Imagine a night sky without the twinkling light from stars and galaxies piercing the darkness. Astronomers believe that this was the bleak picture of the universe for at least a hundred million years after the "big bang," a tremendous explosion that produced time, space, and matter about 14 billion years ago.
Although no stars and galaxies existed just after the big bang, the young cosmos was anything but dull. It was humming with activity.
The big-bang explosion spewed intense radiation and energy. In the beginning, physical conditions were so extreme that matter as we know it today did not exist. When the universe was about one millionth of a second old, the temperatures and densities dropped enough for protons and neutrons the building blocks of atoms to form. Within the next few minutes, the nuclei of light elements, such as hydrogen, helium, lithium, and boron, were created. When the universe cooled to about 5,400 degrees Fahrenheit (3,000 degrees Centigrade), atomic nuclei could finally capture electrons to form atoms. By 300,000 years, the universe was made up mostly of clouds of hydrogen and helium atoms.
As the universe expanded and cooled, some regions of space amassed slightly higher densities of hydrogen. As millions of years passed, the slight differences grew large, as dense areas drew in material because they had more gravity. Researchers have dubbed this period of coalescing the "dark ages."
The dawn of light, called the "cosmic renaissance," began as hydrogen collapsed into small areas, eventually reaching the point at which the effect of gravity became great enough to trigger nuclear fusion reactions and form the first stars. These first-generation stars were probably born at least 100 million years after the big bang.
Today, astronomers who study distant galaxies are beginning to probe the cosmic renaissance. Roughly a thousand galaxies have been identified whose light left them when the universe was about one billion years old. At that epoch stars were forming at a rate about 10 times higher than in the present-day universe. Stars in that early epoch were making heavier elements, such as carbon and oxygen, which mixed with pristine gas from the big bang to create successive generations of stars.
At greater distances (looking farther back in time) our view becomes murkier; the number of known galaxies is smaller and their distances are more uncertain. Using the deepest images from the largest ground- and space-based telescopes, astronomers are just now beginning to see into the era when the first stars may have formed.