The composition of their cores is unknown, but they may consist of a neutron superfluid or some unknown state of matter.
Normal-state helium damps out vibrations better than superfluid helium.
Suddenly, boiling ceases at the transition from the normal phase of helium to the superfluid phase.
British Dictionary definitions for superfluid
(physics) a fluid in a state characterized by a very low viscosity, high thermal conductivity, high capillarity, etc. The only known example is that of liquid helium at temperatures close to absolute zero