Atlanta, GA — To most people, the phrase "Sun-like star" calls to mind images
of a friendly, warm yellow star accompanied by a retinue of planets possibly
capable of nurturing life. But new calculations by Harvard astronomers Volker
Bromm and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics), which were
announced today at the 203rd meeting of the American Astronomical Society in
Atlanta, show that the first Sun-like stars were lonely orbs moving through a
universe devoid of planets or life.

"The window for life opened sometime between 500 million and 2 billion years
after the Big Bang" says Loeb. "Billions of years ago, the first low-mass stars
were lonely places. The reason for that youthful solitude is embedded in the
history of our universe."

In The Beginning

The very first generation of stars were not at all like our Sun. They were
white-hot, massive stars that were very short-lived. Burning for only a few
million years, they collapsed and exploded as brilliant supernovae. Those very
first stars began the seeding process in the universe, spreading vital elements
like carbon and oxygen, which served as planetary building blocks.

"Previously, with Lars Hernquist and Naoki Yoshida (also at the CfA), I have
simulated those first supernova explosions to calculate their evolution and how
much heavy elements (elements heavier than hydrogen or helium) they produced,"
says Bromm. "Now, in this work, Avi Loeb and I have determined that a single
first-generation supernova could produce enough heavy elements to enable the
first Sun-like stars to form."

Bromm and Loeb showed that many second-generation stars had sizes, masses, and
hence temperatures similar to our Sun. Those properties resulted from the
cooling influence of carbon and oxygen when the stars formed. Even elemental
abundances as low as one-ten thousandth those found in the Sun proved sufficient
to allow smaller, low-mass stars like our Sun to be born.

Yet those same low abundances prohibited rocky planets from forming around those
first Sun-like stars due to a lack of raw materials. Only as further generations
of stars lived, died, and enriched the interstellar medium with heavy elements
did the birth of planets, and life itself, become possible.

"Life is a recent phenomenon," Loeb states unequivocally. "We know that it took
many supernova explosions to make all the heavy elements we find here on Earth
and in our Sun and our bodies."

Recent observational evidence corroborates their finding. Studies of known
extrasolar planets have found a strong correlation between the presence of
planets and the abundance of heavy elements ("metals") in their stars. That is,
a star with higher metallicity and more heavy elements is more likely to possess
planets. Conversely, the lower a star’s metallicity, the less likely it is to
have planets.

"We’re now just beginning to investigate the metallicity threshold for planet
formation, so it’s hard to say when exactly the window for life opened. But
clearly, we’re fortunate that the metallicity of the matter that birthed our
solar system was high enough for the Earth to form," says Bromm. "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. As the universe evolved, it progressively seeded itself
with all the heavy elements necessary for planets and life to form. Thus, the
evolution of the universe was a step-by-step process that resulted in a stable
G-2 star capable of sustaining life. A star we call the Sun."

This research was published in the October 23, 2003, issue of the scientific
journal Nature.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists, organized into
six research divisions, study the origin, evolution and ultimate fate of the

Note to Editors:

High resolution images to accompany this release are online at: