neutron star

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A celestial body having a mass of 1.4 to 2.1 solar masses contained within a radius of only about 10 kilometers (6 miles), formed when a massive star collapses with sufficient energy to force all of its electrons into the nuclei that they orbit, where they react with protons, leaving only neutrons. Neutron stars are powerful sources of radio waves and x-rays.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a star that has collapsed under its own gravity; it is composed of neutrons