Neutron Stars

Collapsed cores of massive stars packing 1.4 solar masses into a city-sized sphere of degenerate neutron matter.

Jocelyn Bell Burnell discovered the first pulsar in 1967, a signal so regular it was briefly nicknamed LGM-1 for 'little green men'.
The fastest known pulsar, PSR J1748-2446ad, spins 716 times per second.
Their matter is so dense that a sugar-cube-sized piece would weigh about a billion tonnes on Earth.