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First stars and galaxies
first stars c. 13.6 billion years ago (100–200 million years after the Big Bang); galaxies by c. 13.5 billion (seen by the Webb telescope)· Event· Universe & Earth
Mostly agreedGood evidence; some details, dates or numbers uncertain.
The first stars lit up roughly 100–200 million years after the Big Bang and gathered into the first small galaxies.
See it on the History map →What happened
Gravity, helped by unseen dark matter, pulled clouds of hydrogen and helium together until their cores grew hot enough for nuclear fusion. The first stars were probably huge, perhaps tens to hundreds of times the Sun's mass, and short-lived. Their light ended the cosmic 'dark ages'. Small galaxies formed and merged. The Webb telescope has spotted galaxies from only about 300 million years after the Big Bang.
Why it matters
Stars turned the simple gas of the Big Bang into light, heat and new elements; galaxies became the cities where later stars and planets formed.
How we know
Computer simulations plus deep images from the Hubble and James Webb space telescopes (Webb launched in 2021). No first-generation star has yet been seen directly.
Still debated
Exactly when the first stars formed, how massive they were, and why Webb finds so many bright galaxies so early are open questions.
Inside this (1)
Connections (4)
▸First atoms and the cosmic background light clumped under gravity into First stars and galaxiesWell documented
The connection
Neutral atoms no longer got pushed around by light, so gas could clump. Gravity, aided by invisible dark matter, pulled the slightly denser patches, seen as ripples in the background light, into clouds. Over 100–200 million years those clouds collapsed and grew hot enough to ignite as stars.
When and how
Across the universe, during the 'cosmic dark ages', from about 380,000 to 100–200 million years after the Big Bang.
How we know
Computer simulations that start from the measured ripples in the background light reproduce the pattern of galaxies observed today.
Debates and open questions
What dark matter actually is remains unknown. Exactly when and how the first clouds collapsed is still being tested with the Webb telescope.
▸First stars and galaxies began the Forging of heavy elements in starsWell documented
The connection
The first stars, massive and short-lived, fused hydrogen and helium into carbon, oxygen and heavier elements in their cores. Within a few million years they exploded, scattering these new elements into space, where they mixed into the gas that formed the next generation of stars.
When and how
In the early universe, starting more than 13 billion years ago and continuing in every generation of stars since.
How we know
The oldest known stars contain tiny amounts of heavy elements, apparently inherited from first-generation stars that had already exploded.
Debates and open questions
No first-generation star has been seen directly, so how big they were and how they died is inferred, not observed.
▸First stars and galaxies merged over time to build The Milky Way galaxyMostly agreed
The connection
Small early galaxies, each a swarm of stars and gas, were drawn together by gravity and merged into larger ones. The Milky Way grew this way over billions of years, absorbing many dwarf galaxies. It is still at it, slowly tearing apart the Sagittarius dwarf galaxy today.
When and how
In our corner of the universe, from about 13 billion years ago; the major Gaia-Enceladus merger happened about 10 billion years ago.
How sure are we?
The Gaia satellite traced streams of stars moving together, the remains of swallowed galaxies; their chemistry and ages match.
Debates and open questions
How many galaxies the Milky Way absorbed, and when its disk took shape, are still being worked out.
▸The Big Bang made the hydrogen and helium that fueled First stars and galaxiesWell documented
The connection
In its first few minutes, the Big Bang produced a mix of about three-quarters hydrogen and one-quarter helium, plus a trace of lithium. That was nearly all the matter available, so the first stars formed entirely from it, fusing hydrogen into helium for energy.
When and how
Across the universe: the elements formed within the first 20 minutes or so; the first stars ignited 100–200 million years later.
How we know
Pristine gas clouds, barely touched by stars, contain about 25 percent helium by mass, matching Big Bang calculations begun by Alpher and Gamow in 1948 and refined in the 1960s.
Debates and open questions
Theory predicts about three times more lithium than old stars actually show; this 'lithium problem' remains unsolved.