Neutron stars fascinate me. A star with the mass of the sun, but only 20 or so miles wide. A solid crust of degenerate matter, mostly electrons, being crushed into what can be described as a giant atomic nucleus. When that star hits a weight of 1.44 times the mass of the sun, gravity wins over the strong nuclear force and you get a BOOM.
This impact of two neutron stars created a cloud of gold, platinum and other very heavy elements that can be measured and modeled for future study.
We've only detected gravity waves for two years and we are already learning, and verifying existing theory, about how the universe works.