The first stars. Artwork of the first stars in the universe, thought to have formed over 13 billion years ago, when the universe was only a few million years old. As the universe cooled after the Big Bang, atomic nuclei began to form. After 300,000 years these nuclei combined with electrons to form atoms (mainly hydrogen) and the universe became neutral and opaque (dark ages, bottom). Within a million years dense regions of atoms attracted nearby matter, forming gas clouds and, after 300 to 600 million years, stars and small galaxies began to form. The stars were huge and hot, such as the early supermassive star at right, and lived comparatively short lives before dying in supernova explosions.