An artist depicts a disk of hot gas whipping around a neutron star. The gas in the inner part of the disk whirls around the neutron star at about 40 percent the speed of light, so fast that it experiences effects predicted by Einstein's theories of relativity. Superheated iron atoms in this region emit X-rays at a characteristic wavelength, but the spectral feature is highly distorted by the relativistic effects. NASA/Dana Berry

Neutron stars contain the densest observable matter in the universe.

They cram more than a sun's worth of material into a city-sized sphere, meaning a few cups of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be crammed under the most extreme pressures that nature can offer.

"This is fundamental physics," says Sudip Bhattacharyya of NASA's Goddard Space Flight Center in Greenbelt, Md. and the University of Maryland, College Park. "There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it's impossible to create them in the lab. The only way to find out is to understand neutron stars."