Jefferson Lab



Neutron stars are the remnants of violent supernovae, all that’s left behind when a star tens of times the mass of our Sun ends its nuclear fuel-burning life. These extreme objects pack more mass than our Sun — about 1.4 Suns’ worth of mass, to be more exact — into a stellar remnant about the width of a small city (6–12 miles [10–20 kilometers]).These tiny, distant objects get their name from the fact that they’re almost entirely composed of neutrons. But they do contain a small fraction (about five percent) of protons. Now, new research indicates these protons may have more influence over the properties of the star — such as its size, its temperature, and its “stiffness” — than previously thought. The finding, published August 13 in Nature , was made by members of the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) Collaboration, which included researchers at MIT, Tel Aviv University, and Old Dominion University.The data the team used, however, didn’t come from neutron star observations. Instead, the data came from neutron star analogs — dense atomic nuclei here on Earth. Although atomic nuclei aren’t quite as densely packed as neutron stars, they are easier to observe and can still give insight into the inner workings of some of the most extreme objects in the universe.How can such a small fraction of the material in such a massive object have any sway at all? It’s all thanks to something called short-range correlations. Let’s start simple: In the nucleus of an atom, there are protons and neutrons, surrounded by electrons. All of these particles are packed into a limited space. These particles are continually moving — anything at a temperature above absolute zero is moving, in fact, because temperature and energy are related. As they move, the protons and neutrons can come into contact and interact with each other. This process is called a short-range correlation, and it can significantly affect the properties of the nucleus because of the energy involved.Now, instead of the nucleus of an atom, picture a neutron star. It’s still a packed system of particles constrained into a certain space, but this time the particles are mostly neutrons, with just a few protons, and the space is much larger than an atomic nucleus. The same principle still holds, however — because they are moving, the protons and neutrons can come into contact and interact with each other in short-range correlations, just as in an atomic nucleus.The final key to the puzzle is the fact that in short-range correlations, protons carry more of the energy than neutrons. “We think that when you have a neutron-rich nucleus, the protons move faster than the neutrons, so in some sense protons carry the action on average," said team member Or Hen of MIT in a press release . “Even though protons are the minority in the star, we think the minority rules. Protons seem to be very active, and we think they might determine several properties of the star.”