The first two generations of stars did not create the proper environments for planet formation or the development of life, according to new calculations that suggest the ingredients for life were not present until at least 500 million years after the Big Bang.
The first stars were made almost entirely of hydrogen and perhaps a touch of helium, scientists have long theorized. They would have been huge -- far more massive than what's possible today -- weighing in at up to 200 times the mass of the Sun.
Massive stars die quickly, exploding as colossal supernovas and spitting newly forged elements into the cosmos. The first stars probably did not live more than a few million years.
The initial explosions seeded the expanding universe with enough heavier elements, such as carbon and oxygen, to form low-mass stars similar to our Sun, according to the new model by Volker Bromm and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics.
The research was presented last week in Atlanta at a meeting of the American Astronomical Society. It was also outlined in an October issue of the journal Nature.
But those first Sun-like stars -- similarly low in mass, size and temperature -- were not solar twins. They didn't have enough metals and other heavy elements to spawn planets.
"The first low-mass stars were lonely places," Loeb said. "It took many supernova explosions to make all the heavy elements we find here on Earth and in our Sun and our bodies."
The window for life "opened sometime between 500 million and 2 billion years after the Big Bang," Loeb said. That does not mean life did begin then, it just means the conditions would have been in place. Scientists do not yet know if life exists anywhere besides Earth. Most experts, however, suspect it probably does.
The universe is about 13.7 billion years old.
Going further back in time, there were no stars when the universe first began to unfold. Hydrogen was initially compressed and hot, and it had to expand and cool before stars could form. Only last year did observational evidence pin down when the first stars were born. NASA's WMAP observatory, examining microwave background radiation leftover from that early time, allowed scientists to determine the first star birth occurred about 200 million years after the Big Bang.
Astronomers don't have enough information to pin down with certainty the timing of the first planets, and hence when life might have formed elsewhere, any better than what Loeb and Bromm have outlined.
The new calculations by Loeb and Bromm are in line with discoveries of modern planets around other stars, findings that suggest metal-rich stars are a key to planet formation.
Earth and its metal-rich Sun have been around for about 4.5 billion years. The planet formed from the leftovers of star formation, stuff that swirled around the newborn Sun and gradually stuck together.
"Were now just beginning to investigate the metallicity threshold for planet formation, so its hard to say when exactly the window for life opened," Bromm said. "We owe our existence in a very direct way to all the stars whose life and death preceded the formation of our Sun. And this process began right after the Big Bang with the very first stars."
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