First Stars
For 200 million years the universe was dark. No stars. No light. Just hydrogen and helium slowly falling together under gravity. Then the first stars switched on — and everything changed.
The Cosmic Dark Ages
After the Big Bang, the universe cooled into a dark, featureless fog of hydrogen and helium gas. For roughly 200 million years — longer than dinosaurs existed on Earth — there was no light anywhere. Astronomers call this the Cosmic Dark Ages. Then gravity began pulling clouds of gas together until they compressed so tightly that nuclear fusion ignited. The first stars were born.
Stars unlike anything today
The first stars were nothing like our Sun. They were enormously massive — possibly hundreds of times larger — because they formed from pure hydrogen and helium with no heavier elements to regulate the process. These monster stars burned furiously and died young, exploding as supernovae after just a few million years.
You are made of dead stars
Inside those first stars, hydrogen and helium were being fused into heavier elements — carbon, oxygen, nitrogen, iron, calcium. When the stars exploded, they scattered those elements across space. The next generation of stars formed from this enriched material. And the planets that formed around those stars — including Earth — were built from stellar leftovers. Every atom in your body heavier than hydrogen was forged inside a star that died billions of years ago.
Seeing the first light
The James Webb Space Telescope, launched in 2021, is now detecting some of the earliest galaxies ever observed — formed just a few hundred million years after the Big Bang. The light from those galaxies has been travelling for over 13 billion years to reach us. When we look at the deep sky, we are looking back in time.
The reionisation epoch marks the period when the first stars and galaxies produced enough ultraviolet radiation to ionise the surrounding hydrogen gas, clearing the cosmic fog and making the universe transparent. This transition, which completed about one billion years after the Big Bang, is one of the major phase changes in cosmic history. The 21cm hydrogen line — detectable by radio telescopes — is one of the key signatures astronomers use to study this period.