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Simulating stars with less computing power

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  • A new method of conducting these simulations has been created that substantially reduced the computing effort required.
  • Until now, even the fastest supercomputers in the world could only simulate the very lightest of elements. 
  • The method, which involves simulating particles on a lattice grid instead of a free space, has been implemented on the JUQUEEN supercomputer to simulate the scattering and deflection of two helium nuclei. 

An international collaboration of researchers has developed a new method to simulate the creation of elements inside stars. The process was developed by researchers from the University of Bonn, and University of Bochum, Germany, working with North Carolina State University, Mississippi State University, and the Jülich research center in Germany. The goal of their work was to devise a way to allow simulations of this type to be conducted with less computational power. With this method they were able to model a more complex process that was not previously possible.

A large part of a star’s life is governed by the process of thermonuclear fusion, through which hydrogen atoms are converted into helium at the core of the star. But fusion also creates a host of other elements in the core of the star, produced by the fusion of the nuclei of helium atoms, which are also known as alpha particles.

But when scientists want to observe these processes they come up against a problem: the conditions inside the core of a star (15 million degrees Celsius in the case of our sun) are not reproducible inside a laboratory. Thus, the only way to recreate the processes inside a star is to use ‘ab-initio’ computer simulations.

This particular simulation required . . . only a number of days to run. The same simulation run with older methods would take JUQUEEN several thousand years to complete.

To allow more effective ab-initio simulation, the team devised a new technique that involves simulating the nucleons (the subatomic particles that comprise the nucleus of atoms) on a virtual lattice grid, instead of in free space. This allowed for very efficient calculation by parallel processing from supercomputers, and significantly lowered the computational demand required for the simulation. 

Using this technique, with the help of the supercomputer JUQUEEN at the Jülich Supercomputing Center, a simulation of the scattering and deflection of two helium nuclei was carried out that involved a grand total of eight nucleons. This may not sound extraordinary, but it is in fact unprecedented, as up until now even the fastest supercomputers in the world could only simulate the very lightest of elements involving a maximum of five total nucleons. 

The problem comes when the number of nucleons simulated is increased. Each of these particles interacts with every other particle present, which must be simulated along with the quantum state of each particle. “All existing methods have an exponential scaling of computing resources with the number of particles,” explains Ulf Meißner from the University of Bonn.

The difference this makes to the simulation process is astounding: this particular simulation required roughly two million core hours of computing using the new method, which on a supercomputer as powerful as JUQUEEN could take only a number of days to run. However, the same simulation run with older methods would take JUQUEEN several thousand years to complete.

“This is a major step in nuclear theory” says Meißner. He explains that this new method makes more advanced ab-initio simulations of element generation in stars possible. The next step Meißner and his colleagues are working towards is the ab-initio calculation of the “holy grail of nuclear astrophysics”: the process through which oxygen is generated in stars.

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