The nature of the first stars is a big unknown in astrophysics/cosmology. The most distant known quasar, SDSS-J4010, contains large amounts of heavy elements, which implies that it is made of matter created in even earlier celestial bodies. Previous work has suggested that earlier, primordial stars should be evenly distributed throughout a galaxy, but the same work also put forth the possibility that these early stars were massive and did not live long.
Observations have yet to provide direct evidence for the first stars, but many theories abound. New work published in last week's issue of Science describes detailed computer simulations of the earliest protostars. The authors simulated a box approximately 200 kpc on each side that contained 500,000 solar masses of dark matter and gas. The model examined what happened as the material collapsed through gravitational attraction.
Density distribution around the final protostar. Each side of the
image is 25 solar radii. Image credit: Yoshida et al.
The simulation modeled the collapse of the gas cloud for over 20 decades, right up to the point where strong hydrodynamic shocks began forming within the core, a sign that a protostar had formed. Beyond this point, the physics—specifically the radiative heating and cooling from the high temperature gases—present in the simulation were not sufficient to properly model the evolution further. The simulation run ended with a region of high density and temperature that was deemed to be representative of an early protostar. The protostar had a final mass of approximately 0.01 solar masses, and a radius of about 5×1011 cm. These numbers are in decent agreement with independent theoretical calculations of present-day protostars.
One question that went unanswered is whether the protostar would be spinning fast enough to fragment the core into multiple smaller protostars. In the course of the simulation, a single high-density core formed that did not break up. It is still possible that this protostar could split into multiple masses as it continues to accrete gas, but further calculations have revealed this to be an unlikely scenario. Other research that studied protostar formation suggested that if this one continued to accrete matter at the rate seen in the simulation, it would enter the main sequence carrying hefty 100 solar masses.
While this work doesn't answer all of the questions about the earliest stars, it does provide a realistic scenario for what could be expected in the very early universe. As the authors conclude, "our model provides a viable scenario for the early chemical enrichment in the universe by massive primordial stars, which is necessary for the formation of later populations of ordinary stars." The researchers feel that their work has shown how small density fluctuations could have led to the formation of early stars, and that the properties of dark matter may have played a key role in the early stages of the universe.
Science, 2008. DOI: 10.1126/science.1160259