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 IU Trident Indiana University

Waffle-shaped Nuclear Matter May Lie Deep Inside Neutron Stars

Project Leads: CCharles Horowitz, Professor of Physics and member of the Center for Exploration of Energy and Matter, Indiana University

Scientific Applications and Performance Tuning (SciAPT), Advanced Visualization Lab (AVL), UITS Research Technologies 

Funded by: DOE grants DE-FG02-87ER40365 (Indiana University) and DE-SC0008808 (NUCLEI SciDAC Collaboration)

Neutron star interior simulation
Figure 1. Simulation of the waffle-like structure of a neutron star.  

Molecular dynamics simulations show that nuclear matter about 1,000 meters deep in a neutron star may form waffle-like layers. In simulations performed on IU's Big Red II supercomputer, matter consisting of 30% protons and 70% neutrons formed into perforated plates, dubbed "nuclear waffles," when its density was set to 84 trillion grams per cubic centimeter, and its temperature to 116 billion Kelvin. This proton fraction, density, and temperature exist about 1,000 meters below the surface of a neutron star. Read the New Scientist article at:

The detailed form of matter at this depth is important because it determines much of the overall properties of a neutron star. It also suggests the makeup of the star's surface, which might be some kind of iron with an atmosphere of hydrogen or helium. Physics professor Chuck Horowitz, leader of the group that did the simulations, says that understanding the structure of the interiors of neutron stars provides clues about extraordinary physics that could help in the design of stronger materials like burr-inspired Velcro and swimsuits inspired by shark skins. Nuclear pasta's design may also have an impact in the energy industry.

A neutron star is the remnant of a massive star after it undergoes a core-collapse (or Type II) supernova at the end of its life. What remains is a compact, super-dense ball of nuclear matter containing more mass than the sun in a sphere just 10 kilometers in radius.

Below about 1,000 meters a neutron star consists of uniform neutron matter. Above that depth neutrons and protons bind into individual nuclei that are neutron-rich isotopes of elements found on earth, except completely ionized and packed to densities of order ten-trillion grams per cubic centimeter. However, physicists have long known that there is a transition layer about 100 meters thick between these two domains. At the top of the layer, the nearly spherical nuclei begin to merge, their neutrons and protons combining to form long cylindrical shapes. As density increases further down, the shapes transform to slabs, then to cylindrical voids, then spherical voids, or bubbles. Finally, at the bottom of the layer, the bubbles decrease and matter becomes uniform. Because of the shapes, this 100-meter-thick layer has been called "nuclear pasta."

Nuclear pasta is impossible to study on earth, and cannot be observed directly, so physicists have turned to theoretical calculations and computer simulations such as molecular dynamics (MD) to understand it. MD simulations allow one to follow the motion of individual neutrons and protons as they interact with each other. Such simulations have shed much light on the nature of nuclear pasta. The recent discovery by Horowitz and his colleagues of the waffle phase via large-scale molecular dynamics simulations on Big Red II will help physicists better understand nuclear pasta and neutron stars.

The Scientific Applications and Performance Tuning (SciAPT) group delivers and supports software tools that promote effective and efficient use of IU’s advanced cyberinfrastructure – which in turn improves research and enables discoveries.

IU's Advanced Visualization Lab is funded through the Office of the Vice President for Information Technology; is a unit of UITS Research Technologies; and is affiliated with the Pervasive Technology Institute. For questions about this project or to request a consultation, please contact

NSF GSS Codes:

Primary Field: Astronomy (201) - Astrophysics

Secondary Field: Computer Science (401) - Computer Systems Networking and Telecommunications