Black holes
  Black holes are defined as a region in spacetime where gravity is so strong
that particles and electromagnetic radiation cannot escape. According to their
mass, they are classified into three types: stellar-mass black holes,
intermediate-mass black holes, and supermassive black holes. This entry
describes how to weight and detect these three types of black holes, summarizes
key research findings such as the universality of black hole accretion and
black hole-galaxy co-evolution, and gives an outlook to what the next
generation of observational facilities will provide.
