neutron star

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A celestial body consisting of the superdense remains of a massive star that has collapsed with sufficient force to push all of its electrons into the nuclei that they orbit, thus leaving only neutrons, and having a powerful gravitational attraction from which only neutrinos and high-energy photons can escape, rendering the body detectable only by x-ray.

Wiktionary

1. n. astronomy A degenerate star that has been so collapsed by gravity that its electrons and protons have been merged into neutrons by the intense pressure. The solid mass of neutrons is sometimes called neutronium.

WordNet 3.0

1. n. a star that has collapsed under its own gravity; it is composed of neutrons