Cracking The Crusts Of Neutron Stars

NSCL professor Bill Lynch inspects the mini-ball, a detector at the MSU laboratory used to analyze fragments produced when nuclei collide at high velocities. (Credit: Harley Seeley, MSU)

Research by Michigan State University scientists is helping shed light on neutron stars, city-sized globs of ultra-dense matter that occasionally collapse into black holes.
A team led by Betty Tsang, a professor at MSU’s National Superconducting Cyclotron Laboratory, has had some success in measuring a key nuclear quality that may make it easier to describe the outer crusts of such stars.

A neutron star is produced when a massive star explodes as a supernova and then collapses onto itself. The result is one of the oddballs of the universe, a star that is roughly 15 miles in diameter but more massive than the sun. On Earth, a teaspoon of a neutron star – think of a dense pudding of nuclear matter, most of it neutrons and all of it packed tightly together – would weigh about 1 billion metric tons if it were taken from the inner crust of the neutron star. If the teaspoon were taken from the denser interior where neutrons are more tightly packed, the matter could weigh up to 10 billion metric tons.

Atomic nuclei are composed of positively charged protons and neutral neutrons. Proton-neutron forces help to bind a nucleus together while proton-proton and neutron-neutron interactions exert a pressure that tend to push a nucleus apart or support a neutron star against collapse into a black hole. This pressure can be obtained by determining how the symmetry energy, which is difference between the energy of a system of only neutrons and another with equal numbers of neutrons and protons, depends on the density.

Tsang, along with Bill Lynch and Pawel Danielewicz, also professors in the NSCL, were interested in refining the understanding of symmetry energy, estimates of which have ranged widely in most theoretical models describing neutron stars.

To do this work, the researchers studied third-of-the-speed-of-light collisions of tin nuclei wherein nuclear densities were varied during a series of experiments at the NSCL's Coupled Cyclotron Facility.

Tsang's result, to be published in Physical Review Letters, helps to describe the crust of neutron stars where the density of nuclear matter is about half of normal nuclear density. New and planned more powerful accelerator facilities in Japan, Germany and the United States will help to further characterize symmetry energy in the ultra-dense cores of such stars.

Among those facilities is the Facility for Rare Isotope Beams, a $550 million project scheduled to be built at MSU.

Tsang's research is supported in part by the National Science Foundation, which provides funding both for NSCL and the Joint Institute for Nuclear Astrophysics.