from atoms into electrons and protons—a period known as the epoch of reionization. Scientists think that the first stars were massive and short-lived. They quickly exploded as supernovae, creating and dispersing the first elements with nuclei heavier than those of hydrogen, helium, and lithium, and leaving behind the first black holes. Astronomers must now search the sky for these infant galaxies and find out how they behaved and interacted with their surroundings.
After the cosmic dawn, more and more galaxies formed, merged, and evolved as their gas turned into stars and those stars aged. Many of the faintest images from current telescopes are of these growing infant galaxies. Their properties are just starting to be revealed. In particular, it is now known that such galaxies quickly grow black holes in their nuclei with masses that can exceed a billion times the mass of the Sun and become extraordinarily luminous quasars. How this happens is a mystery.
We also know that the giant galaxies we see around us today were built up from the mergers of smaller galaxies and the accretion of cold gas. Not only do the stars and gas commingle, but the central black holes also merge. Amazingly, it should be possible to detect waves in the fabric of space-time—gravitational waves—that result from the dramatic unions when galaxies and black holes are young and relatively small.
Another approach to understanding our cosmic dawn is to carry out “cosmic paleontology” by finding the rare stars that have the lowest concentrations of heavy elements and were formed at the earliest times. Today, we can scrutinize only stars in our galaxy; in the future, we will be able to explore other nearby galaxies to uncover stellar fossils and use them to reconstruct the assembly of young galaxies.
Exploring the first stars, galaxies, and quasars is a tremendous challenge, but one astronomers and astrophysicists are ready to tackle and overcome, thereby continuing the story of how our universe came to be.
On Christmas Eve, 1968, Apollo 8 astronaut William Anders took an iconic photograph of the rising Earth from his vantage point orbiting the Moon. It highlighted, to more people than ever before, that we humans share a common home that is both small and fragile. It also brought into focus the question, What does Earth look like from much farther away? Remarkable discoveries over the past 15 years have led us to the point that we can ask and hope to answer the question, Can we find another planet like Earth orbiting a nearby star? To find such a planet would complete the revolution, started by Copernicus nearly 500 years ago, that displaced Earth as the center of the universe.
Almost two decades ago, astronomers found evidence for planets around neutron stars, and then, in 1995, a star just like the Sun in the constellation Pegasus