When a star the size of 20 suns dies, it becomes, in the words of astrophysicist Zaven Arzoumanian, “the most outrageous object that most people have never heard of”—a city-size body of improbable density known as a neutron star. A chunk of neutron star the size of a Ping-Pong ball would weigh more than a billion metric tons. Below the star’s surface, under the crush of gravity, protons and electrons melt into one another to form a bulk of mostly neutrons—hence the name. At least, that is what we think. The issue is far from settled. Astronomers have never seen a neutron star up close, and no laboratory on Earth can create anything even approaching the same density, so the inner structure of these objects is one of the greatest mysteries in space. “They are matter at the highest stable density that nature allows, in a configuration that we don’t understand,” says Arzoumanian, who works at NASA’s Goddard Space Flight Center. They are also the most strongly gravitating form of matter known—add just a bit more mass, and they would be black holes, which are not matter at all but rather purely curved space. “What goes on at that threshold,” Arzoumanian says, “is what we’re trying to explore.”