Sizing Up Neutron Stars

 A neutron star is the lingering leftovers of a massive star that has ended its nuclear-fusing "life" in the brilliant and fatal fireworks of a supernova explosion. These extremely dense city-sized objects are actually the collapsed cores of dead stars which, before their violent "deaths", weighed-in at between 10 to 29 times the mass of our Sun. These bizarre, lingering relics of heavy stars are so extremely dense that a teaspoon full of neutron star material can weigh as much as a herd of elephants. In March 2020, an international research team of astronomers announced that they have obtained new measurements of how big these oddball stars are. They also found that neutron stars unlucky enough to merge with voracious black holes are likely to be swallowed whole--unless the black hole is both small and/or rapidly spinning.

The international research team, led by members of the Max Planck Institute for Gravitational Physics (Einstein Institute: AEI) in Germany, obtained their new measurements by combining a general first principles description of the mysterious behavior of neutron star material with multi-messenger observations of the binary merger of a duo of neutron stars dubbed GW170817. Their findings, published in the March 10, 2020 issue of the journal Nature Astronomy are more stringent by a factor of two than earlier limits and demonstrate that a typical neutron star has a radius close to 11 kilometers. In addition, they found that because such unlucky stars are swallowed whole during a catastrophic merger with a black hole, these mergers might not be observable as gravitational wave sources, and would also be invisible in the electromagnetic spectrum. Theoretical work in physics and other sciences is said to be from first principles (ab initio) if it originates directly at the level of established science and does not make assumptions such as empirical model and parameter fitting.

Gravitational waves are ripples in the fabric of Spacetime. Imagine the ripples that propagate on the surface of a pond after a pebble is thrown into the water. Gravitational waves are disturbances in the curvature of Spacetime. They are generated by accelerated masses, that propagate as waves outward from their source at the speed of light. Gravitational waves provide a new and important tool for astronomers to use because they reveal phenomena that observations using the electromagnetic spectrum cannot. However, in the case of neutron star/black hole mergers, neither gravitational wave observations nor observations using the electromagnetic spectrum can be used. This is why such mergers may not be observable.

"Binary neutron star mergers are a gold mine of information. Neutron stars contain the densest matter in the observable Universe. In fact, they are so dense and compact, that you can think of the entire star as a single atomic nucleus, scaled up to the size of a city. By measuring these objects' properties, we learn about the fundamental physics that governs matter at the sub-atomic level," explained Dr. Collin Capano in a March 10, 2020 Max Planck Institute Press Release. Dr. Capano is a researcher at the AEI in Hannover.

"We find that the typical neutron star, which is about 1.4 times as heavy as our Sun has a radius of about 11 kilometers. Our results limit the radius to likely be somewhere between 10.4 and 11.9 kilometers. This is a factor of two more stringent than previous results," noted Dr. Badri Krishnan in the same Max Planck Institute Press Release. Dr. Krishnan leads the research team at the AEI.

Strange Beasts In The Stellar Zoo

Neutron stars are born as the result of the fatal supernova explosion of a massive star, combined with gravitational collapse, that compresses the core to the density of an atomic nucleus. How the neutron-rich, extremely dense matter behaves is a scientific mystery. This is because it is impossible to create the necessary conditions in any lab on Earth. Although physicists have proposed various models (equations of state), it remains unknown which (if any) of these models actually describes neutron star matter.

Once the neutron star is born from the wreckage of its progenitor star, that has gone supernova, it can no longer actively churn out heat. As a result, these stellar oddballs cool as time goes by. However, they still have the potential to evolve further by way of collision or accretion. Most of the basic models suggest that neutron stars are made up almost entirely of neutrons. Neutrons, along with protons, compose the nuclei of atoms. Neutrons have no net electrical charge, and have a slightly larger mass than protons. The electrons and protons in normal atomic matter combine to create neutrons at the conditions of a neutron star.