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Galaxies and Galactic Structure
Developed for undergraduate astronomy students, this volume introduces observational and theoretical perspectives on galaxies, and their formation, structure, evolution and distribution. Twelve chapters cover galaxy morphology and kinematics galaxy classification, star light, integrated and differential galaxy light, gases, rotation, the kinematics of the Milky Way and spiral wave galaxies, large-scale star formation, starbursts and active galaxies, and large-scale distributions. Basic physics and calculus ideas are introduced in the text where relevant. Annotation c. by Book News, Inc., Portland, Or.
306 pages, Hardcover
First published December 22, 1997
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June 29, 2024
This 1998 book says all galaxies were formed within the first billion years after the big bang, and galactic structures since then are due to galactic collisions and mergers. I wonder what subsequent Hubble and Webb findings might say about that general statement.
This book pretty much stays within Hubble’s classification scheme from a hundred years ago. As with other books, it is descriptive, but short on theoretical explanation. Given the outstanding photos of galaxies - the points of light seen are not stars but billions of stars - from Hubble and now Webb, more discussion and speculation need to account for the variable and distinctive shapes that are seen, which suggests something more fundamental is going on versus just randomized mergers.
What is missing mainly is gravity’s role in all of this. It’s generally viewed that initial galaxy formation occurred via clumping, which makes sense early on after the big bang when matter and energy were closer together, but that still begs the question as to why gravitational effects are not on-going occurrences in galactic formation. Maybe this is what Elmegreen means about subsequent collisions and mergers where gravity moves galaxies close enough to other galaxies to collide and merge. But what about gas and dust left over from the big bang and supernova type of explosions? Why wouldn’t gravity, even in a dispersed intergalactic space environment, pull this space stuff together to form new galaxies and why wouldn’t this be an on-going creation process?
Then there are the spiral galaxies with their distinctive arms. If gravity is at work, shouldn’t the arms be pulled into the galactic center, which is what Einstein’s general theory of relativity might suggest (the movement of bodies in geometric space toward a larger gravitational mass), which is different than this book’s reference to galaxies as orbital systems (orbits being a stabilized balance between outward, straight-line inertial movement and the geometric movement inward toward the gravitational center). Then there is the question whether the movement inward is compensated by outward movement via angular momentum, so that one arm moves inward and the other arm spits outward (or does this have anything to do with the jets that are seen coming out of galaxies)? From the Hubble and Webb photos it does seem that spirals have two main arms entering from two axial points, thus, associated with galactic rotation (prompting another question - what causes such rotation and its spin direction?), and each main arm has tangential arms that flow into them, with progressively large distances that separate them when looking farther away from the galactic center. That suggests too that gravity is at work as there is increased concentration of the spirals with movement toward the center, which then culminates in the bulge of light seen at the center. Also, the author speaks of spirals having an accretion disk which lies perpendicular to the axis of rotation, so then the questions might be, do the spiral arms enter the galactic bulge area via the accretion disk, or via the axial points, or do they, as the author and others suggest, become a randomized movement that enters the center from all directions?
Then there are the ellipticals with no arms, and barred galaxies that seem like remnant arms that precede ellipticals (drawn in arms closer to the galactic center), and the amorphous clouds of dust and gas that might precede spiral galaxies before they condense into the accretion arms of spiral galaxies, is it possible that there is a sequence here created by the geometric flow of mass-energy toward a gravitational center? Regarding the galactic center, what is the role of black holes that are speculated to lie at the center of most if not all galaxies? Elmegreen doesn’t get into that question. We know about the no-return effects of a black hole’s event horizon, but is there a gravitational effect of black holes on the outside of its event horizon, and it is this that ultimately causes movement toward the center of each galaxy?
These are some of the questions prompted by reading this book that Elmegreen does not get into. Despite a wealth of detail, it seems that not a lot is really known about the formation of galactic structure. But just identifying and asking these questions is a good start.
This book pretty much stays within Hubble’s classification scheme from a hundred years ago. As with other books, it is descriptive, but short on theoretical explanation. Given the outstanding photos of galaxies - the points of light seen are not stars but billions of stars - from Hubble and now Webb, more discussion and speculation need to account for the variable and distinctive shapes that are seen, which suggests something more fundamental is going on versus just randomized mergers.
What is missing mainly is gravity’s role in all of this. It’s generally viewed that initial galaxy formation occurred via clumping, which makes sense early on after the big bang when matter and energy were closer together, but that still begs the question as to why gravitational effects are not on-going occurrences in galactic formation. Maybe this is what Elmegreen means about subsequent collisions and mergers where gravity moves galaxies close enough to other galaxies to collide and merge. But what about gas and dust left over from the big bang and supernova type of explosions? Why wouldn’t gravity, even in a dispersed intergalactic space environment, pull this space stuff together to form new galaxies and why wouldn’t this be an on-going creation process?
Then there are the spiral galaxies with their distinctive arms. If gravity is at work, shouldn’t the arms be pulled into the galactic center, which is what Einstein’s general theory of relativity might suggest (the movement of bodies in geometric space toward a larger gravitational mass), which is different than this book’s reference to galaxies as orbital systems (orbits being a stabilized balance between outward, straight-line inertial movement and the geometric movement inward toward the gravitational center). Then there is the question whether the movement inward is compensated by outward movement via angular momentum, so that one arm moves inward and the other arm spits outward (or does this have anything to do with the jets that are seen coming out of galaxies)? From the Hubble and Webb photos it does seem that spirals have two main arms entering from two axial points, thus, associated with galactic rotation (prompting another question - what causes such rotation and its spin direction?), and each main arm has tangential arms that flow into them, with progressively large distances that separate them when looking farther away from the galactic center. That suggests too that gravity is at work as there is increased concentration of the spirals with movement toward the center, which then culminates in the bulge of light seen at the center. Also, the author speaks of spirals having an accretion disk which lies perpendicular to the axis of rotation, so then the questions might be, do the spiral arms enter the galactic bulge area via the accretion disk, or via the axial points, or do they, as the author and others suggest, become a randomized movement that enters the center from all directions?
Then there are the ellipticals with no arms, and barred galaxies that seem like remnant arms that precede ellipticals (drawn in arms closer to the galactic center), and the amorphous clouds of dust and gas that might precede spiral galaxies before they condense into the accretion arms of spiral galaxies, is it possible that there is a sequence here created by the geometric flow of mass-energy toward a gravitational center? Regarding the galactic center, what is the role of black holes that are speculated to lie at the center of most if not all galaxies? Elmegreen doesn’t get into that question. We know about the no-return effects of a black hole’s event horizon, but is there a gravitational effect of black holes on the outside of its event horizon, and it is this that ultimately causes movement toward the center of each galaxy?
These are some of the questions prompted by reading this book that Elmegreen does not get into. Despite a wealth of detail, it seems that not a lot is really known about the formation of galactic structure. But just identifying and asking these questions is a good start.
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