What Black Holes Bring to the Galaxy

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 the invisible depths, a process so intense that particles light up beautifully. And they have felt them in the ripples in the fabric of space-time, the gravitational waves that fan out across the universe when two black holes collide. Black holes, it turns out, are everywhere, in the center of most galaxies and spread throughout them, and they come in different sizes. (Some even appear to be so big that, theoretically, they shouldn’t exist.) Earlier this year, astronomers found the closest known black hole to Earth 1,000 light-years away, almost on our doorstep by cosmic measures, in a constellation that can be seen with the naked eye. The variety in discoveries is quite impressive for an object best known for its nothingness.

That nearby black hole is no threat to Earth. No known black hole is. If anything, we benefit from their existence. The stellar explosions that produce black holes also spew elements such as carbon, nitrogen, and oxygen into space. The collisions of black holes and neutron stars help spread heavier elements, such as gold and platinum. These elements make up our Earth, and our own selves.

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