Astronomers have pinned down part of the history of the early Universe. The team, led by Luca Pasquini of the European Southern Observatory (ESO), provides proof that the very first stars were formed when the universe was less than 200 million years old. Their evidence? Minuscule amounts of beryllium atoms in the outer layers of two faint stars 7200 light-years from Earth.
According to the big bang theory, the first stars--formed from a primordial gas of hydrogen and helium--were hot, massive, and short-lived. Shortly after their birth, they exploded as supernovas, ejecting newly formed carbon, oxygen, and nitrogen atoms into space. When those atoms are hit by energetic cosmic ray particles, they break up in fragments, including beryllium nuclei, which can't be produced by any other means. First-generation stars have never been observed so far, but they must have left their beryllium mark on the interstellar medium, from which later generations of stars condensed.
Using a sensitive spectrograph on ESO's 8-meter Very Large Telescope in Chile, Pasquini and his colleagues have now detected trace amounts of beryllium in two stars that are part of a globular cluster known as NGC 6397. Because globular clusters are the oldest known structures in the universe, the beryllium must have been produced by the long-sought first generation of stars.
From the observed amount of beryllium (just one beryllium atom for every trillion hydrogen atoms), the team determined that some 200 million to 300 million years must have elapsed between the death of the first generation of stars in the Milky Way and the birth of the stars in the 13.4-billion-year-old cluster. Because the big bang happened 13.7 billion years ago, and taking various uncertainties in account, this implies that the first stars in the Milky Way formed when the universe was less than 200 million years old, the team says in a paper accepted for publication in Astronomy & Astrophysics.
Timothy Beers of Michigan State University in East Lansing says the results are "quite exciting. This is a real observational challenge, impossible to accomplish in the past." But although it's comforting that the beryllium results are fully compatible with current theoretical ideas about the early universe, Beers warns that it's not quite certain that all of Pasquini's team's assumptions are valid.