Hubble Searches for Rare Black Holes Close to Earth

NASA's Hubble Space Telescope has detected a possible intermediate-mass black hole of roughly 800 solar masses in the core of the globular star cluster Messier 4, located 6,000 light-years away from Earth. The black hole's mass is calculated by studying the motion of stars caught in its gravitational field. Hubble's data tend to rule out alternative theories for this object, such as a compact central cluster of unresolved stellar remnants like neutron stars, or smaller black holes swirling around each other.