neutrons packed together as closely as the particles in an atomic nucleus. This is a very unusual and extremely dense form of matter, as most atoms consist mainly of empty space. A neutron star just twenty kilometers across would weigh twice as much as our sun, and a teaspoon of neutron-star stuff would weigh a billion tons.5 There is some evidence that many of the heavier elements in the periodic table may have been formed, not in standard supernovas,
