Neutron Stars

Neutron stars are, as the name describes, stars that consist almost entirely of neutrons. Because neutrons have more mass than protons, these stars are extremely dense. These stars can result from a super-massive star collapsing during a supernova. This event is described in the video below (this depiction is actually that of a pulsar, a type of neutron star that emits a beam of extremely bright electromagnetic radiation).

When a star's core compresses into a neutron star, as shown above, its rotation increases incredibly rapidly. However, this effect wears down with time. Neutron stars can suffer gravitational collapse and become a black hole as well. Neil Degrasse Tyson describes neutron stars as so dense that "if you cram 50 million elephants into a thimble, you get the density [of a neutron star]."