Illuminating Black Holes in the Milky Way
Smithsonian, April 2008
My comments are appended in red in italics.
Without question, the Milky Way's black hole is the strangest thing in our galaxy - a three-dimensional cavity in space just ten times the physical size of our sun but with four million times the mass, a virtual bottomless pit from which nothing can escape. Every major galaxy; it turns out, has a black hole at its core. Now, for the first time, scientists have the chance to study the havoc these mind-boggling entities wreak. For the next decade, Keck astronomers will track thousands of stars caught in the gravity of the Milky Way's black hole. They will try to figure out how stars are born close to the black hole and how it distorts the fabric of space itself. Note the words "cavity in space" and "distorted fabric of space", which come from assuming what might be happening before you see the data.
To be sure, the evidence for black holes is entirely indirect; astronomers have never actually seen one. Albert Einstein's general theory of relativity predicted that the gravity of an extremely dense body could bend a ray of light so severely that it could not escape. Something the mass of our sun, for instance, could trap light if it shrank into a ball just one and a half miles across. For Earth to become a black hole, its entire mass would have to fit into a sphere no bigger than a pea. Schwartzchild derived this radius without using general relativity, by simply assuming an escape velocity equal to the speed of light.
In 1939, J. Robert Oppenheimer and another physicist calculated that such drastic compression could happen to the biggest stars after they ran out of hydrogen and other fuel. Once the stars sputtered out, the scientists posited, the remaining gas would collapse under its own gravity into an infinitely dense point. Telescope observations backed up the theory in the 1960s and 1970s. Astronomers discovered quasars - extremely bright beacons billions of light years away. A few researchers suggested the only possible power source for something so luminous would be a concentration of millions of suns in a small volume pulled together by what scientists later dubbed a super-massive black hole. Astronomers then found stars that seemed to whip around invisible companions in our Milky Way, and they concluded that only the pull of gravity from small black holes could keep the stars in such tight orbits. Containing several times the mass of our sun, these are called stellar-mass black holes. Note that the singularity was posited, but is not necessary to get the results seen.
The Hubble Space Telescope added to the evidence for black holes in the 1990s by measuring how quickly the innermost parts of other galaxies rotate-up to 1.1 million miles per hour in big galaxies. The startling speeds pointed to cores containing up to a billion times the mass of the sun. The discovery that super-massive black holes are at the core of most, if not all, galaxies was one of Hubble's greatest achievements. At the beginning of the Hubble survey, we would have said black holes are rare, maybe in one galaxy in 10 or 100, and that something went wrong in the history of that galaxy. Now we've shown they are Standard equipment. It's the most remarkable thing. Maybe there are black holes in all galaxies for a reason.
Even from Hubble, though, the Milky Way's core remained elusive. If our galaxy harbored a super-massive black hole, it was quiet, lacking the belches of energy seen from others. Hubble can track groups of stars near the centers of distant galaxies, but because of its narrow angle of view and our galaxy's thick dust clouds, it can't take the same pictures in our galaxy. Another approach would be to track individual stars in the black hole's vicinity using infrared light, which travels through dust, but the stars were too faint and too crowded for most ground-based telescopes to resolve. Still, some astronomers in the 1960s ventured that observations of the Milky Way's core might be possible, proving beyond doubt that a black hole exists there. A number of tantalizing questions could then be addressed: How do stars live and die in that wild setting? What does a black hole consume? And can we witness, at the heart of the Milky Way, the warped space and time predicted by Einstein nearly a century ago? Again, here is an assumption about how it all works.
Several years ago American and the German teams independently deduced that only a giant black hole could explain the behaviors of stars at the Milky Way's core. Stars circling a hefty mass - whether a black hole or some large star - travel through space much faster than those circling a smaller mass. In visual terms, the larger mass creates a deeper funnel in the fabric of space around which the stars revolve; like leaves circling a whirlpool, the deeper the whirl- pool, the faster the leaves spin. Other astronomers had seen fast-moving stars and clouds of gas near the center of the Milky Way. What does a 3D funnel look like? I cannot imagine one.
