The Secret Life of Neutron Stars – Ars Technica

Composite X-ray, visible light and radio image of the Circinus X-1 X-ray binary system. This object is a neutron star in a common orbit with an ordinary star. Such systems could have been responsible for the reionization of the cosmos in the age of the first stars and galaxies.

Forget archaeologists and their lost civilizations or paleontologists with their fossils – astrophysicist Heloise Stevance examines the past on a whole different scale. When astronomers catch a glimpse of an unusual signal in the sky, perhaps light from an exploding star, Stevance takes that signal and turns the clock back billions of years. She works at the University of Auckland in New Zealand, tracking the past lives of dead and dying stars, a process she calls stellar genealogy. “There’s a lot of drama in celebrity life,” she says.

On August 17, 2017, astrophysicists saw the remnants of two dead stars known as neutron stars collide in a distant galaxy. Known as a neutron star merger, they detected this event via ripples in spacetime — known as gravitational waves — and light created by the resulting explosion. This was the first and only time scientists observed such a gravitational-wave event. From these signals, they concluded that the neutron stars had 1.1 to 1.6 times the mass of the Sun. They also found that such collisions create some of the heavier natural elements in the universe, such as gold and platinum. But overall, the signals offered more puzzles than answers.

Researchers don’t know how common these mergers are, and they can’t say whether they’re responsible for the formation of all the heavy elements in the universe or just a fraction. But if astrophysicists could observe more of these mergers, they could answer these questions and even deeper ones – like how old the universe is. This is where star genealogy can help.

In a study published in Nature Astronomy in January, Stevance and her colleagues used observations of the collision to delve into the past of neutron stars. They deduce details about the billions of years before the collision, when the two objects, still hydrogen in their cores, merged as two regular stars and orbited each other as a single entity known as a binary star system. By gaining a better understanding of these binary stars and their evolution, her team is striving to find out how these merger events can be searched for and thus understood more systematically.

According to Stevance and her team’s analysis, the two neutron stars in the collision were the remains of a star 13 to 24 times the mass of the Sun and another star 10 to 12 times the mass of the Sun. Both began to glow between 5 and 12.5 billion years ago, and at that time only 1 percent of the structure of stars consisted of elements heavier than hydrogen and helium.

The work also describes interactions between the two stars before they burned up their fuel to become neutron stars. They started tens of millions of kilometers apart, which sounds far, but is actually far less than the distance between the Earth and the Sun. Each star’s exterior was surrounded by gas known as the stellar envelope. Stevance and her team’s models found that over the lifetime of stars, one star’s envelope would envelop the other – that is, their outer gases merged into a single common envelope – at least twice.

There’s a lot of detail at stake about two distant objects, especially considering the astrophysicists only directly observed their extremely violent demise. The team reconstructed a city from a pile of dust. To infer so much from so little, they combined observations of the neutron stars with insights from studying 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 interiors to their surfaces, and how these properties change as each star burns fuel and eventually dies. In addition, the model can simulate entire galaxies, each containing multiple clusters of stars of different ages and chemical compositions.

To uncover the past of the neutron star merger, Stevance and her colleagues worked to replicate the data observed for the neutron stars in their model, which could then give them the most likely scenarios of what happened before the two stars merged. For example, they concluded that the stars shared an envelope several times because the two objects took so long to collide. When two binary star envelopes merge, the gases in this shared envelope create a drag force that slows the stars’ orbit, which then causes the stars to spiral toward each other, rapidly reducing the distance between them. In order to merge as quickly as their remaining cores, the stars had to split their envelopes several times.

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

Additionally, Stevance and her team have made their work open source, allowing more researchers to turn back the clock on other stellar activity. Researchers could use the framework to study supernovae, the brilliant explosions of massive stars, says Northwestern University’s Peter Blanchard, who was not involved in the work. As astrophysicists study more of these different types of explosions, which are predicted to produce many types of heavy elements, they can better explain where all the elements in the universe come from. It’s likely that the death of stars forged the gold and uranium that eventually fused with other elements to form Earth billions of years before we fashioned them into jewelry or weapons.

In order to predict the genealogy of neutron stars, Stevance’s model also had to infer properties of the galaxy that hosted them, such as: B. the types of elements the galaxy contains and whether they are evenly distributed across it. This knowledge will guide where to look for other mergers in the future, says University of Texas at Austin astrophysicist Hsin-Yu Chen, who was not involved in the work.

If researchers find more neutron star mergers, Chen hopes to find out how fast the universe is expanding, which is needed to calculate its age. Chen can use the gravitational-wave signal from a merger to calculate the distance from Earth to these neutron stars. Then, by analyzing the light emitted during the merger, she can estimate how fast the neutron stars are moving away – giving the expansion rate. Astrophysicists have so far calculated two opposing rates of universe expansion using different methods, so they’d like to see more mergers to try to resolve the conflict.

The Laser Interferometer Gravitational Wave Observatory collaboration, which discovered neutron star mergers with its two detectors in the US states of Washington and Louisiana, is scheduled to return to service in May 2023 after a two-year upgrade. If that’s the case, researchers expect to detect 10 neutron star mergers a year – which should provide plenty of opportunities to dig deeper into how old the universe is. “It’s going to be very exciting over the next few years,” says Blanchard. It was also a very exciting few billion years.

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