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Evolving Stars, Evolving Methods

Almost everyone is fascinated by black holes. Dr. David Neilsen studies black holes, and he is also interested in processes that create black holes — like colliding neutron stars.

“Black holes are not really made of anything,” he said.

But neutron stars? Now those are interesting. And when they crash into each other? Even better.

“They’re some of the most energetic explosions in the universe, so everyone would love that,” Neilsen said.

Neutron stars contain the densest matter in the universe. Neutron stars are about 7 miles in radius and approximately 100 trillion times as dense as the sun. In other words, it’s as if two suns were compressed into a sphere 12.4 miles in diameter—a mile short of the length of Manhatten Island.

Neutron stars are created when a star’s core runs out of fuel for nuclear fusion and gravity overcomes the natural stability of atoms. The nucleus disintegrates and the electrons react with the newly free protons to create a star comprised almost solely of neutrons.

Neilsen is most interested in what happens when two neutron stars collide together. One possible result is a hypermassive star, which is too large to be stable, but can exist for short times when hot and rapidly rotating. Eventually, however, the hypermassive star cools, friction slows it down, and it collapses into a black hole.

Neilsen used supercomputers to solve complicated equations from general relativity and nuclear physics to model neutron star collisions. These models allow Neilsen to study what we can learn about neutron stars from future observations of their mergers.

“One of the big questions is that we don’t know the exact properties of the matter inside neutron stars because we can’t create matter like that in a laboratory,” Neilsen said. “There is still some mystery about what neutron stars are made of.”

Using realistic nuclear equations of state, Neilsen and his team can make predictions about the dynamics of neutron star mergers that can be checked in future observations. This will allow us to better understand neutron stars.

Neilsen studied neutron stars as a graduate student at the University of Texas and in his early years teaching at BYU. At that time, limitations in computational power meant that he and collaborators had to use overly simplistic equations to describe nuclear matter. They did simulations and calculations, but very few times could he say, “Yes, this is something we can actually detect.”

“A lot of it didn’t have a connection to the real world or a connection to things we can observe,” he said.

Recent developments in technology, including more advanced telescopes, allow scientists to detect and observe astrophysical processes like the creation of hypermassive stars. These detections provide more accurate data and equations of state for scientists like Neilsen.

“It’s hard to visualize (these processes), but that’s part of the fun of the discovery,” Neilsen said. “I like thinking about these really extreme events and matter behaving in completely strange ways. We don’t have a lot of connection to these phenomena because we can’t live in those environments.”

But through his research and highly sophisticated computer simulations, Neilsen is providing a better vision of these astrophysical phenomena. As Neilsen has shown, studies in astrophysics no longer need be “ideal,” but can be based on real-life observations and accurate calculations.

By Jessica Olsen Posted on