Neutron Star

Neutron Stars  are incredibly dense stars composed entirely of degenerate neutrons. They are formed after a star of 8 M☉ to 20 M☉ explodes in a type Ib, Ic or II supernova. The nature of their formation leaves them rotating incredibly quickly – with a rotational frequency of 0.033 rotations per second to a whopping 716 rotations per second. The period of these rotations remains very regular. These rotations do slow down, however, by about 10-15 seconds per rotation. Thus a neutron star rotating with a period of 1 second will have a period of 1.03 seconds after about 1 million years.

Neutron stars that are highly magnetized emit beams of electromagnetic radiation which can only be seen if pointed directly at the observer. Thus these neutron stars, known as pulsars, are incredibly accurate timekeepers.

About 5% of neutron stars are part of a binary system.

Neutron stars have a mass of about 1.4 M☉ to 3 M☉ and a radius of between 10 km to 13 km. The relationship between the mass and the radius of a neutron star is not currently understood, but the values will definitely lie between these ranges.

Habitability (for life as we know it) around a neutron star is almost certainly impossible. Other, non life-bearing worlds could easily exist in a neutron star system however.

Worldbuilding in Practice
"Far away from human territory, in the deep depths of uncharted space, PSR J0048-7C6, a small pulsar, rotates at 1.833 revolutions per second, sending consistent pulses of radiation at the Tyson cluster. One of many pulsars in the galaxy, it is almost completely unremarkable, other than the fact that its rotational frequency happens to be almost 110 revolutions per minute, which matches the 110 bpm speed of the 20th century human musical group Queen's 'Another One Bites the Dust'. A popular internet meme is a video of the pulsar as viewed through the Laswell Telescope accompanied by a ten hour loop of the aforementioned song. It has 17 trillion views."