Neutron stars were stars created by the collapse of much larger stars.
The Tythonians set a neutron star on a collision course with Chloris in retaliation for Lady Adrasta imprisoning Erato. After the Fourth Doctor freed him, he spun an aluminium shell around the star, allowing the Doctor to use his TARDIS' tractor beam to pull it off course and save Chloris. (TV: The Creature from the Pit)
The sun of Proamon had become a neutron star three thousand years after having been a red giant and two thousand years after having become a supernova. (TV: Dragonfire)
According to the Seventh Doctor, the original homeworld of the Quoth was created when three neutron stars collided "in exactly the right way." (PROSE: The Death of Art)
New Alexandria orbited around a neutron star. (PROSE: The Crystal Bucephalus)