The Enigmatic Center of Our Galaxy

Twenty years ago, evidence began to accumulate that the center of the Milky Way galaxy contained a dark mass that could not be accounted for as normal stellar remnants.55 Star motions in the center were so rapid they indicated a huge black hole, about four million times the mass of the Sun. The evidence for this supermas­sive black hole is now better than for the more conventional black holes that result from the death of a massive star. The mass is not just based on stellar velocities but on tracking the entire stellar orbits as they loop around the central dark object.56

Chandra did not contribute the evidence that cemented the case for a large black hole in the galactic center, but it showed that the black hole was unusually anemic, emitting far less high-energy ra-

diation than other massive black holes. Even as it devours 1019 kg of material each second, it radiates energy a million times less ef­ficiently than a set of stars of equivalent mass. Yet, while the black hole in the galactic center is very feeble, it’s not boring. Ten years ago, an X-ray flare was seen coming from the vicinity of the black hole and since then, hundreds of flares have been seen, occurring almost daily.57 They raise the level of X-ray emission tens or hun­dreds of times above the quiescent state (figure 10.3). Such rapid flares must be created within ten times the event horizon scale, close to the point of no return. The galactic center also harbors a source of energy intense enough to create anti-matter in the form of positrons.58

The real mystery is the inactivity of the galactic center. The cen­ter of our galaxy harbors a massive star cluster with star densities thousands of times higher than those seen in the solar neighbor­hood, and there is plenty of gas available within 10-20 light-years. So why is the black hole so quiet? The best guess is that it’s cur­rently starved because explosive events have cleared away much of the gas from around it. Chandra has provided evidence for this explanation. There are lobes of X-ray emission that indicate the black hole was more active 5,000 or so years ago, and blobs of plasma emerging from the center argue for quasi-periodic activity. Remarkable observations a few years ago saw a cloud of gas near the black hole brighten and fade in only a few years, responding to an X-ray pulse that had traveled for three hundred years to get there. We can infer that the black hole was a million times brighter three hundred years ago, when Queen Anne had just ascended to the throne in England and the largest town in her American colo­nies was Boston, with a population of seven thousand.59 However, the galactic center is about 27,000 light-years from Earth, so all of this action really happened in the late Stone Age before humans settled down into civilizations and the information is just reaching us now.