 The
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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The Black Hole at the Center of our Galaxy at Amazon.com!
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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