On the heels of Einstein’s general theory of relativity, German theoretical astrophysicist Karl Schwarzschild provided a detailed proposal on the existence of black holes in 1916. The concept of black holes goes all the way back to the 1780s, when John Michell and Pierre Simon Laplace envisioned “dark stars” whose gravity was so strong that not even light could escape. As with many startling ideas, the acceptance of black holes as real objects took a long time.
It wasn’t until astronomers were able to observe lots of galaxies and massive binary star systems in the 1970s and early 1980s that it became obvious black holes must exist. In the 1990s, it became clear to astronomers that black holes not only exist, but are plentiful.
A black hole is a region of space-time affected by such a dense gravitational field that nothing, not even light, can escape it. Consider the escape velocity on Earth: If you could throw a baseball at a velocity of 7 miles per second, you could hurl it into space, overcoming Earth’s gravitational tug. As massive objects are crushed into smaller volumes, their gravitational tug increases dramatically. In a black hole, the escape velocity exceeds 186,000 miles per second — the speed of light — and everything inside the hole is trapped.
So, if black holes are black, how do astronomers know they exist? Not directly visible, black holes must be detected by their effects on nearby stars, gas, or dust. In the Milky Way, many stellar black holes — with masses in the range of about 10 times that of the Sun — exist in binary star systems.
When a massive star dies, it explodes as a supernova. But the core of the exploded star remains behind as either a neutron star or, if it’s heavy enough, a black hole. Black holes become visible when they exist in X-ray binaries, twin star systems in which one of the stars has become a black hole and the other is still there. The black hole shreds or perturbs its companion, the result of which unleashes X-ray energy.