1965

Black Holes

Roger Penrose (b. 1931)

Even though they are among the strangest, most exotic, and perhaps most misunderstood objects in the universe, black holes can really be thought of simply as collapsed stars. Part of their mysterious appeal comes from their fundamentally unobservable nature and the fact that we can only learn about them by observing the strange and beautiful ways that they modify and interact with their surroundings.

Once a big enough star, maybe 5 to 10 times the mass of our Sun, converts all its hydrogen to helium and other heavier elements, nuclear fusion reactions can no longer counterbalance gravitational forces, and the star collapses. The collapse eventually causes an enormous supernova explosion, which ejects much of the star’s material into space. Some of that explosive energy goes into further compressing the dense remaining core of the star, however, which continues to contract, to grow more dense, and to radiate more energy. If the collapsed star’s mass continues to grow (perhaps by stealing material from a companion star), at some point not even light may be able to escape from the object’s gravity. The region around the core would then appear, from the outside, to look like a black hole. Physicists know of no force in nature that can stop the collapse; in 1965 the British astrophysicist Roger Penrose proved mathematically that black holes can be formed from collapsing stars and that massive stellar should shrink to an infinitely small point known as a singularity. Weird stuff, indeed.

But since gravity falls off as one moves away from such an object, beyond some distance (called the event horizon) any light or other radiation related to the black hole can escape and be observed. Much of this observed radiation is a result of gas or dust being accelerated to enormous velocities by the black hole’s huge gravitational and magnetic fields. Quasars are thought to form in such areas.

Einstein’s theory of relativity predicts that many strange things happen near a black hole’s event horizon, including time itself appearing to stand still as viewed by an observer from the outside. Unfortunately, since information can never escape from a black hole, it may never be possible to truly know what they are like up close.

An artist’s impression of a supermassive black hole “stealing” gas from a companion in a binary star system. The gas forms an accretion disk around the black hole that can radiate enormous amounts of energy into strong polar jets as the gas falls into the black hole’s gravity well.