Summarize the following passage.A neutron star is the core formed by the collapse of a massive supergiant star, with a total mass between 10 and 25 times that of the Sun, or possibly more if the star is particularly metal-rich. Neutron stars are the smallest and densest stars known, except for black holes and hypothetical objects such as white holes, quark stars, and strange stars. The radius of a neutron star is about 10 kilometers (6.2 miles), with a mass of approximately 1.4 solar masses. They are formed by a combination of a supernova explosion of a massive star and gravitational collapse, which compresses the core to a density beyond that of white dwarfs, reaching nuclear density.