Summary
Highlights
A black hole is a region in space with gravity so strong that nothing, not even light, can escape. The boundary is called the event horizon, and beyond it, matter collapses into an infinitely small, dense singularity where physics breaks down.
There are different types of black holes. Stellar black holes, 10-20 times the sun's mass, form from collapsed massive stars and are common. Supermassive black holes, millions to billions of times the sun's mass, are found at the center of most large galaxies, including the Milky Way's Sagittarius A.
Since black holes are invisible, scientists detect them by observing their effects on nearby matter. This includes studying accretion disks, which are swirling gas and dust falling into a black hole, and quasars, which are powerful jets of particles blasting from supermassive black holes.
Black holes were largely unknown until the 20th century. In 1916, Karl Schwarzschild, using Einstein's theory of relativity, calculated that any mass could become a black hole if compressed enough. The first black hole was discovered in 1971 in the constellation Cygnus.