A neutron star is the remnant of an exploded star, compressed so tightly that a teaspoon of its material would weigh as much as a mountain. Inside such an object, protons and neutrons are packed tightly together, and under enormous pressure, hyperons, particles containing a strange quark, can appear. Until now, no one knew exactly how these hyperons affect the star's behavior.
The authors of the new study built the first model that simultaneously tracks the number of electrons and strange particles in neutron star matter. When two such stars spiral around each other before merging, their tidal forces squeeze and stretch the matter, like kneading dough. In response, nuclear reactions kick in: some particles transform into others. It turns out that reactions involving hyperons proceed very quickly, while electron reactions are slow. Because of this difference, the matter doesn't have time to reach equilibrium and starts behaving like very thick syrup. The viscosity of this "syrup" is tens of times higher than old models predicted.
This viscosity dampens the star's oscillations, like a shock absorber dampens a car's vibrations. But for gravitational waves, it's not just a loss of energy: the very rhythm of the stars' approach changes. The scientists calculated that during the final orbits before collision, the phase of the gravitational wave could shift by 0.14 radians. This is a noticeable amount for LIGO and Virgo detectors. If such a shift can be measured, it would be the first direct evidence that hyperons truly exist inside neutron stars.
So strange particles, which so far can only be created in accelerators, might give themselves away in space — through a barely perceptible glitch in the dance of dying stars.
Hyperons live less than one billionth of a second in the lab, but inside a neutron star they can survive for millions of years — pressure prevents them from decaying.
In Vernor Vinge's novel "A Fire Upon the Deep," neutron stars are home to superintelligent beings that sense gravitational waves.