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The Black Hole at the Center of our GalaxyThe Black Hole at the Center of our Galaxy

Fulvio Melia, Princeton University Press, Princeton, N.J., 2003, 189 pages, $29.95.

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Reviewed by Terrell Kent Holmes

In 1974 astronomers discovered a bright radio source at the center of the Milky Way Galaxy, in the direction of the constellation Sagittarius. It was an unknown quantity then, but astronomers now know that the object is a supermassive black hole causing a maelstrom of activity at the galactic center. Fulvio Melia, Professor of Physics and Astronomy at the University of Arizona, has written The Black Hole at the Center of Our Galaxy, a detailed and richly illustrated biography of the object he describes as "the big gorilla that remains unfazed while all around it flail in frenzy."

The area where the object is located is known as the Sagittarius A complex, and the black hole itself has since been designated Sagittarius A*. Whereas the typical mass of a black hole that forms from a collapsing star is around 5 to 10 solar masses, the estimated mass of Sagittarius A* is 2.6 million solar masses. But this tremendous amount of mass is concentrated into an area the size of the solar system. Astronomers determined Sagittarius A*'s mass by measuring the speed at which stars orbit around it, a blinding 5 million kilometers per hour.

Melia points out that interstellar dust dims our view of the galactic center by a factor of 100 million, and that were it not for that dust, the center of the Milky Way would glow as brightly as the full Moon. This dust renders even the most powerful optical telescopes useless for studying Sagittarius A*, so the black hole's surroundings have been "seen" with radio and X-ray imaging and spectroscopy. Space-based scopes such as the Compton Gamma-Ray Obsevatory and the Chandra X-ray Observatory have played a starring role, as well as the Very Large Array radio interferometer in New Mexico. Even so, determining the true size of Sagittarius A* has been problematic because it appears to be different sizes at different wavelengths of light.

An intriguing aspect of the galactic center is that it contains two different stellar generations. One generation formed 100 million years ago; the other is a bunch of whippersnappers born a scant 10 million years ago. Astronomers think this generation gap is the result of several epochs of star formation in that area, which might have been triggered by the sporadic infall of fresh material toward the strong gravitational forces of the black hole. Interestingly, in the wake of this book's publication, it was revealed that there may be a second, smaller black hole at the galactic center, orbiting Sagittarius A* once per century and acting as sort of a Judas goat, dragging young, unsuspecting stars into the maw of the monster, which might partially explain the primary's prodigious size.

The book vacillates between reading like a dry textbook and a novel. Melia thrives on extended metaphors (he is particularly enamored with a metaphor comparing wavelengths of light to waves lapping against a gondola in Venice) and flowery prose, such as this passage concerning the consequences of "massless matter": "[C]ondensations such as stars and planets would not occur; wintry landscapes with powdery blankets of snow would be unknown; Kandinsky would have never graced the world of art; and Mozart would not have written a single note of music." Unfortunately, Melia does not seem to be quite sure for whom he is writing. He takes care at the beginning to explain a light-year, and he includes other analogies that a lay reader could grasp. But the book is clearly geared toward the advanced reader, which at times leaves the novice in the (interstellar) dust.

Given all of the variables involved, it's clear that astronomers have yet to reveal the true nature of Sagittarius A*, and astronomers, in their tradition, will continue to peek around corners and cast at shadows. Near the end of the book, Melia even describes how astronomers in the near future might be able to image the "shadow" of Sagittarius A*. The Black Hole at the Center of Our Galaxy is the story of the discovery of an astonishing object that has presented yet another new challenge to our understanding of astronomy, specifically galactic evolution. As Melia writes, "Once again we are faced with one of those situations where nature is telling us something we don't understand, but the acceptance of which leads us to a greater enlightenment."

Freelance writer TERRELL KENT HOLMES (terrellholmes@hotmail.com) is a member of the Amateur Astronomers Association of New York. Besides writing about astronomy, he also writes about travel and jazz.

 
 

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