For centuries, black holes were merely theoretically speculative ideas.
The concept first appeared in 1783, when John Michell proposed them.
If you maintained the Sun’s density but increased its mass, light couldn’t escape above ~500 solar masses.
Although none were observed, the idea resurged with Karl Schwarzschild’s 1916 solution within Einstein’s General Relativity.
With enough mass in a given spatial volume, collapse to a black hole becomes unavoidable.
In 1963, Roy Kerr enhanced Schwarzschild’s solution to incorporate rotation.
Contemporaneously, suggestive “black hole” evidence appeared with the discovery of the first quasars.
These extragalactic QUAsi-StellAr Radio Sources (QUASARs) were ultra-distant, but shone brilliantly in radio light and beyond.
Then Cygnus X-1, an X-ray emitting black hole candidate, was found within the Milky Way.
Meanwhile, Roger Penrose demonstrated, astrophysically, how black holes could pragmatically form in our Universe.
John Wheeler gave the name “black holes” in 1968.
Once speculative, the modern case for them is overwhelming.
X-ray emissions appear from accelerating, infalling, accreted matter.
Individual stars orbit these massive, non-luminous objects.
Gravitational waves arise from both inspirals
And photon emissions now reveal their horizons,
directly. Welcome to the golden age for black holes.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talkless; smile more.