Neutron stars pack more than the mass of the Sun into a ball roughly the size of a city. Their matter is so dense that a sugar-cube sample would weigh around a billion tonnes.

Their surfaces are smoother than polished marble, with mountains only millimetres high. Some spin hundreds of times a second and sweep beams across space like lighthouses.

Why it is in the vault: it is the kind of find worth coming back to. Add sources, corrections and related links in the discussion thread.