Protostar simulation

Physicists in Japan and the US have performed a simulation of structure formation in the early universe that suggests the first baby stars or “protostars” were hundreds of times less massive than previously thought.

Led by Naoki Yoshida of Nagoya University, the researchers say that their simulation provides the clearest picture yet of how tiny fluctuations in the early universe grew into protostars, and could be a stepping stone to explaining how the structures evolved into the stars and galaxies we see today.

“It is exciting because we have been unable to look at the structure of a protostar and its surroundings at the time a star is about to be born,” astrophysicist Ling Gao of Durham University in the UK, who was not involved in the research, told “This gives a significant input to understanding the formation of the first-generation stars.”

Cosmic dark ages

Although popular representations of the Big Bang usually involve the sky becoming awash with light almost instantaneously, in reality the first billion years of the universe were starless. From measurements of the cosmic background radiation we know that electrons and nuclei started combining into atoms, mostly of hydrogen, after about 380,000 years. Over the next 300 million years these hydrogen atoms accumulated into vast gas clouds.

Eventually, tiny density fluctuations in these clouds began to draw in matter and expand. Although definitions vary of when enough matter has accreted to become a protostar, Yoshida says the crucial stage comes when the gravitational forces driving the accretion balance the matter’s thermal pressure.

Past simulations of protostars have predicted that the mass of the first protostars would have been as much as 500 times that of our Sun. This is because simple elements like hydrogen and helium retain their heat for a long time, which produces high thermal pressures. To balance these pressures, the protostars would need high gravitational forces, and hence a lot of mass.

However, Yoshida’s team claim that these previous simulations have not fully taken into account the process of “radiative transfer”, which redistributes energy in the protostar. By including radiative transfer in their simulation, the researchers have found that protostars can start out just 1% as massive as our Sun (Science 321 669) — although they would quickly grow into stars a hundred times as big.

Rosetta stone or stepping stone?

Rachid Ouyed, an astrophysicist at the University of Calgary, points out that the nature of the simulation meant it had to be terminated after the formation of the protostar, leaving the fate of the structure uncertain. “This is to be taken seriously, since in past work it was shown that the [process] they see tended to quickly dissipate the so-called proto-stars they found.”

Nevertheless, Yoshida’s team would now like to expand their simulation into later periods of star formation. In the next period, gravity would overwhelm thermal pressure at the centre, leading to compression and raised temperatures of around a million Kelvin. At this point, deuterium molecules would begin to fuse — but Yoshida and his team have not found a way to model nuclear fusion reactions in three dimensions.

Describing this as his “life’s work”, Yoshida predicts that we’ll see a full simulation of complete star formation within a decade and galaxy formation within 20 years.