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Because they were so hot, the first stars would have poured out not just visible light, but also copious amounts of high-energy ultraviolet radiation. One effect of that radiation would have been to knock apart hydrogen atoms, thus destroying their ability to block light. That process is known as reionization, and those stars, forming perhaps 100 million years into the Dark Ages, or roughly at the era's midpoint, might have rendered the universe transparent on their own if they had lived long enough. But unlike the sun, which has survived 5 billion years so far and should live another 5 billion, those stars lasted only a paltry million years. If the first stars formed 100 million years after the Dark Ages began, they were gone by 101 million years. As they died, the smaller of the stars exploded and spewed their contents back into space, while the bigger ones formed black holes.
BURNING AWAY THE COSMIC FOG
As in any other fusion reaction, the fires that powered these short-lived stars worked by forcing simple hydrogen and helium atoms to meld into heavier, more complex elements. The stars that died explosively spiked the surrounding gas clouds with elements like oxygen and carbon, which had never existed before. Billions of years later, the elements forged in stars like these would be assembled into planets, organic molecules and, ultimately, human beings. At the time, though, they served simply to change the chemistry of the clouds, allowing them to collapse into far smaller objects than they could before. The second generation of stars, incorporating the ashes of the first, arose almost immediately. They were much more like the sun, in both composition and size. And like the sun, they would have started out generating lots of ultraviolet light before settling down to a more sedate existence.
It's this radiation--the ultraviolet light from hot, newly formed stars--that many theorists suspect finally reionized the remaining hydrogen, making it transparent again and bringing the Dark Ages to a close. Others suggest that the process may have been powered instead by black holes spewing out X-rays and ultraviolet light. Or it may have been a combination of hot stars and black holes that cleared the hydrogen and put an end to the Dark Ages.
In any event, the accepted scenario is that this new generation of small galaxies, containing no more than a million second-generation stars, gradually collided, merging to form ever bigger objects that eventually reached the size of the Milky Way. One piece of evidence: the faintest and oldest galaxies found in any great number by the Hubble telescope tend to be small and irregular in shape, not the majestic spirals and huge elliptical galaxies that formed later. Another hint that the merger theory is correct is that the collisions are still going on today. Astronomers can see hundreds of colliding galaxies in their telescopes, and our own Milky Way is still slowly gobbling up the half a dozen or so dwarf galaxies that surround it.