What remains after the supernova depends on how massive the surviving remnant
is (which itself depends on how massive the star was to begin with). If the
collapsed core is between about 1.4 and two times as massive as our sun, it
will become a neutron star. Composed primarily of neutrons—its atoms have
all succumbed to the extreme pressure—a neutron star is so dense that a
teaspoonful of its matter would weigh about a billion tons.