During the first few seconds of the universe, it was so dense and hot that particles crashed into each other in a chaos of creation and annihilation. By 100 seconds, the universe had cooled to one billion degrees, the temperature inside large stars, and protons and neutrons began to cling together and form the lightest atoms, especially helium. As the universe expanded, areas of slightly greater density began to coalesce from gravity until galaxies formed. Inside them, gas clouds continued to contract into stars dense enough for nuclear fusion to take place, which radiated enough energy to stop the star from collapsing further. Bigger stars burn hotter, use up their nuclear fuel faster, and create many of the heavier elements. When stars run out of nuclear fuel, they contract further. The biggest stars contract so violently that they explode much of their mass out into space, where they combine with nearby gas clouds to form new stars like Earth’s sun. Some of the heavier elements formed and dispersed by giant stars coalesce into planets like Earth. Life began here as macromolecules, some of which could replicate themselves. The ones that did better at it survived longer; ever since, life forms that do better at surviving and reproducing tend to become dominant.