Nothing is stranger than a black hole. The darkened corpse of a former sun from which not even light can escape, a black hole forms when a massive, dying star crumples under its own gravity. It shrinks until all of its mass is contained in an infinitely dense point, called a singularity. Its gravity is so intense, if anything ventures within an invisible border around the singularity, called the event horizon, it cannot escape.
Just outside the event horizon whirls high-temperature material — the accretion disk — waiting to “fall into” the black hole like water spiraling down a drain. The disk emits X-rays, a high-energy form of light, because the matter moves so fast that its friction generates a lot of heat. Jets of energy and matter, whose formations remain a mystery, can stretch away from the accretion disk for hundreds of thousands of light-years.
Nudging up against the event horizon, a ring of photons surrounds the black hole. This loop of light, called the innermost stable circular orbit, outlines the edge of the black hole like a bull’s-eye. And from its dead center, the black hole evaporates energy called Hawking radiation, causing the whole thing to shrink ever so slightly and slowly. Billions or trillions of years after its birth, the black hole will evaporate entirely.