Pour one out for ol' space and time: A theoretical physicist has used iron’s signature qualities to trace forward to the end of the universe via the increasingly spectacular deaths of the stars. The research appears in Monthly Notices of the Royal Astronomical Society.
Matt Caplan, a computer-aided cosmologist who researches and teaches at Illinois State University (ISU), studies “astromaterials.” These are the almost unfathomably dense materials produced by stars that begin to die, contract extremely, and then freeze solid. To study these incredible materials, Caplan uses high-level simulations.
In this research, Caplan examines how stars constrict and die, in a process that almost mimics biodegradation of a living thing. He explains:
“In the far future long after star formation has ceased the universe will be populated by sparse degenerate remnants, mostly white dwarfs, though their ultimate fate is an open question. These white dwarfs will cool and freeze solid into black dwarfs while pycnonuclear fusion will slowly process their composition to iron-56.”
In other words, the accumulating, extremely dense star stuff induces a nuclear reaction: pycno-, meaning thick, where in this case, the density itself touches off the reaction. Contrast this with thermonuclear reactions, where extreme heat is the catalyst. As the iron isotope accumulates, the rest of the star dies away, and the presence of the iron then continues to choke out the remaining elements.
Iron is what triggers a supernova, but smaller stars simply don’t have the catalytic iron to get that reaction going. Large stars explode into supernovae because they have enough iron, and Caplan says this is what most stars we see in supernova form today are embodying. But in smaller stars, the far lower rate of accumulation of iron and the extremely slow fusion reaction in their cores mean they'll sit, dormant, long after the rest of the universe has gone dark.
Then, Caplan says, the last remnants—the long-simmering white dwarfs—will reignite like trick birthday cake candles as their centers are finally dense and ferrous enough to react. “As white dwarfs cool down over the next few trillion years, they’ll grow dimmer, eventually freeze solid, and become ‘black dwarf’ stars that no longer shine,” he says in an ISU statement.
It’s this reaction that Caplan is simulating, both to measure the rate of accumulation of iron in the stars and the tipping point where the amount of iron triggers a timely death in stars of different sizes. The last, smallest “trick candle” supernovae will happen about 10 to the 32,000th years in the future, somewhere in the nebulous stretch between a googol and a googolplex.
“Galaxies will have dispersed, black holes will have evaporated, and the expansion of the universe will have pulled all remaining objects so far apart that none will ever see any of the others explode,” Caplan says in the statement. “It won’t even be physically possible for light to travel that far.”
The last stars will, like the proverbial tree in a forest, fall with no one around to hear the sound—not even other stars.