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Double trouble

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  • A neutron star is the incredibly dense remnant of a supernova
  • Unlike a black hole, neutron stars are made of matter and contain clues about how nuclear forces work
  • Simulating neutron star systems gives insight into the universe—and sometimes new technology

Few natural phenomena are as awe-inspiring as the night sky. One especially intriguing astronomical object is the binary neutron star system. This is where two stars explode into supernovae around the same time, which often creates two pulsars that scientists can track.

When stars collide. Visualization of a binary neutron star merger simulated with the superfast WhiskyTHC code for fully general relativistic hydrodynamics. Courtesy RelAstro at Institute of Theoretical Physics.

Dr. Luciano Rezzolla, chair of theoretical astrophysics of Goethe University presented his work on binary neutron star systems at PRACEdays18. As he told his audience in Ljubljana, he relies on high performance computing (HPC) to simulate these fascinating systems and learn more about them.

The lifecycle of a neutron star

Although their lives are measured in millions of years, stars have a finite lifespan. In its youth, a star’s gravity pushes inwards, while the pressure created by thermonuclear fusion processes pushes outwards. As it ages, the amount of material available for the thermonuclear processes wanes, and gravity begins to dominate the force balance that up until then had regulated the stellar equilibrium.

<strong>Crushing mass</strong>. A neutron star is the densest object astronomers can observe directly, cramming half a million times Earth's mass into a sphere about the size of Manhattan. A sugar-cube sized amont of its material would weigh more than 1 billion tons. Courtesy NASA.For a star that is no longer in equilibrium and that is sufficiently massive, gravity forces the star to collapse. This implosion occurs with so much force that it rebounds, wiping out the external layers of the star and creating a supernova. The remnants become a neutron star. These incredibly dense spheres measure only around 20 km across and have some very interesting properties.

“Black holes are the object with the strongest gravity,” says Rezzolla. “But neutron stars have the second largest gravity, and they are made of matter as opposed to black holes which are made of nothing, of vacuum. While you can fall into a black hole, when falling onto a neutron star you will eventually stop when you reach its hard surface.”

If conditions are just right, a neutron star will begin to spin, discharging radio waves and radioactive particles from its poles. Rezzolla likens these pulsars to a lighthouse, emitting a beam of radiation that can be seen from Earth.

<strong>Mutual attraction. </strong> The magnetic fields of two neutron stars begin to entangle. Courtesy NASA/AEI/ZIB/M. Koppitz and L. Rezzolla.“If we happen to be in the beam of this lighthouse, then we see neutron stars as pulsars,” says Rezzolla. “Because they are rotating, the beam hits us every now and then. The fastest rotating pulsar spins at 700 Hz, so just imagine an object with a radius of 10 kilometers which is rotating at 700 times a second.”

In a binary system, each neutron star inches closer to the other, due to their high gravitational pull and the loss of energy via gravitational waves. Over time, these pulsars eventually collide. When they smash, they give off an electromagnetic pulse strong enough to be observed from farthest corners of the Universe. These “fireworks,” as Rezzolla calls them, also produce a short gamma ray burst and gravitational waves.

Super blackboard

The uniqueness of neutron stars makes them a valuable resource for scientists. Through observation, researchers can draw conclusions about other physical phenomena.

“Neutron stars are made of matter,” says Rezzolla. “This means that they are made of the same elements that you and I are made of, it's just that they are in extreme conditions. If we want to understand how nuclear matter works, how nuclear forces and how elements are held together – the details of the strong force – then we need to study these objects.”

But how can scientists study something so far away and that can’t be replicated in an accelerator? HPC simulations, of course.

“Accelerators take two particles and smash them together, but the high density of nuclear matter means that you need billions of particles interacting. You cannot produce that in a laboratory,” says Rezzolla.

<strong>Gamma ray burst.</strong> After neutron stars collide, they may form a black hole from which a jet-like magnetic field emerges. Courtesy NASA/AEI/ZIB/M. Koppitz and L. Rezzolla.“Researchers look at real neutron stars to compare their predictions with the observation. If your predictions match, then you are happy. If they don't, then you go back to the blackboard.”

In this case, the ‘blackboard’ is a supercomputer.

“Gravitational waves are the signal produced by the dynamics of this binary system as they merge— you can think of this like smoke from a fire,” says Rezzolla.

“By reading the signals in the smoke, we can tell what the wood was made of. In order to go from wood to smoke, from gravitational waves to nuclear physics, we need numerical simulations and high performance computing.”

The complexity and asymmetry of the problem that needs to be simulated “just completely explodes the computational requirements,” according to Rezzolla. “This something you cannot do on your laptop or a desktop. You need a supercomputer to evolve this very massive set of equations.”

When the smoke clears

And lest you think that explosions in outer space have little relevance for our lives down here on Earth, Rezzolla points out that a lot of current technology that we now take for granted, such as cellphone cameras, Wi-Fi, and GPS positioning, all grew out of astronomical research, that is, research that did not have any specific technological application.

“The technology for taking pictures, all the selfies that populate the internet, that very technology comes from the CCD camera, which was developed in the 1980s to take pictures of stars,” says Rezzolla.

While scientists are calculating equations to comprehend the inner workings of neutron stars, increasing computing power is getting them closer every day. And when the smoke clears, we’ll hopefully have a greater understanding of the universe—and some cool new tech for interacting with our immediate surroundings.

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