The clincher came in 2002, when both teams sharpened their images using adaptive optics, technology that compensates for the atmosphere's blur. The scientists followed stars that orbit perilously close to the galaxy's center and found that the fastest star's top speed was 3 percent of the speed of light - about 20 million miles per hour. That's a startling speed for a globe of gas far bigger than our sun, and it convinced even the skeptics that a super-massive black hole was responsible.
Identifying a black hole is one thing; describing it is another. It's difficult to paint a picture that relates to the world as we understand it, without using mathematical complexity. A "sinkhole" makes an apt metaphor for a black hole, particularly "a three-dimensional sinkhole. If you were in space near the black hole, you would see things disappear into it from all directions.
In this magnificent setting, the laws of physics are wonderfully twisted. Astronomers hope to gather the first evidence that stars do indeed travel along the weird orbital paths predicted by Einstein's relativity theory. If so, each star would trace something like a Spirograph pattern over time, gradually altering the point of its closest approach to the black hole. They think they are about eight years away from spotting that shift. Again, warped space-time is the expected result, and not something to see and then interpret.
With each new finding, the Milky Way's core becomes more perplexing and fascinating. The teams were startled to discover many massive young stars in the black hole's neighborhood. There are scores of them, all just five to ten million year old-infants, in cosmic terms and they are roughly ten times as massive as our sun. No one is entirely sure how they got so close to the black hole. Elsewhere in the galaxy, gestating stars require a cold, calm womb within a large cloud of dust and gas. The galactic core is anything but calm: intense radiation floods the area, and the black hole's gravity should shred gaseous nurseries before anything incubates there. Those young stars "have no damn right to be there." It's possible some of them were born farther out and migrated inward, but most theorists think they are too young for that scenario. They think the intense gravity compresses spiraling gas into a disk around the black hole, creating the new suns in a type of star birth not seen in any other galactic environment.
These young stars will self-destruct a few million years from now. And when they do, the most massive ones will leave behind small black holes. They theorize that hundreds of thousands of these stellar-mass black holes, accumulated from past generations of stars, swarm around the central super-massive black hole. The stellar-mass black holes are only about 20 miles wide, so collisions between them would be rare. Instead, you'll have black holes swinging past each other in the night, and stars moving through this destruction derby. A near miss between one of the black holes and a star could scatter the star into the super-massive black hole or out of the galactic center entirely. Theorists think the super-massive black hole may gobble a star once every tens of thousands of years - an event that would ignite the center of the galaxy with radiation. It would be a spectacular event.
Theorists are painting a new picture of how the universe and its 100 billion galaxies have evolved since the big bang 13.7 billion years ago. They believe that all galaxies started with as-yet-unexplained "seed" black holes - tens to thousands of times the mass of our sun-that grew exponentially during violent feeding cycles when galaxies collided, which they did more frequently when the universe was younger and galaxies were closer together. In a collision, some stars catapult into deep space and other stars and gases plummet into the combined black hole at the galaxies' center. As the black hole grows, it turns into a raging quasar with gas heated to billions of degrees. The quasar then blasts the rest of the gas out of the galaxy entirely. After the gas is depleted, the super-massive black hole sits at the center of the galaxy, dormant and starved. The big bang and its theoretical age are assumed to be true. The new idea is that black holes are the seeds of galaxies; they may be correct but for reasons they do not yet understand.
Our galaxy's fearsome future aside, they hope that soon - perhaps within a decade - we'll have the first image of the Milky Way's super-massive black hole, thanks to an emerging global network of "millimeter wave" telescopes. Named for the wavelength of infrared light they detect, the instruments technically won't see the black hole itself. Rather, they'll act in concert to photograph the shadow the black hole casts on a curtain of hot gas behind it. If all goes well, the image should show a black shadow, possibly with a distinctive shape. Theorists expect the black hole to be spinning. If so, according to the counterintuitive dragging of space predicted by Einstein's general theory of relativity, our view of the shadow will be distorted into something like a lopsided and squashed teardrop. It would be the most remarkable picture we could have. The truly remarkable thing would be if it looked anything at all like they expect it to look, and when it looks different than their expectations, what they would conclude about their theories!