Giant Baby Stars Need to Spin Slowly

Early universe’s biggest stars must rotate very little to form.

New research reveals that the colossal "baby" stars of the early universe, believed to be the precursors of today's supermassive black holes, had to spin incredibly slowly to form and grow.

Simulating Supermassive Star Growth

Scientists focused on understanding the rotational speed of these early, supermassive stars. They simulated the growth of a Population III (Pop III) supermassive star, starting small and gradually adding mass until it reached 100,000 times the sun’s mass.

Their findings indicated a critical condition for the formation of these behemoths: the incoming material could only carry a tiny amount of spin.

The "Geneva Code" and Spin Constraints

The researchers utilized a specialized computer program, "the Geneva code," to model the intricate behavior of a growing star, including its rotation. They initiated the simulation with a star 10 times the sun's mass and "fed" it new material, akin to a cosmic snowball accumulating mass, until computational limits prevented further simulation.

The results were striking:

• Only 1 percent of the incoming material’s potential spin could be absorbed by the star.
• If the star attempted to spin faster, it simply couldn't form.

Rotational Dynamics: Surface vs. Core

The star's surface spun at a remarkably leisurely pace, operating at less than 10 to 20 percent of the speed that would cause it to tear apart. In stark contrast, the internal core spun up to 10,000 to 100,000 times faster than the surface.

"Supermassive stars have to be slow rotators," the authors state. "Since SMSs evolve at close to the Eddington limit, the ΩΓ-limit imposes tight constraints on their rotation velocity."

This suggests that the star’s powerful outward push from light (the Eddington limit) played a crucial role in preventing its surface from spinning too rapidly.

The Mystery of Cosmic "Spin Control"

This finding strongly suggests the existence of an unknown mechanism in the early universe, actively removing spin from the gas accreting onto these giant stars. This cosmic "spin control" might have involved:

• Magnetic fields
• Other as-yet-undetermined forces

Such a mechanism would be essential, ensuring these supermassive stars didn't rip themselves apart before they could grow to immense sizes and eventually evolve into the monstrous black holes observed today.