Finding Strange Stars with Primordial Black Holes

It sounds like something out of science fiction: a miniature black hole barrels through the densely packed remnant of a dead star, consuming it from the inside. Strange though it may seem, this scenario might hold the key to discerning between stars composed of different kinds of exotic matter.

Strange Stars, Indeed

illustration of a neutron star on a map of manhattan

Comparison of the size of a neutron star to the size of Manhattan. A strange star of the same mass would be slightly smaller. [NASA/Goddard Space Flight Center]

When a massive star expires, its outer layers explode away from its collapsed core. This core is so dense that electrons and protons are squished together, forming a star composed of neutrons. However, a neutron star’s evolution might not stop there: it’s possible that the extreme conditions inside the star could convert the neutrons into elementary particles called strange quarks. The resulting strange star would be only slightly smaller than the original neutron star, making it difficult to distinguish between them observationally. Since we can’t peer into their interiors, how can we tell if a compact stellar remnant is a neutron star or a strange star?

In new article, Ze-Cheng Zou (邹泽城) and Yong-Feng Huang (黄永锋) from Nanjing University, China, propose an intriguing way to detect strange stars: by watching them be consumed by a tiny black hole. Specifically, Zou and Huang posit that we can differentiate between the gravitational waves emitted as a neutron star or strange star encounters a planet-mass primordial black hole — one that formed spontaneously from a dense spot in the universe less that one second after the Big Bang.

plot of black hole mass over time

Demonstration of the how the mass of the black hole changes as it accretes matter from the neutron star or strange star. [Zou & Huang 2022]

Insights from Inspiraling

Zou and Huang modeled the outcome of a clash between a neutron star or strange star and a planet-mass primordial black hole and predicted the gravitational waves that would be emitted as a result. The authors consider black holes from slightly more massive than Mercury to as massive as Jupiter — with radii of 0.3 to 300 centimeters — and strange stars and neutron stars of 1.4 solar masses.

As the black hole tunnels into the star, its progress gradually slows as a result of accreting stellar material and emitting gravitational waves. Though the modeled neutron star and strange star have the same mass and similar radii (12.6 and 11.0 km, respectively), they are theorized to have different internal structures, which alters the rate at which the black hole spirals inward. This distinction is imprinted on the emitted gravitational-wave signal — but can we tell the difference?

Revealed by Gravitational Waves

Amplitude of gravitational-wave signals as a function of frequency and black hole mass (left) and distance (right). Sensitivities of existing and future detectors are shown for comparison. Click to enlarge. [Adapted from Zou & Huang 2022]

Though the two scenarios produce gravitational waves with similar amplitudes, the shapes of the signals are different — and detecting them is not outside the reach of our current gravitational-wave observatories. The Laser Interferometer Gravitational-Wave Observatory (LIGO) should be able to detect these events up to 32,000 light-years away, depending on the black hole’s mass, while future detectors should be sensitive to events 10,000 times more distant.

Observing these curious collisions might tell us more than just the composition of the dense stellar remnants — it also might tell us something about the composition of dark matter. If we observe primordial black holes interacting with neutron stars — or strange stars — it could help constrain the abundance of primordial black holes and determine what fraction of the total mass of dark matter they comprise. With strange stars, exciting results are clearly the norm!


“Gravitational-wave Emission from a Primordial Black Hole Inspiraling inside a Compact Star: A Novel Probe for Dense Matter Equation of State,” Ze-Cheng Zou and Yong-Feng Huang 2022 ApJL 928 L13. doi:10.3847/2041-8213/ac5ea6