Black holes slowly consume material from nearby stars, creating some of the universe's brightest X-ray sources.

X-ray binaries consist of a compact object—either a black hole or neutron star—orbiting closely with a companion star. Cygnus X-1, discovered in 1964, was the first such system widely accepted to contain a black hole. It features a black hole approximately 21 times the mass of the Sun in orbit with a blue supergiant star called HDE 226868, separated by roughly 0.2 astronomical units (about 20% of the Earth-Sun distance).As the black hole's gravity pulls material from the donor star, conservation of angular momentum forces the gas to spiral inward rather than fall straight in, creating an accretion disk. Friction and magnetic forces within the disk convert gravitational potential energy into intense heat. Temperatures in the inner disk regions exceed 10 million degrees Celsius, causing the superheated plasma to emit brilliant X-ray radiation.A 2021 study published in Science refined measurements of Cygnus X-1, placing it approximately 7,200 light-years from Earth. X-ray binaries are invaluable laboratories for testing general relativity and understanding matter under extreme conditions. The X-rays they produce are so intense that orbiting observatories like NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton can detect them, allowing astronomers to determine the spin and mass of black holes that would otherwise remain invisible.