Science News - July 30, 2011

After the Bang the universe was a quiet place for millions of years after the big bang, plunged into darkness when electrons and protons cooled enough to combine into neutral hydrogen atoms. today, scientists are reconstructing the series of events that led to the first stars, galaxies and ultimately the universe as seen today.

0 years

big bang

17 million years dark ages begin; universe is cold and dark

100 million years

first stars form

~200 million years second generation of stars begins

300–500 million years first galaxies of second-generation stars

9 billion years solar system forms

13.75 billion years today

meetings and still there’s no consensus,” notes Tumlinson.

Ground truth

As the theorists continue to debate their models, observations to test their ideas are about to begin.

from left: pederk/istockphoto; t. dubÉ; detlev van ravenswaay/photo researchers, inc.; hubble heritage team/ stsci/aura/nasa; John h. wise; nasa, Jpl-caltech, t. pyle/ssc; nasa, esa, hubble heritage team/stsci/aura

New arrays of radio telescopes will look for imprints that the first stars left behind on the clouds of hydrogen atoms surrounding them. Radio astronomers can tune in to radio waves from hydrogen atoms that existed at different epochs of the Dark Ages — before, during and after the first stars formed — thanks to shifts in wavelength caused by the expansion of the universe.

In particular, astronomers will look for radio emissions with wavelengths of 21 centimeters, which neutral hydrogen emits but ionized hydrogen cannot. If the Swiss cheese model is correct and the first stars were massive loners, observers should see the holes created when the stars broke apart the neutral hydrogen atoms.

By using 21-centimeter radiation to pinpoint if and when holes formed and merged, low-frequency radio telescopes such as LOFAR, a set of radio dishes spread across the Netherlands and other parts of Europe, will map out the history of the first stars, says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Such maps should indicate whether the first stars were massive loners after all.

Last year in Physical Review D, Loeb and his Harvard-Smithsonian colleague

Jonathan Pritchard calculated that even a relatively inexpensive single radio dish that would record the intensity of the 21-centimeter radio emission averaged over the entire sky could indicate when the first stars were born and how quickly they ionized helium and hydrogen atoms by emitting ultraviolet light or X-rays.

Other researchers are attempting to read a fossil record of the elements cast into space by the very first generation of stars. Theorist John Wise of Princeton University and his colleagues are trying to simulate the second generation of stars, dubbed Pop II, which are the first stars that got incorporated into galaxies. Because Pop II stars are small enough to be relatively long-lived, researchers can examine them to see what they inherited from their parents’ generation.

“Astronomers are actually able to see Pop II stars in galaxies” and learn about their predecessors, says Wise. In addition to giant, 30-meter ground-based telescopes that astronomers are now planning to build, the James Webb Space Telescope, which researchers hope will launch late this decade, will closely examine Pop II stars from the first galaxies.

But researchers aren’t just waiting
for Webb to be launched. Astronomers
using the European Southern Observa-
tory’s Very Large Telescope in Chile are
getting a head start by re-examining the
surfaces of eight elderly Milky Way stars.
The stars are at least 12 billion years old
and are probably members of the Pop
II generation, Cristina Chiappini of
the Leibniz Institute for Astrophysics
Potsdam in Germany and her colleagues
report in the April 28 Nature.