Neutron stars have an immense density--they are only about 12.5 miles in diameter but are about 1.5 times heavier than Earth's sun. According to National Geographic, a piece of a neutron star the size of a sugar cube weighs about 100 million tons. In addition, because of their incredible density, neutron stars have a gravitational field about 100 billion times more powerful than Earth's, according to Georgia State University.
The first official observation of a neutron star occurred on November 28, 1967, states Franco Pacini of the University of Florence. However, astrophysicists in the 1930s started the investigation into the final state of collapsed stars. Discovery of another pulsar in 1968 in the Crab Nebula led scientists to delve further into the subject of neutron stars.
Before a star becomes a neutron star, it swells to several times its original size and then explodes in a process called "going supernova," according to National Geographic. After going supernova, a star loses its outer shell and cannot produce fusion any more, letting its core collapse in on itself and attain great density.
Nobody really knows what goes on in the core of a neutron star, according to National Geographic. Because of the massive density that combines protons and electrons into neutrons, it probably contains a neutron superfluid (which has no viscosity, so it can flow endlessly) or a phase of matter we do not yet know about.
Theoretically, neutron stars could help scientists create antimatter--matter with reverse properties--that would provide the energy for interstellar travel, according to The Register UK. However, as of 2008, nobody knows how to come into possession of a neutron star to create antimatter.