Neutron star collisions push the limits of extreme physics

When neutron stars collide, they create one of the most spectacular and complex events in the universe. Neutron stars, remnants of collapsed stars, are incredibly dense and small.

When two such stars are in close proximity, they spiral towards each other and eventually collide. This collision generates extreme heat and fascinating physical phenomena.

What is a neutron star?

A neutron star is a compact remnant of a massive star that has undergone a supernova explosion.

When a star with a mass between about 8 and 20 times that of our Sun exhausts its nuclear fuel, it collapses under its own gravity. The core compresses so much that protons and electrons combine to form neutrons, resulting in a neutron star.

These stars are only about 20 kilometers (12 miles) in diameter but have masses of up to twice that of the Sun. To illustrate their density, a single teaspoon of neutron star material would weigh around a billion tons on Earth.

Neutron stars possess extremely strong magnetic fields and can rotate rapidly, emitting beams of radiation detectable as pulsars.

Despite their small size, neutron stars provide a unique laboratory for studying the behavior of matter under extreme conditions, contributing to our understanding of fundamental physics.

Hidden physics of neutron star mergers

Recent simulations conducted by physicists from Penn State University have provided new insights into neutron star collisions. The simulations revealed that hot neutrinos, which are tiny and nearly massless particles, can be briefly trapped at the interface where the stars merge.

This happens for just 2 to 3 milliseconds, during which the neutrinos interact with the star matter, helping drive particles back toward equilibrium.

“For the first time in 2017, we observed signals, including gravitational waves, from a binary neutron star merger,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley, who led the research.

“This discovery sparked immense interest in binary neutron star astrophysics. Since we cannot replicate these events in a lab, simulations based on Einstein’s theory of general relativity are our best tool for understanding them.”

Nature of neutron stars

Neutron stars are thought to be composed almost entirely of neutrons. Their incredible density, surpassed only by black holes, results from the fusion of protons and electrons into neutrons.

“Before they merge, neutron stars are effectively cold, despite their temperatures reaching billions of degrees Kelvin,” explained Professor David Radice, a leader of the research team.

“Their density means this heat adds very little to the system’s energy. Upon collision, however, the interface can reach temperatures in the trillions of degrees Kelvin. Photons cannot escape this dense environment to dissipate the heat, so the stars cool down by emitting neutrinos.”

Post-collision interactions

During the collision, neutrons in the stars break apart into protons, electrons, and neutrinos.

The immediate aftermath of this process has long been a mystery in astrophysics. To address this, the research team created detailed simulations that model the merger and the resulting physics.

These simulations, requiring immense computing power, showed that even neutrinos can be briefly trapped by the collision’s heat and density.

Out of equilibrium with the cooler star cores, these hot neutrinos interact with the star matter.

“These extreme events push the limits of our understanding of physics,” noted Professor Radice. “The brief 2 to 3 milliseconds out-of-equilibrium phase is when the most intriguing physics occurs. Once equilibrium is restored, the physics becomes more comprehensible.”

Implications for observing the mergers

The interactions during the merger can influence the signals we detect on Earth from these events.

“How neutrinos interact with star matter and are emitted affects the oscillations of the merged remnants,” explained Espino.

“This impacts the electromagnetic and gravitational wave signals observed on Earth. Next-generation gravitational-wave detectors could be designed to detect these signal differences. Thus, our simulations not only enhance our understanding but also guide future experiments and observations.”

These groundbreaking simulations open new windows into the physics of neutron star collisions, helping us understand one of the most extreme and fascinating phenomena in the universe.

The study is published in the journal Physical Review Letters.

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