Since it was first published in 1987, Galactic Dynamics has become the most widely used advanced textbook on the structure and dynamics of galaxies and one of the most cited references in astrophysics. Now, in this extensively revised and updated edition, James Binney and Scott Tremaine describe the dramatic recent advances in this subject, making Galactic Dynamics the most authoritative introduction to galactic astrophysics available to advanced undergraduate students, graduate students, and researchers. Every part of the book has been thoroughly overhauled, and many sections have been completely rewritten. Many new topics are covered, including N-body simulation methods, black holes in stellar systems, linear stability and response theory, and galaxy formation in the cosmological context. Binney and Tremaine, two of the world's leading astrophysicists, use the tools of theoretical physics to describe how galaxies and other stellar systems work, succinctly and lucidly explaining theoretical principles and their applications to observational phenomena. They provide readers with an understanding of stellar dynamics at the level needed to reach the frontiers of the subject. This new edition of the classic text is the definitive introduction to the field. * A complete revision and update of one of the most cited references in astrophysics * Provides a comprehensive description of the dynamical structure and evolution of galaxies and other stellar systems * Serves as both a graduate textbook and a resource for researchers * Includes 20 color illustrations, 205 figures, and more than 200 problems * Covers the gravitational N-body problem, hierarchical galaxy formation, galaxy mergers, dark matter, spiral structure, numerical simulations, orbits and chaos, equilibrium and stability of stellar systems, evolution of binary stars and star clusters, and much more * Companion volume to Galactic Astronomy, the definitive book on the phenomenology of galaxies and star clusters
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James Binney is professor of physics at the University of Oxford. His books include "Galactic Astronomy". Scott Tremaine is the Richard Black Professor of Astrophysics at the Institute for Advanced Study and a member of the National Academy of Sciences. Both are fellows of the Royal Society.
"Henceforth, one must have a copy of Binney and Tremaine's classic in order to do astrophysics. . . . We will all study from it and come to know it better than the Bible."--John N. Bahcall, Institute for Advanced Study
"I consider Galactic Dynamics to be the most important single book published in the field of astronomy in the last ten years. In its subject, it is the most important book published in a generation."--William Press, Harvard University
A stellar system is a gravitationally bound assembly of stars or other point masses. Stellar systems vary over more than fourteen orders of magnitude in size and mass, from binary stars, to star clusters containing [10.sup.2] to [10.sup.6] stars, through galaxies containing [10.sup.5] to [10.sup.12] stars, to vast clusters containing thousands of galaxies.
The behavior of these systems is determined by Newton's laws of motion and Newton's law of gravity, and the study of this behavior is the branch of theoretical physics called stellar dynamics. Stellar dynamics is directly related to at least three other areas of theoretical physics. Superficially, it is closest to celestial mechanics, the theory of planetary motions-both involve the study of orbits in a gravitational field-however, much of the formalism of celestial mechanics is of little use in stellar dynamics, since it is based on perturbation expansions that do not converge when applied to most stellar systems. The most fundamental connections of stellar dynamics are with classical statistical mechanics, since the number of stars in a star cluster or galaxy is often so large that a statistical treatment of the dynamics is necessary. Finally, many of the mathematical tools that have been developed to study stellar systems are borrowed from plasma physics, which also involves the study of large numbers of particles interacting via long-range forces.
For an initial orientation, it is useful to summarize a few orders of magnitude for a typical stellar system, the one to which we belong. Our Sun is located in a stellar system called the Milky Way or simply the Galaxy. The Galaxy contains four principal constituents:
(1) There are about [10.sup.11] stars, having a total mass [??] 5 x [10.sup.10] solar masses (written 5 x [10.sup.10] [[??].sub.[??]]; 1 [[??].sub.[??]] = 1.99 x [10.sup.30] kg). Most of the stars in the Galaxy travel on nearly circular orbits in a thin disk whose radius is roughly [10.sup.4] parsecs (1 parsec [equivalent to] 1 pc [equivalent to] 3.086 x [10.sup.16] m), or 10 kiloparsecs (kpc). The thickness of the disk is roughly 0.5 kpc and the Sun is located near its midplane, about 8 kpc from the center.
(ii) The disk also contains gas, mostly atomic and molecular hydrogen, concentrated into clouds with a wide range of masses and sizes, as well as small solid particles ("dust"), which render interstellar gas opaque at visible wavelengths over distances of several kpc. Most of the atomic hydrogen is neutral rather than ionized, and so is denoted HI. Together, the gas and dust are called the interstellar medium (ISM). The total ISM mass is only about 10% of the mass in stars, so the ISM has little direct influence on the dynamics of the Galaxy. However, it plays a central role in the chemistry of galaxies, since dense gas clouds are the sites of star formation, while dying stars eject chemically enriched material back into the interstellar gas. The nuclei of the atoms in our bodies were assembled in stars that were widely distributed through the Galaxy.
