“Homing in on Black Holes”
by Rob Irion
Smithsonian, April 2008
The weird beasts of the cosmic zoo have wonderfully self-explanatory names, from brown dwarfs to pulsars to gamma ray bursts. Strangest of all, but most simply named, are black holes: the invisible corpses of stars that exploded long ago. Nothing can escape these bottomless pits, not even light. For years it was easy to dismiss black holes as the stuff of science fiction. Now astronomers believe our Milky Way galaxy is chock- full of them — including a giant one at the center, darkness at the heart of a whirlpool of stars.
The evidence comes from the world’s biggest telescopes, which use infrared vision to peer through thick dust shrouding the Milky Way’s core. This vision — sharpened in recent years by new laser technology — reveals hundreds of stars dashing around a point exactly at the galaxy’s center. Some stars dive so near this vortex that they travel on cigar-shaped paths at a remarkable 3% the speed of light. According to the laws of physics, only one object could exert such influence: a supermassive black hole, pulling with the gravity of nearly 4 million suns.
In a 3,000-word feature for Smithsonian, I will introduce the astronomers who built this case, year by year, during the last decade. I will join Andrea Ghez of UCLA for four nights in July at the summit of Mauna Kea in Hawaii to observe the galaxy’s center. Ghez uses one of the twin Keck Telescopes, which stare at the heavens with segmented mirrors 33 feet wide. Her team maps the motions of dozens of bright stars that crowd the galaxy’s center in a tight, dazzling swarm. Charted over the years, these motions make a time- lapse movie that graphically reveals the black hole’s relentless pull. A rival group in Germany, using an exceptional array of telescopes in Chile, has created a similar film.
The Germans also see signs that giant stars are born within the turbulent gas whirling around the black hole. This discovery has startled theorists, who thought all stars arose in calm nurseries of gas and dust.
To track these stars, astronomers learned how to erase the blurring of our atmosphere. Normally, Earth’s blanket of air makes a star’s image shimmer, just as water in a pool distorts our view of the bottom. But by shining a powerful laser into the sky,
astronomers create an artificial “star” high overhead. Computers at the telescope analyze this twinkling speck and adjust a flexible mirror dozens of times every second to cancel its jitters. The rest of the telescope’s image then becomes crisp and clear. This wizardry, called “laser adaptive optics,” brings the galaxy’s center into focus — exposing the black hole’s sway beyond doubt. I will examine the innards of this system by daylight and describe it in action, when a sharp beam pierces the night over Mauna Kea.
My article also will take readers into the cores of other galaxies, where similar studies point to gargantuan black holes. Indeed, the Milky Way’s hole is modest; others appear thousands of times as massive. Gas and stars spiral into the most active black holes in feeding frenzies that create white-hot disks of matter. Radiation from the blazing disks blasts outward through the galaxies — dictating how large they grow and when they form stars. This is a new picture of the universe, where black holes and their host galaxies grow up hand in hand. Observers and theorists agreed on this scenario only recently, but they still don’t understand how it works. An ultracompact black hole is extraordinarily small on the vast scale of a galaxy, yet it seems to control how the entire galaxy evolves. “It’s almost magical,” says Nickolay Gnedin of the University of Colorado. “It’s as if one particular germ in your body knows you have two arms and two legs.”
In a sidebar, I intend to pose simple questions about black holes to the leading theorists and observers: When you think of a supermassive black hole, what does it look like?
How did it form? What does it do to the familiar laws of physics, and to space and time itself? Readers might know, for instance, that the extreme gravity of a black hole would slow down the clock of an approaching traveler — from our perspective on the outside — and that the hole would rend the doomed voyager into a string of atoms. But really, this is so bizarre that it doesn’t help us imagine what a black hole is. I want to call upon the brightest minds to answer that question for our fascinated but mystified audience.
I have explored supermassive black holes for Sky & Telescope (cover story), Muse (cover story), and Science (several articles). I’ve met the key figures: Andrea Ghez, Reinhard Genzel (leader of the German group), and prominent theorist Avi Loeb of the Harvard- Smithsonian Center for Astrophysics. I know the director and press officer at Keck, so access to the mountaintop for several days will pose no problem. Moreover, I’ve followed laser adaptive optics since its inception at Lick Observatory in California, and it’s now ready for prime time. The images — from the laser beam above the dome to the clarified views of the crowded core of the galaxy — will create a striking spread.
Working with the editors at Smithsonian on my recent story on exoplanets was a great experience. I’d be delighted to take your readers on a journey a bit deeper into space, but no less enticing.