A neutron star is a stellar remnant--a super-compressed object left over when stars with a mass between 1.4 and about 3 times the mass of our Sun exhaust their nuclear fuel and collapse inwards. The result is a condensed sphere of matter about 20 km (12 miles) across, with a gravitational field approximately 2 x 10^11 times stronger than that of Earth's.

The density of a neutron star is so great that the protons and electrons making up the atoms fuse to form electrically neutral neutrons, the primary particles making up the neutron star. Because they are electrically neutral, such particles can be packed very closely together, resulting in a celestial object with similar density to that of the atomic nucleus.

The neutron star is an exotic astronomical object whose existence was predicted by theory 35 years before one was actually discovered in 1968. The escape velocity for a neutron star is approximately half the speed of light. The tallest "mountains" on such a star measure in the millimeters (fractions of an inch) rather than kilometers (feet). Because the rotation speed of the star accelerates as it collapses, tremendous rates of angular velocity may be achieved, on the order of 30,000 km/sec (18,640 mi/sec), or one rotation every millisecond or two. When these rapidly rotating stars emit electromagnetic radiation that can be detected on Earth, they are received in continuous pulses, prompting the title "pulsar."

Formed from the core of expired suns, the neutron star is home to exotic forms of matter found nowhere else in the universe: Nuclei composed of huge amounts of neutrons with no orbiting electrons, free neutrons floating in a superdense "neutronium" soup, and possibly exotic forms of matter such as pions or kaons. These are particles composed of unusual configurations or types of quarks, the constituents of subatomic particles. Because conventional atomic forms of matter would be ripped to shreds by the immense gravity and pressure of a neutron star, we may never be able to perform experiments or observations on such objects directly. The primary types of neutron stars include the x-ray burster , pulsar, and magnetar.