Neutron Star

A star (approximately sun-sized or larger), a remnant of a supernova explosion, in which gravity has caused all matter to collapse to a giant nucleus, composed only of neutrons. The collapse is also expected to greatly amplify any magnetic field present in the pre-collapse star, as well as speed up enormously any rate of rotation. It is believed that pulsars, pulsating radio sources with very precise pulsation periods, are neutron stars of radius about 10 km and rotation period about 1 second. Their magnetic axis spins and beams radio waves, in a way similar to the way a lighthouse beams its light. We detect pulsars when the Earth is in one of the directions swept by the beams.

A sugarcube of neutron-star stuff on Earth would weigh as much as all of humanity!

See also: Chandrasekhar Limit, Star, White Dwarf.