Early stars were smaller than thought

Posted on December 14, 2011

Early UniverseRather than the bloated monsters unlike anything found in the night sky today that the first stars have long been thought to be, it now appears that they may have been less than half the size they were thought to be. This new size limit may resolve one of astronomy’s oldest mysteries – why some elements are more abundant than theory predicts.

English: The oldest star of our Galaxy: HE 152...

What we know so far is that in the first few hundred millions of years after the big bang the earliest stars formed from atomic hydrogen, helium and miniscule amounts of other light elements hanging around at the time. Up to now, it was thought that these stars grew and grew to a size between perhaps 100 times to 200 times the size of our own sun. The new work by a team led by Takashi Hosokawa at NASA’s JPL (Jet Propulsion Laboratory) in Pasadena, California has used computer simulations to test the theory.

What they have found is that the gas clouds from which these early stars would have formed should be much hotter than was previously thought. It is the behaviour of the hot gas that has changed the estimates of the mass of these early stars.

English: The Progenitor of a Type Ia Supernova...

“That hot gas,” says Harold Yorke, a member of the JPL team, “expands and doesn’t accrete onto the disc [that will eventually form the early star].” As a result of this, the masses of those early stars must have been closer to perhaps 40 times the mass of our own sun. Stars of around this size would explain the distribution of elements that we see in the Universe today. When the first stars exploded as supernovae they would spew out new elements in proportions that depended on the mass of the exploding star.

Stars of around 100 solar masses or greater could not have produced the proportions of elements that we actually see in the real world. By contrast though, the ratios expected from the much smaller supernovae – if it is possible to consider something as huge as a supernova as being small – then the proportions of the elements produced are exactly what you would expect from the smaller early stars predicted by Hosokawa’s team.

It is not conclusive, of course, but the reality of the universe around us and the way that the stars within it actually do behave would appear to lend considerable credibility to the new simulations.