Neutron Stars: Billions of Times Stronger Than Steel

Massive imperfection. A bump (red) on a super-dense and rapidly spinning neutron star (blue) can disturb spacetime.

Charles Horowitz and Kai Kadau/Indiana University and LANL

Talk about a hard body. New supercomputer simulations of the crusts of neutron stars--the rapidly spinning ashes left over from supernova explosions--reveal that they contain the densest and strongest material in the universe. So dense, in fact, that the gravity of the mountain-sized imperfections on the surfaces of these stars might actually jiggle spacetime itself. If so, neutron stars could offer new insights into a mysterious phenomenon known as gravity waves.

Astrophysicists already know that neutron stars are very dense. The property results from the way they form: When a giant star runs out of fuel and can no longer fight against the crushing force of its gravity, its core shrinks to the size of an asteroid, and most of its mass is blasted away in a titanic explosion called a supernova. What's left is a relic containing gigantic amounts of matter packed into a very small space that can rotate hundreds of times per second. Calculations reveal that the stars weigh as much as 90 million metric tons per teaspoon. But until now, no one has figured out the material's strength.

That's what theoretical astrophysicist Charles Horowitz and materials scientist Kai Kadau have done. Horowitz, of Indiana University in Bloomington, and Kadau, of Los Alamos National Laboratory in New Mexico, ran supercomputer simulations of how the material constituting neutron stars forms at the atomic level. Computing the effects of the star's titanic gravity on the structure of its constituent atoms, the researchers report in an upcoming issue of Physical Review Letters that the material in the star's crust is at least 10 billion times stronger than the toughest steel. It has to be, Horowitz says, to contain the immense electromagnetic forces building up within the whirring star. For example, he says, gamma-ray bursts originating from magnetars--the most highly magnetized versions of neutron stars--arise when energy buildups periodically cause the crust to rupture, in phenomena called starquakes. To hold in that much energy, Horowitz explains, their crusts must be as strong as the simulations suggest.

That incredible strength also means that when neutron stars form they can tolerate some imperfections on their surfaces. In this case, such imperfections can be mountain-sized bumps as heavy as Earth. As those bumps ride the fast-spinning stars, their mass disturbs spacetime enough to generate gravity waves, the simulations by Horowitz and Kadau show. First predicted by Albert Einstein, the waves are disruptions that radiate through the very fabric of spacetime. They travel as fast as light and can stretch every atom they encounter. Scientists have deployed new instruments in recent years in an attempt to observe the waves, but so far they have remained elusive.

The calculations by Horowitz and Kadau could change that, says physicist Benjamin Owen of Pennsylvania State University, University Park. The research shows that neutron stars "could emit a hundred times more energy in gravitational waves than we thought," he says. And that could make the waves easier to detect in the future.