When the universe was young, galaxies were much smaller amorphous blobs of stars and gases. It was only over time that they merged to form bigger and shapelier blobs, gradually acquiring the contours that have become familiar. Now a team of observers, using several of the world’s most powerful telescopes, reports that they’ve seen this process in its earliest stages, as light from ancient stars has reached us after thirteen billion years. They’ve captured images of three small galaxies in the act of merging only nine hundred million years after the Big Bang, when the universe was only about seven per cent of its present age.
The first stars, and the galaxies that harbored them, were the earliest steps in the process of cosmic evolution that led to the modern universe. For astronomers, finding the first stars would be the equivalent of biologists finding the first organisms that arose on Earth.
According to the standard model of cosmology, the universe faded from the incandescent brilliance of the Big Bang into utter blackness about four hundred thousand years after the cosmos burst into existence. It was a nearly uniform sea of cold hydrogen and helium gas. Over the course of several hundred million years, some of that gas collapsed into distinct clumps. Inside those clumps, the very first stars flared into existence in what astronomers call the Cosmic Dawn.
Theorists believe that those early stars were huge, up to a hundred times the mass of the Sun, and that they lived only a few million years before exploding. (The sun is now four and a half billion years old, and will live for another few billion years.) The debris from those explosions then formed into the second generation of stars, most of them more modest in size, many of which are still burning today.
The first stars were made only of hydrogen and helium, since those were the only elements produced by the Big Bang, along with a tiny bit of lithium. But as they burned, the thermonuclear reactions in their cores created heavier elements, including oxygen, nitrogen, and carbon. Any stars that came after the first generation, including the sun, have detectable traces of those heavier elements.
Once ALMA is complete, he said, “we should be able to find many more of these first-generation star systems.” And with those in hand, astronomers might finally be able understand exactly how the universe went from darkness into light.