(iii) At the center of the disk is a black hole, of mass [??] 4 x [10.sup.6] [[??].sub.[??]]. The black hole is sometimes called Sagittarius [A.sup.*] or Sgr [A.sup.*], after the radio source that is believed to mark its position, which in turn is named after the constellation in which it is found.
(iv) By far the largest component, both in size and mass, is the dark halo, which has a radius of about 200 kpc and a mass of about [10.sup.12] [[??].sub.[??]] (both these values are quite uncertain). The dark halo is probably composed of some weakly interacting elementary particle that has yet to be detected in the laboratory. For most purposes, the halo interacts with the other components of the Galaxy only through the gravitational force that it exerts, and hence stellar dynamics is one of the few tools we have to study this mysterious yet crucial constituent of the universe.
The typical speed of a star on a circular orbit in the disk is about 200 km [s.sup.-1]. It is worth remembering that 1 km [s.sup.-1] is almost exactly 1 pc (actually 1.023) in 1 megayear (1 megayear [equivalent to] 1 Myr = [10.sup.6] years). Thus the time required to complete one orbit at the solar radius of 8 kpc is 250 Myr. Since the age of the Galaxy is about 10 gigayears (1 gigayear [equivalent to] 1 Gyr = [10.sup.9] yr), most disk stars have completed over forty revolutions, and it is reasonable to assume that the Galaxy is now in an approximately steady state. The steady-state approximation allows us to decouple the questions of the present-day equilibrium and structure of the Galaxy, to which most of this book is devoted, from the thornier issue of the formation of the Galaxy, which we discuss only in the last chapter of this book.
Since the orbital period of stars near the Sun is several million times longer than the history of accurate astronomical observations, we are forced to base our investigation of Galactic structure on what amounts to an instantaneous snapshot of the system. To a limited extent, the snapshot can be supplemented by measurements of the angular velocities (or proper motions) of stars that are so close that their position on the sky has changed noticeably over the last few years; and by line-of-sight velocities of stars, measured from Doppler shifts in their spectra. Thus the positions and velocities of some stars can be determined, but their accelerations are almost always undetectable with current observational techniques.
Using the rough values for the dimensions of the Galaxy given above, we can estimate the mean free path of a star between collisions with another star. For an assembly of particles moving on straight-line orbits, the mean free path is [lambda] =1/(n]sigma]), where n is the number density and [sigma] is the cross-section. Let us make the crude assumption that all stars are like the Sun so the cross-section for collision is [sigma] = [pi][(2 [R.sub.[??]]).sup.2], where [R.sub.[??]] =6.96 x [10.sup.8] m = 2.26 x [10.sup.-8] pc is the solar radius. If we spread [10.sup.11] stars uniformly over a disk of radius 10 kpc and thickness 0.5 kpc, then the number density of stars in the disk is 0.6 [pc.sup.-3] and the mean free path is [lambda] [??] 2 x [10.sup.14] pc. The interval between collisions is approximately [lambda]/[upsilon], where [upsilon] is the random velocity of stars at a given location. Near the Sun, the random velocities of stars are typically about 50 km [s.sup.-1]. With this velocity, the collision interval is about 5x[10.sup.18] yr, over [10.sup.8] times longer than the age of the Galaxy. Evidently, near the Sun collisions between stars are so rare that they are irrelevant-which is fortunate, since the passage of a star within even [10.sup.3] solar radii would have disastrous consequences for life on Earth. For similar reasons, hydrodynamic interactions between the stars and the interstellar gas have a negligible effect on stellar orbits.
Thus, each star's motion is determined solely by the gravitational attraction of the mass in the galaxy-other stars, gas, and dark matter. Since the motions of weakly interacting dark-matter particles are also determined by gravitational forces alone, the tools that we develop in this book are equally applicable to both stars and dark matter, despite the difference of 70 or more orders of magnitude in mass.
We show in 1.2 that a useful first approximation for the gravitational field in a galaxy is obtained by imagining that the mass is continuously distributed, rather than concentrated into discrete mass points (the stars and dark-matter particles) and clouds (the gas). Thus we begin Chapter 2 with a description of Newtonian potential theory, developing methods to describe the smoothed gravitational fields of stellar systems having a variety of shapes. In Chapter 3 we develop both quantitative and qualitative tools to describe the behavior of particle orbits in gravitational fields. In Chapter 4 we study the statistical mechanics of large numbers of orbiting particles to find equilibrium distributions of stars in phase space that match the observed properties of galaxies, and learn how to use observations of galaxies to infer the properties of the underlying gravitational field.
The models constructed in Chapter 4 are stationary, that is, the density at each point is constant in time because the rates of arrival and departure of stars in every volume element balance exactly. Stationary models are appropriate to describe a galaxy that is many revolutions old and hence presumably in a steady state. However, some stationary systems are unstable, in that the smallest perturbation causes the system to evolve to some quite different configuration. Such systems cannot be found in nature. Chapter 5 studies the stability of stellar systems. In Chapter 6 we describe some of the complex phenomena that are peculiar to galactic disks. These include the beautiful spiral patterns that are usually seen in disk galaxies; the prominent bar-like structures seen at the centers of about half of all disks; and the warps that are present in many spiral galaxies, including our own.
