Many millions of years after the big bang, a point of light arose in a dark universe. This first star began to blast out radiation, which knocked electrons off the surrounding fog of hydrogen. More stars formed, turning nearly all the neutral hydrogen atoms, which had previously absorbed some wavelengths of light, into a fully transparent broth of ionised hydrogen. Light could then travel freely through the expanding cosmos: this was the end of the cosmic dark ages and the start of galaxy formation.
These first stars and the galaxies they formed were very different from anything we see…
Article amended on 6 March 2025
We clarified the extent to which hydrogen absorbed light during the cosmic dark ages.
