�鶹APP

When a massive star dies, it often leaves behind one of the most enigmatic objects in the universe: a black hole. Despite knowing about their existence for decades, the nature of black holes—they are so dense that even light can’t escape their gravitational pull—means astronomers have never been able to watch one form.

That’s where simulations and computational astrophysics come in.

In a new paper published in Astrophysical Journal Letters titled “,” a team led by Illinois Tech Assistant Teaching Professor of Physics Goni Halevi has created the first 3D simulation of a collapsing massive star forming a black hole by using general relativity—Albert Einstein’s theory of gravity that describes the curvature of spacetime caused by mass and energy.

“People think about science as things you can do in a lab, but astronomy works differently. In astronomy, we can’t make a black hole in a lab,” says Halevi. “Using computers is our way of doing numerical experiments and seeing what happens when a star dies. Astronomers are either observers looking at the universe and trying to interpret it, or we’re computational astronomers doing these numerical experiments.”

When massive stars reach the end of their lives, their cores collapse under the weight of their own gravity. What the star then turns into depends on a wide range of factors—sometimes it leaves behind a dense neutron star, while other times it vanishes into an even denser black hole that’s nearly impossible to spot in the night sky.

That’s why simulations are so crucial in astronomy: they allow astronomers to bridge the gap between what telescopes can observe and what theory predicts. Until Halevi’s simulation, however, simulating a black hole while incorporating the effects of general relativity and neutrinos in 3D had never been accomplished.

“Black holes can only exist because of general relativity, and most of these (previous) simulations don’t include general relativity,” says Halevi. “They can’t actually form the black hole on the computer without including the way that space-time is curved due to mass. People have done similar things, but [it was] always with more approximations that we’re making here.”

Part of what makes this new simulation so unique is because it includes physics at very small scales—that of neutrinos and nuclear particles—while modeling the enormous force of gravity on a star weighing nearly 50 times as much as our Sun.

“There are these turbulent, complicated fluid motions that can affect whether a black hole forms and how long it takes,” says Halevi. “You’re solving a very complex system of equations that are all coupled to one another, and every new ingredient of physics that you add makes it slower and more expensive to do that calculation.”

Halevi’s simulation is powered by a new graphics processing unit-accelerated code called GRaM-X that follows the collapse, core-bounce, shock propagation, and eventual formation of a black hole in full 3D. Halevi credits her co-author Swapnil Shankar with the crucial work of writing and adapting the code to work on GPUs. Developing that infrastructure was necessary before the simulation could be created.

While the simulation is the first of its kind, it is still limited because it ends immediately after the black hole is formed. Halevi’s next step is to extend the simulation’s lifespan—specifically, observing what happens when the black hole evolves as time goes on.

Creating additional simulations with different stars in order to see differences in how those stars collapse into black holes—as well as the properties of those black holes—will be explored as the project moves along.

“We call it a parameter study, to explore those initial conditions and see what the results are,” says Halevi. “Now that we’ve been able to show this proof of concept, we want to get more of a population-level statistical understanding of what happens to stars once they die.”