Black holes are among the most mysterious phenomena in the universe. Forged from the cores of dead stars, they are so dense that nothing can escape their gravitational pull, not even light, which renders them invisible. Entire stars, once luminous, can be extinguished if they cross a black hole’s boundary, and pass the point of no return.
Albert Einstein predicted more than a century ago, based on his theories untangling the nature of gravity, that such strange objects could exist, but he thought the idea was too far-fetched. In 1965, after Einstein’s death, Penrose, the Oxford professor, published a paper showing, mathematically, that the forces of the universe could indeed produce black holes, and that inside their impenetrable depths resides something called a singularity, an inscrutable point which no known laws of physics can describe.
Such a thing might still seem too incredible to exist, but without black holes, the movements of faraway stars in our galaxy don’t always make sense. Genzel and Ghez spent many years poking into the cosmic cloud of interstellar gas and dust at the very center of the galaxy, with the world’s largest telescopes. They discovered stars orbiting a seemingly empty spot at startling speeds, a chaotic environment that could make sense only in the presence of a supermassive black hole. This region in our galaxy, known as Sagittarius A* (pronounced ay-star), has a mass 4 million times that of our sun, squeezed into a space smaller than our solar system.
Astronomers have found other black holes, too, by watching for the dizzying orbits of the unlucky stars around them. They have seen black holes in the glow coming from matter as it plunges into