The secret life of neutron stars

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There are a lot of details about two distant objects, especially considering that astrophysicists only directly observed their extremely turbulent end. The team reconstructed the city from a pile of rubbish. To infer so much from so little, they combined observations of neutron stars with findings from studies of other stars and galaxies, creating a gigantic mathematical model of both observed and hypothetical stars. The model contains detailed descriptions of the temperature, chemical composition and other characteristics of 250,000 different types of stars, from their interior to the surface, and how these properties change as each star burns fuel and eventually dies. In addition, the model can simulate entire galaxies, each containing several collections of stars of different ages and chemical compositions.

And so, in order to uncover the past of neutron star mergers, Stevans and her colleagues worked to replicate the data observed for neutron stars within their model, which could then tell them the most likely scenarios of what happened before the two stars merged. For example, they came to the conclusion that the stars divided the shell several times due to how long it took two objects to collide. When two double stars merge into shells, the gases in that common shell create a drag force that slows the stars’ orbits, which then causes the stars to spiral toward each other, rapidly closing the distance between them. To merge as quickly as their core remnants did, the stars had to exchange shells several times.

The work on this neutron star merger is based on decades of astronomical research. Stevens’ colleagues began formulating their star model 15 years ago to study celestial objects in very distant galaxies, says Ian Eldridge, professor of astrophysics at the University of Auckland and one of Stevens’ collaborators. “When we first created this, we were years away from finding gravitational waves,” says Eldridge. This 15-year-old model, in turn, is based on star models created by astronomers in the 1970s. The work illustrates a long, often circuitous scientific process: generations of astronomers working on circumstantial questions about the stars inadvertently contribute to a new discovery decades later.

In addition, Stevans and her team have made their work open source, allowing additional researchers to rewind the clock to other stellar activities. The researchers could use this structure to study supernovae, the bright explosions of massive stars, says Peter Blanchard of Northwestern University, who was not involved in the work. As astrophysicists learn more about these different types of explosions that are predicted to produce many types of heavy elements, they may better explain where all the elements in the universe came from. It is likely that the death of the stars forged the gold and uranium that eventually combined with other elements to form the Earth, billions of years before we turned them into jewelry or weapons.

To predict the genealogy of neutron stars, Stevans’ model also had to infer properties of the galaxy they were in, such as the types of elements that galaxy contained and whether they were evenly distributed throughout it. This knowledge will help determine where to look for other mergers in the future, says astrophysicist Xin-Yu Chen of the University of Texas at Austin, who was not involved in the work.

If researchers can find more neutron star mergers, Chen wants to use them to figure out how fast the universe is expanding, which is needed to calculate its age. Chen can use the merger’s gravitational wave signal to calculate the distance from Earth to these neutron stars. Then, by analyzing the light emitted by the merger, it can estimate how fast the neutron stars are receding, determining the rate of expansion. By now, astrophysicists have calculated two conflicting expansion rates of the universe using different methods, so they would like to observe more mergers to try to reconcile the conflict.

The Laser Interferometric Gravitational Wave Observatory, which detected neutron star mergers with its two detectors in the US states of Washington and Louisiana, is scheduled to resume operations in May 2023 after two years of updates. When that happens, researchers expect to find 10 neutron star mergers a year, which should provide plenty of opportunity to delve deeper into questions about how old the universe is. “The next few years are going to be very interesting,” says Blanchard. It’s been a very exciting few billion years.

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