The Black Hole at the Center of Our Galaxy , Fulvio Melia , Princeton U. Press, Princeton, NJ, 2003. $29.95 (189 pp.). ISBN 0-691-09505-1
In 1974, radio astronomers discovered a moderately strong, pointlike radio source at the center of our Milky Way galaxy. The discovery of that source—now known by the insufficiently epic name of Sagittarius A*—made respectable the few-years-old idea that our galaxy contains a central, supermassive black hole. The idea garnered further support from dynamical studies, starting about 1980, of gas and stars; the findings implied substantially more mass present at the galactic center than could be accounted for in terms of observable stars.
The trick has been to demonstrate unambiguously that the central mass is truly a black hole and not some other exotic, dark object or objects masquerading as one. The demonstrations are under way: Diffraction-limited observations with the world’s largest telescopes are pushing the implied density of the dark mass into a realm that defies an alternative description. And ever more sophisticated models of the accretion flow onto a black hole are approaching consistency with direct measurements at millimeter, IR, and x-ray wavelengths of the mass’s immediate environment. Exciting as those developments are, the real excitement is just beginning. As Fulvio Melia points out in The Black Hole at the Center of Our Galaxy , astrophysicists can foresee that, in the next decade or so, they’ll have the means to effectively image the black hole’s event horizon.
The mind- and spacetime-bending subject of this timely book is bound to have a broad appeal, and Melia amplifies that appeal with a carefully crafted, lyrical writing style and a striking collection of handsome color images. The book, however, has no diagrams to illustrate its numerous geometric discussions, so the reader is obliged to pay close attention to the prose. Fortunately, Melia is an excellent wordsmith. His book is aimed at the lay public and is ideally suited as a supplemental reading for students in a general education course in physics or astronomy.
To set the stage for understanding cosmic black holes, Melia guides the reader through a brief history of gravity, a tour that includes a primer on general relativity. He effectively illustrates some important relativistic effects, such as the frame dragging associated with black hole spin. He also shows how that dragging affects the orbits of nearby objects in observable ways and thus gives astrophysicists a handle on a black hole’s spin. In his explanations, Melia avoids math. Instead, he makes abundant use of analogies and thought experiments. Some of those are the classical invocations of elevators, trains, and airplanes; others are of Melia’s own invention. Rich with cultural allusions, his analogies are sometimes as elaborate as the physics he is trying to convey, and they may be rather distracting to the lay reader. Nonetheless, I found them to be a laudable effort to bring a remote subject to the terrain of real-world experience.
Melia’s history tour also illuminates the subject of collapsed stars, knowledge of which is essential if one is to understand how nature effects the incomprehensible compression and vast times required to make a stellar-mass black hole. (To get a feel for that compression, imagine trying to squeeze Earth to the size of a golf ball.) Subramanyan Chandrasekhar was mentioned in a few contexts, in particular as the namesake of the Chandra X-ray Observatory , which enabled the discovery of x-ray emission from the accretion flow onto the galactic black hole. However, he could have been more prominently credited for his circa 1930 work in which he showed—notwithstanding the disbelief of some of his notable contemporaries—that sufficiently massive stars would inevitably collapse.
In any case, the galactic black hole, with a mass 4 million times greater than that of our Sun, is more than just a collapsed star. It is, as Melia points out, inexorably growing by disrupting hapless stars that wander within its tidal grasp and by sucking gas out of the nearby interstellar medium through the intermediary of an accretion disk. Indeed, Melia has been a prolific member of the cadre of theorists working to explain the accretion flow and its emergent spectrum. Naturally, his own work and ideas are emphasized in the book. The reader should be cautioned, though, that some of Melia’s theories (for example, the production of the radio source Sagittarius A East by the tidal disruption of a star) are presented as accepted fact and a few important theories (such as the importance and role of a jet emanating from the galactic black hole’s accretion disk) are not mentioned.
The galactic black hole is but the title character in Melia’s story of extreme gravity; it just happens to be the most accessible representative of a vast population of supermassive black holes lurking in galactic nuclei. The most extreme examples are quasars, which are abundantly distributed across the universe and powered by gas accretion onto black holes up to 1000 times more massive than our comparatively puny galactic black hole. Researchers still do not agree on what mechanism forms those supermassive black holes. They are currently attempting to decide from among the possibilities: seeding by primordial black holes; dynamical collapse of a stellar cluster, either before or after the epoch of galaxy formation; and a succession of direct stellar collisions in a high-density environment.
Melia opts not to discuss the origins of supermassive black holes, although a section on the topic would have deepened readers’ appreciation of what can be learned from these monsters. He does make it clear, however, that for some time to come, our own galactic black hole promises to be astrophysicists’ best laboratory for investigating how a black hole interacts with its environment.
