This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange). GW190521 is highlighted in the middle of the graphic as the merger of two black holes that produced a remnant that is the most massive black hole observed yet in gravitational waves. Image credit: LIGO-Virgo/ Northwestern U. / Frank Elavsky & Aaron Geller.