, tens to hundreds of times the Sun's mass, with surfaces near 100,000 degrees. Big stars burn fast: they lived only a few million years, then exploded or collapsed into black holes. Their light ended the dark ages and their debris carried the first carbon, oxygen and iron.
Why it matters
They lit the universe, began its chemistry, and may have planted the seeds of the giant black holes found in surprisingly young galaxies.
How we know
None seen directly. They are inferred from simulations and from Milky Way stars with almost no iron, such as one reported in 2014 with under a ten-millionth of the Sun's iron, whose makeup fits debris from one first-star explosion. JWST has reported candidate hints.
Still a mystery
How massive were they really? Could small, long-lived ones survive to today? None has been found. Did their remnants grow into the giant black holes JWST sees?
Connections (4)
▸The cosmic dark ages ends with The first starsWell supported
The relationship
Through the dark ages, dark matter clumps grew and merged. Once some reached about a million times the Sun's mass, around 100 to 200 million years, the gas inside was dense enough to form molecular hydrogen, which radiates heat away. Cooling let the gas collapse further until its densest cores ignited fusion. The first starlight marks 'cosmic dawn', the end of the dark ages.
How the link was found
Max Tegmark, Joseph Silk, Martin Rees and colleagues calculated in 1997 which clumps could cool and form stars. Tom Abel, Greg Bryan and Michael Norman simulated a first star's birth in 2002.
Is it proven?
Not directly observed. Supercomputer simulations from standard cosmology agree on this picture, and JWST galaxies at about 300 million years require that stars started earlier.
Open questions and mysteries
The start time is uncertain by about 100 million years. Gas streaming past dark matter, magnetic fields or unusual dark-matter properties could delay or speed it.
▸The first stars start Reionization: starlight clears the fogWell supported
The relationship
The first stars were so hot, with surfaces near 100,000 degrees, that most of their light was ultraviolet, energetic enough to strip electrons from hydrogen. Each carved out a bubble of ionized gas thousands of light-years across. These first bubbles were scattered, and many recombined after their star died; lasting, universe-wide reionization needed whole galaxies.
How the link was found
Volker Bromm, Rolf-Peter Kudritzki and Abraham Loeb calculated the first stars' ultraviolet output in 2001. Tom Abel, John Wise and Greg Bryan simulated their ionized bubbles in 2007.
Is it proven?
Inferred from simulations. Microwave-background polarization shows reionization was underway early, but cannot separate the first stars' share from galaxies'.
Open questions and mysteries
How much did first stars contribute compared with later galaxies or black holes? Probably a small share, but nobody knows.
▸The first stars began Stars forge the heavier elementsWell supported
The relationship
The first stars were the first factories for elements beyond helium and lithium. Their cores made carbon, oxygen and iron, and their explosions scattered them. Stars of roughly 140 to 260 Suns may have died in 'pair-instability' supernovae, blowing apart completely and leaving a distinctive chemical pattern. Even traces of these elements let later gas cool more easily, forming smaller, long-lived stars like the Sun.
How the link was found
Pair-instability explosions were predicted by Zalman Barkat, Gideon Rakavy and Nissan Sack in 1967 and tied to first stars by Alexander Heger and Stan Woosley in 2002.
Is it proven?
Ancient Milky Way stars with tiny iron show patterns matching first-star supernovae. In 2023 a star, LAMOST J1010+2358, was reported with a pair-instability pattern; that claim has been disputed.
Open questions and mysteries
The rarity of clear pair-instability patterns may mean first stars were less massive than thought. How fast their elements spread through early galaxies is unknown.
▸The first stars gather into The first galaxies (seen by JWST)Well supported
The relationship
First stars formed alone or in small groups inside minor dark matter clumps. Their explosions blew gas out and enriched it. Over tens of millions of years those clumps merged into bigger ones, of tens to hundreds of millions of Suns, whose gravity could hold hot gas and keep forming stars continuously. Those merged systems, with mixed generations of stars, are the first galaxies.
How the link was found
The 'small things merge into big things' picture comes from Simon White and Martin Rees in 1978. Volker Bromm and Naoki Yoshida reviewed how the first galaxies assembled in 2011.
Is it proven?
JWST galaxies at about 300 million years already contain oxygen, which requires earlier stars. Simulations and JWST's images of clumpy early galaxies show merging.
Open questions and mysteries
Did pure first-generation stars survive inside galaxies bright enough for JWST to see? How did the earliest galaxies grow so bright so fast?