Cosmic Primitive: Old star sheds light on early stellar formation

When the faint star HE1327-2326 was born, the universe was a much simpler place. Stars were few, galaxies were tiny, and heavy elements such as iron were scarce. Now, some 13.5 billion years later, researchers have identified this dim Milky Way resident as one of the oldest and most chemically primitive stars in the cosmos. By investigating the composition of this star as well as that of a similar primitive star found 2 years ago, astronomers may be revealing the conditions in which the first stars formed in the universe.

Because the Big Bang forged only hydrogen, helium, and trace amounts of lithium, the first stars coalesced in the absence of heavier atoms. That earliest generation of stars then produced the rest of the elements, from carbon to uranium.

Anna Frebel of the Australian National University in Weston Creek and her colleagues identified HE1327-2326 from a survey of 1,777 primitive, iron-poor stars in the southern sky. Follow-up spectra taken at the European Southern Observatory in Paranal, Chile, and the near-infrared Subaru Telescope atop Mauna Kea in Hawaii revealed that the star contains only 1/250,000 the abundance of iron as does the sun. The new figure establishes the lowest amount of iron ever recorded for a star, Frebel’s team reports in the April 14 Nature.

Residing about 4,700 light-years from Earth and about as heavy as the sun, HE1327-2326 provides “the earliest observational evidence of the ongoing star-formation process in the universe,” says Frebel. Computer simulations have suggested that the universe’s first stars were extremely massive. Such stars would have burned their nuclear fuel rapidly and died in supernova explosions in just a few million years.

If the simulations are accurate, then HE1327-2326 and the similar star found 2 years ago may belong to the universe’s second generation of stars. Most stars of this generation would have much lower masses than those in the previous generation did and lifetimes that could span billions of years. Furthermore, the composition of second-generation stars reflects the atoms forged in the first generation and then released into space during the universe’s initial round of supernova explosions.

By analyzing the spectra of HE1327-2326, the team may be seeing fingerprints of the earlier stars’ supernovas, says Frebel. Already, the researchers have discerned that the star is extremely low in iron but relatively rich in carbon, oxygen, and strontium, all of which are expected for second-generation stars. One surprise is the absence of lithium, which ought to be present in such a star.

With these data in hand, researchers who model early supernovas can put their simulations to a direct test, says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

In an alternative model, the star could be a low-mass, rather than a high-mass, member of the universe’s first generation. In this scenario, HE1327-2326 would have received its allotment of heavy elements from a partner too faint to be seen as well as from material that it captured from interstellar space.

“It’s truly remarkable that stars stick around, preserving some of this information for us to look at” from the early universe, comments Tom Abel of Stanford University.

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