• Star-forming galaxies bursting with young stars
• "Dead" systems with little or no ongoing star formation
• Stellar graveyards littered with black holes
• Glowing clouds of pure gas with no stars at all

The last category is unusual: the simulation produces galaxy-sized objects that shine without containing any stars. Some may be the remnants of systems whose stars exploded or collapsed into black holes; others may have always held only gas.

How do Population III stars explain the iron puzzle?

The simulation also tackles a long-standing puzzle in stellar astrophysics — the study of how stars form, live, and die. In the Milky Way today, smaller and fainter galaxies normally carry less iron. In the faintest systems, however, iron levels appear flat, regardless of mass. Earlier simulations could not reproduce that plateau.

MEGATRON traces the culprit to Population III stars — a hypothetical, extremely ancient class made only of hydrogen and helium, the first two elements forged after the Big Bang. No telescope has ever imaged one. When these stars explode, they can produce more iron than other supernovae, the team found.

A galaxy big enough retains the iron through gravity. A galaxy too small loses it to space. That difference explains the flat iron signal in the faintest dwarfs.

One of the six papers is the first to model how Population III stars could form inside a Milky Way–like environment and to predict where any survivors would most likely sit — a guide for future searches.

What's new about the physics?

"What's unique about our simulation is that it's the first time we have modeled the enrichment of individual chemical elements from individual stars after the Big Bang, coupled to detailed models for gravity, chemistry, radiation and stellar processes," Katz said.

MEGATRON also pioneers detailed "non-equilibrium physics" at galaxy scale. Most models assume a galaxy's chemistry has settled into a steady state because running the full, time-dependent calculation slows computation enormously. Two of the six papers show the detail matters. Adding non-equilibrium physics changes predictions about the circumgalactic medium — the halo of gas surrounding a galaxy. Researchers can now read James Webb data more sharply and narrow the range of plausible histories for the universe.

What are the limits?

Katz is candid about what MEGATRON does not yet get right.

"Looking at these results, it's very clear that the physics happening right after the Big Bang has a direct impact on what we see today in the local universe," he said.

"But there are also things we're not getting right, which is interesting too — what are the parts we're still missing? That can lead you into new directions and new questions."

The team plans to keep comparing model output against James Webb readings. Future runs will likely test different recipes for how stars form and how they push material back into space — variables that remain uncertain across cosmology.