In a 1783 paper English scholar John Michell envisioned a voracious cosmic monster: a star that was massive enough that its gravity would swallow light. He speculated that many such behemoths might exist, detectable only by their gravitational effects. Two centuries later, in 1967, American physicist John Wheeler gave the idea an evocative name: black hole. Just a few years afterward, in 1974, British astrophysicist Stephen Hawking taught us that black holes aren't so black after all: they emit radiation and will eventually evaporate.
Yet we've learned a lot since then about these destructive cosmic engines, as you'll find in “The Benevolence of Black Holes,” this issue's cover story by Caleb Scharf of Columbia University—and an excerpt from the latest entry in our Scientific American book imprint series with Farrar, Straus and Giroux. Starting on page 34, the feature article explains how the feeding habits of black holes can have surprising effects on the galaxy they occupy.
Too little black hole activity, and a galaxy might produce a surfeit of youthful stars exploding as supernovae. Too much, and it would suffer from reduced star formation, robbing it of the star-fused heavy elements such as iron, silicon and oxygen that form our own planet. Fortunately, our Milky Way sports a supermassive black hole with four million solar masses that is “just right”: active enough to churn things up productively for star formation but not so much that it eliminated the possibility of our own solar system's existence. In fact, Scharf argues, our galaxy's black hole had much to do with our ability to live in this place at this time. “The entire chain of events leading to you and me would be different” without such black holes, he writes. “We owe so much to them.”
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