Universe's first stars were more suns than supergiants

THE earliest stars may have been less than half as large as previously thought. The new size limit could resolve one of astronomy's oldest mysteries: why some elements are more abundant than theory predicts.

In the first hundreds of millions of years after the big bang, the earliest stars formed from atomic hydrogen, helium and tiny amounts of other light elements. Initial calculations showed that these stars would have grown to between 100 and 200 times the mass of our sun.

Now a team led by Takashi Hosokawa at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, has used computer simulations to show the gas clouds from which the stars formed would have been much hotter than thought.

"That hot gas expands and doesn't accrete onto the disc [that eventually forms the star]," says Harold Yorke, a member of the JPL team. Consequently, the early stars must have had masses closer to 40 times that of our sun (Science, DOI: 10.1126/science.1207433).

Stars around this size help to explain the distribution of elements we see today. When the first stars exploded as supernovas, they spewed out new elements in proportions that depended on the mass of the explosion.

However, the explosive deaths of stars of around 100 solar masses or more could not have produced the elements in the proportions that astronomers see. By contrast, the ratios are exactly what you would expect from the smaller supernovas predicted by Hosokawa's team.

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