The universe lights up
A major clue comes courtesy of the cosmic microwave background (CMB), the relic radiation that formed in the hot, early universe. This radiation has been cooling ever since it was emitted as the cosmos expands — currently, the back-ground temperature is about 2.73 kelvins (–455 F, –270 C). And measurements of the CMB show that it is incredibly consistent, corresponding to density variations of only 1 part in 100,000. But those variations, literal ripples in the structure of the universe, are revealing how the first stars formed.
Computer models show that the minuscule density fluctuations in the early universe acted as starting points for immense clouds of gas. Without these variations in structure, nothing would have formed. The whole cosmos would have evolved into an ever-thinning homogeneous cloud of hydrogen, helium, and that tiny bit of lithium. Thanks to gravity, however, the fluctuations became gathering points: huge clouds where gas continued to collect. Eventually, the clouds contracted. As they did, they heated up to more than 1,300 F (700 C).
That temperature would be far too high for a star-forming region today to form stars. Indeed, if a cloud is hotter than about 10 kelvins (–442 F, –263 C), the speed of the atoms inside it will be too fast for them to stick together and eventually form stars.
But the clouds in the early universe were larger and much more densely packed than modern-day nebulae. Within them, some hydrogen atoms paired up to become hydrogen molecules. And because molecules are better emitters of infrared radiation (heat), the temperature dropped and clumps inside the clouds could contract further.
Each of the regions was probably several hundred times as massive as the Sun. That much mass, and its corresponding gravity, could overcome the outward pressure from radiation. The clumps didn’t split as they contracted, so only a single star formed from each one. The result was that the first stars were potentially colossal — estimates range from several tens of solar masses up to 1,000 solar masses — and luminous, perhaps millions of times as bright as the Sun.
Because the universe was smaller and denser, vast numbers of these stars formed near each of its density variations. Eventually, the gravitational pull from these stars would attract other stars, and the numbers grew from there. Astronomers think this took a few hundred million years, but, at the end of that time, the first galaxies had formed.