The book `Galactic Astronomy: Structure and Kinematics', by Dimitri Mihalas and James Binney, is a classic, basic text for those interested beyond the basic level of what a galaxy is and how it is structured.
This is a revision of a text first written in 1967 -- as scientific advances have proceeded at an ever increasing speed, this text too is lacking in many of the most recent discoveries and interpretations. However, it still provides a significant introduction to many basic ideas and patterns that have not changed over time. There has been a follow-up by James Binney and Michael Merrifield published recently.
Galactic astronomy, with the increasing understanding of large-scale physics and the increasing data from telescopes, both visual and extra-visual, has made great strides in understanding the composition, motions and developments of galaxies -- not only spiral galaxies such as our own, but other galactic shapes and structures, too. This book addresses the structure and kinematics. A word of explanation: kinematics is the study of motion without reference to the forces which cause the motion -- dynamics addresses that, and that is not included in this text, but rather left for a second volume.
This volume begins with a basic overview of the our conception of our own galaxy, giving a brief historical overview of explanations about the Milky Way and how this developed into the idea of a disc of stars; the recognition of our Sun being not in the centre but rather in a more unremarkable spot on one of the arms of the galaxy; the increasing knowledge from better and better observational data of the number of stars and other constituents of the galaxy, and the plotting of movement of individual stars and the collective motion of the galaxy. With the additional charting of dark matter, globular clusters, and other pieces, an increasingly accurate portrait of our home galaxy has been made.
The volume then gives a 'refresher' on astronomical terminology and concepts as related to galactic astronomy. Positions and coordinate systems, motions, measurements using parallax, stellar spectra, magnitudes and colours, energy distributions, and a refresher on the cataloguing system help to put the rest of the text in perspective. Particularly as galaxies are primarily made up of stars, the physical properties stars--masses, evolution, types of stars--are examined in detail, along with information about interstellar absorption and other galactic constituents.
Then, galactic astronomy begins in earnest, with a discussion of the distribution of stars and other elements in our own galaxy. Methods of star counting, apparent versus real distribution of stars and elements, distribution in the disc and distribution in the spheroidal component, as well as stellar populations are covered. From here, a description of other types of galaxies (morphology, magnitudes, distances, spectrophotometric properties) enlarges the ideas of galactic ideas. In examining other galactic types, Mihalas and Binney give a cautionary tale, one that is true in observational astronomy generally -- one thing may look like another, even with the most sophisticated of measuring tools. People may make assumptions about observational data trends that can skew large-scale ideas of the entire universe.
`Biases of the kind...may affect our understanding of the Universe in two important ways. (1) We may be unaware of the existence of numerous compact galaxies or diffuse, faint galaxies, which together might contribute significantly to the mass density and dynamics of the Universe, and which might feature prominently in the world views of observers whose skies are either darker or less hazy than our own. (2) We may seriously misjudge the nature and size of 'normal' galaxies.'
While our data today is yet again much better than that used when this text was compiled, these caveats are still of importance, as the hunt for dark matter, less luminous objects, and hidden material still continues.
In discussing the kinematics, the motion of galaxies, Mihalas and Binney begin with solar and stellar motions, and then proceed from there. Using relative parallax measurements and motion around the galactic centre, they proceed to describe the kinematics of stars both in the disk and the halo (the spheroidal-component), also observing the varying velocities. Estimation and application of general rotation laws from our own galaxy to other galaxies is a rather difficult one to observe, given the continuing difficulties of pinpointing individual constituents in vastly distant galaxies for measurement.
This is a book on how galaxies are, and some basic information about the motion of galaxies. A follow-up volume, published in 1987, dealt with galactic dynamics. Many questions remained unanswered at the time of publication, and still remain unanswered. However, for the serious astronomer (amateur or professional -- and astronomy is one of the few sciences in which to be an amateur is a respected status), this book gives insight and information on a fascinating branch of the study of the universe. Intended for late undergraduate and graduate-level study.
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Stefano M.
Buono
Reviewed in Italy on 18 July 2019
Stefano M.
Reviewed in Italy on 18 July 2019
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