Neutron Stars Are Atoms the Size of Mountains
Science channel Kurzgesagt, German for "in a nutshell," explained the nature of neutron stars, which they call "some of the strangest things in the Universe." They are not-quite black holes, and as a result, their properties are incomprehensibly extreme. They are essentially atoms, but on a massive scale.
As they explain in the video, neutron stars are "like giant atom cores" that are "unbelievably dense and violent." They are exactly like atom cores with one notable exception: atom cores are held together by strong interaction between subatomic particles, while neutron stars are held together by gravity.
They arise by the same mechanism as black holes: when the gravitational pull begins to outweigh the star's radiation pressure. Neutron stars are massive enough for this process to occur, but not massive enough to become black holes, so they become a bundle of neutrons that are "packed as densely as an atomic nucleus." It has the mass of three suns that is condensed into an object 25 kilometers wide, approximately the diameter of Manhattan. One cubic centimeter of a neutron star contains roughly the mass of Mount Everest.
A neutron star's other properties are equally extreme, such as its gravitational pull; if one were to drop an object from one meter above the surface, it would hit the ground at 7,200,000 km/hr. Its surface temperature is 1,000,000 Kelvin, or approximately 170 times the temperature of our Sun, and its magnetic field is a trillion times stronger than Earth's. The magnetic field is so strong, in fact, that atoms are literally bent when they enter it. Neutron stars also have an incredibly fast rotational speed; young neutron stars spin several times per second. If there is another star nearby to provide it with energy, then it can spin up to several hundred times per second. One such star has been observed as spinning at 252,000,000 km/hr, or 24% the speed of light. This object spins so fast, its circular shape is warped slightly into an elliptical.
Its core is a mystery to scientists, but their best guess is "superfluid neutron degenerate matter," or an ultra-dense quark matter called "quark gluon plasma." Neither of these substances "make any sense in the traditional way, [they] can only exist in such an ultra-extreme environment."