First Stars After the Big Bang Were Not Huge, Lonely Objects

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The first stars in the universe were not as solitary as previously thought. In fact, they could have formed alongside numerous companions when the gas disks (image)  that surrounded them broke up during formation, giving birth to sibling stars in the fragments, according to computer simulations by researchers at Heidelberg University’s Center for Astronomy along with colleagues at the Max Planck Institute for Astrophysics and the University of Texas.


Stars evolve from cosmic gas clouds in a fierce and complex battle between gravity and internal gas pressure. The density of the gas increases due to its own gravitational pull. This causes the gas to heat up, as a consequence the pressure rises, and the compression process comes to a halt. If the gas manages to get rid of the thermal energy, compression can continue and a new star is born.

This cooling process works especially well if the gas contains chemical elements like carbon or oxygen. Stars forming in this way are normally low in mass, like our Sun. But in the early universe these elements had yet to emerge, so the primordial cosmic gas could not cool down very well. Accordingly, most theoretical models predict the masses of primordial stars to be about a hundred times greater than that of the Sun.

Heidelberg astrophysicist Paul Clark and his colleagues investigated these processes with the help of  high resolution computer simulations that indicated that this simple picture needs to be revised and that the early universe was not only populated by huge, solitary stars.

The reason is the underlying physics of the so called accretion disks accompanying the birth of the very first stars. The gas from which a new star forms rotates, and so the gas is unable to fall directly onto the star, but first builds up a disk-like structure. Only as a result of internal friction can the gas continue to flow onto the star. If more mass falls onto this disk than it can transport inwards, it becomes unstable and breaks into several fragments. So instead of forming just one star at the centre, a group of several stars is formed. The distances between some of the stars can be as small as that between the Earth and the Sun.

Time evolution of the accretion disk that surrounds the nascent central star. The disk becomes gravitationally unstable and develops spiral arms of enhanced density. These arms fragments and build up new companion stars. Within only 110 years already three protostars have been born (as indicated by the symbols) and a fourth one is beginning to form (in the upper left region of the last figure in the sequence).

This realisation opens up exciting new avenues for detecting the first stars in the universe. In the final stages of their lives, binaries or multiple stellar systems can produce intense bursts of X-rays or gamma rays.

Future space missions are being planned specifically to investigate such bursts from the early universe. It is also conceivable that some of the first stars may have been catapulted out of their birth group through collisions with their neighbours before they were able to accumulate a great deal of mass.

Unlike short-lived high-mass stars, low-mass stars may survive for billions of years. “Intriguingly,” says Clark, “some low-mass primordial stars may even have survived to the present day, allowing us to probe the earliest stages of star and galaxy formation right in our own cosmic backyard.”

The Daily Galaxy via www.ita.uni-heidelberg.de/research/klessen/science/starformation.shtml.

Image top of page shows  artist's concept of the first stars in the Universe turning on. Wilkinson Microwave Anisotropy Probe (WMAP) data reveals that this era occurred 200 million years after the Big Bang, much earlier that many scientists had suspected. Credit: NASA/WMAP Science Team

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