The rapid spin of some astrophysical objects, such as millisecond pulsars, is initiated and maintained through a combination of factors. Firstly, the object must be small and dense, with a radius less than 30 miles (48 kilometers) to allow for sub-light-speed rotation. Neutron stars and black holes are the only stellar-mass objects that meet these criteria.
The process of reaching millisecond spin periods often involves accretion, where the object gains mass by pulling in material from a nearby companion. As this accreted material spirals in, it imparts angular momentum to the object, causing it to spin faster. This can occur as part of the collapse of a slowly spinning star into a neutron star or black hole.
Once these objects achieve rapid spin, their spin can be maintained with remarkable precision due to their flywheel-like nature. The spinning neutron star or black hole, now with a considerable amount of angular momentum, experiences minimal resistance in the vacuum of space.
However, millisecond pulsars, which are neutron stars, exhibit a slight slowing of their spin over time due to the weak magnetic field connecting them to the universe. This magnetic field introduces a small amount of “friction,” allowing scientists to predict changes in pulse arrival time with high precision for various experiments, including measurements of pulsar orbits and searches for gravitational waves.