Final answer:
Neutron stars can remain stable in size due to the conservation of angular momentum.
Step-by-step explanation:
Neutron stars can remain stable in size because of the conservation of angular momentum. When a star collapses to form a neutron star, its size becomes much smaller, allowing it to spin more rapidly. The principle of conservation of angular momentum states that if an object gets smaller, it can spin faster. Therefore, even if the parent star was rotating slowly on the main sequence, its rotation would speed up as it collapsed to form a neutron star. With a diameter of only 10 to 20 kilometers, a neutron star can complete one full spin in only a fraction of a second.