Neutron Cool

Just over 300 years ago, people looking into the night sky may have seen a spectacular explosion, with embers of light lasting for weeks.

People at the time may not have knew it, but scientists now know the explosion was a supernova in the distant Cassiopeia A galaxy that eventually left behind a neutron star.

In the 330 years since, the star’s thermal evolution has behaved unexpectedly, dropping from around 20 billion degrees Fahrenheit at birth to its current temperature of around 4 million degrees.

Strange cooling

“Just over the last 10 years alone the temperature of this neutron star fell by 150 thousand degrees, which is even more puzzling,” said Fridolin Weber, a theoretical astrophysicist in San Diego State University’s College of Sciences.

Weber, along with overseas collaborators, published an article in The Physical Review, providing a theoretical explanation of the puzzle by suggesting the existence of certain novel states of matter in the cores of neutron stars.

Neutron stars explained

Neutron stars are very particular entities, Weber said. They are dense, neutron-packed remnants of massive stars that blew apart in supernova explosions. They are typically about 20 kilometers across and spin rapidly, often making many hundred rotations per second.

“Imagine — if you could hold on to the equator of such a rapidly rotating object and withstand the tremendous forces pulling on you, you would move past the stars in the universe at more than half the speed of light,” Weber said.

Many of the bizarre properties of neutron stars come from their enormous densities. A single thimble of matter from a neutron star, Weber said, would carry a mass of one billion tons — nearly the same as the Mount Everest massif — in the same space of a few teaspoons.  

Under such extraordinary conditions atoms themselves collapse, and atomic nuclei are squeezed so tightly together that new fundamental particles are generated and novel states of matter are created. On Earth, these particles and matter states can only be created by the most powerful particle collider machines, said Weber.

These features, together with the unprecedented progress in observational astrophysics, make neutron stars exceptional astrophysical laboratories for a broad range of physical studies, such as the one carried out in Weber’s recent paper.

The big picture

It is believed that in the Milky Way Galaxy alone there may be up to one billion neutron stars. A significant fraction of the matter in this galaxy—and probably in any other galaxy as well—is therefore not simply in the form of molecules and atomic nuclei, as we may be inclined to think, but has given way to new and exotic states of matter residing deep inside of neutron stars.

“What many people believe to be well understood – neutron stars, black holes, etc., really are not,” Weber said.

“Truthfully, even the matter that living things are created from is not greatly understood.”

Of course, that is largely what Weber and his colleagues are seeking to find — a greater understanding of matter and the structure of neutron stars, which could greatly affect human life on Earth.