Even though stellar collisions are extremely rare, the gravitational fields of passing stars exert a series of small tugs that slowly randomize the orbits of stars. Gravitational encounters of this kind in a stellar system are analogous to collisions of molecules in a gas or Brownian motion of small particles in a fluid-all these processes drive the system towards energy equipartition and a thermally relaxed state. Relaxation by gravitational encounters operates so slowly that it can generally be neglected in galaxies, except very close to their centers (see 1.2); however, this process plays a central role in determining the evolution and present form of many star clusters. Chapter 7 describes the kinetic theory of stellar systems, that is, the study of the evolution of stellar systems towards thermodynamic equilibrium as a result of gravitational encounters. The results can be directly applied to observations of star clusters in our Galaxy, and also have implications for the evolution of clusters of galaxies and the centers of galaxies.
Chapter 8 is devoted to the interplay between stellar systems. We describe the physics of collisions and mergers of galaxies, and the influence of the surrounding galaxy on the evolution of smaller stellar systems orbiting within it, through such processes as dynamical friction, tidal stripping, and shock heating. We also study the effect of irregularities in the galactic gravitational field-generated, for example, by gas clouds or spiral arms-on the orbits of disk stars.
Throughout much of the twentieth century, galaxies were regarded as "island universes"-distinct stellar systems occupying secluded positions in space. Explicitly or implicitly, they were seen as isolated, permanent structures, each a dynamical and chemical unit that was formed in the distant past and did not interact with its neighbors. A major conceptual revolution in extragalactic astronomy-the study of the universe beyond the edges of our own Galaxy-was the recognition in the 1970s that this view is incorrect. We now believe in a model of hierarchical galaxy formation, the main features of which are that: (i) encounters and mergers of galaxies play a central role in their evolution, and in fact galaxies are formed by the mergers of smaller galaxies; (ii) even apparently isolated galaxies are surrounded by much larger dark halos whose outermost tendrils are linked to the halos of neighboring galaxies; (iii) gas, stars, and dark matter are being accreted onto galaxies up to the present time. A summary of the modern view of galaxy formation and its cosmological context is in Chapter 9.
1.1 An overview of the observations
1.1.1 Stars
The luminosity of the Sun is [L.sub.[??]] = 3.84 x [10.sup.26] W. More precisely, this is the bolometric luminosity, the total rate of energy output integrated over all wavelengths. The bolometric luminosity is difficult to determine accurately, in part because the Earth's atmosphere is opaque at most wavelengths. Hence astronomical luminosities are usually measured in one or more specified wavelength bands, such as the blue or B band centered on [lambda] = 450 nm; the visual or V band at [lambda] = 550 nm; the R band at [lambda] = 660 nm; the near-infrared I band at [lambda] = 810 nm; and the infrared K band centered on the relatively transparent atmospheric window at [lambda] = 2200nm = 2.2 m, all with width [DELTA][lambda]/[lambda] [??] 0.2 (see Binney & Merrifield 1998, 2.3; hereafter this book is abbreviated as BM). For example, the brightest star in the sky, Sirius, has luminosities
[L.sub.V] = 22 [L.sub.[??]V] ; [L.sub.R] = 15 [L.sub.[??]R], (1.1)
while the nearest star, Proxima Centauri, has luminosities
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.2)
This notation is usually simplified by dropping the subscript from [L.sub.[??]], when the band to which it refers is clear from the context.
Luminosities are often expressed in a logarithmic scale, by defining the absolute magnitude
M [equivalent to] -2.5 [log.sub.10] L + constant. (1.3)
The constant is chosen separately and arbitrarily for each wavelength band. The solar absolute magnitude is
[M.sub.[??]B] = 5.48 ; [M.sub.[??]V] = 4.83 ; [M.sub.[??]R] = 4.42. (1.4)
Sirius has absolute magnitude [M.sub.V] = 1.46, [M.sub.R] = 1.47, and Proxima Centauri has [M.sub.V] = 15.5, [M.sub.R] = 13.9. The flux from a star of luminosity L at distance d is f = L/(4[pi][d.sup.2]), and a logarithmic measure of the flux is provided by the apparent magnitude
m [equivalent to] M + 5 [log.sub.10](d/10pc) = -2.5 [log.sub.10] [L](10 pc/d).sup.2]] + constant; (1.5)
thus, the absolute magnitude is the apparent magnitude that the star would have if it were at a distance of 10 parsecs. Note that faint stars have large magnitudes. Sirius is at a distance of (2.64 0.01) pc and has apparent magnitude [m.sub.V] = -1.43, while Proxima Centauri is at (1.295 0.004) pc and has apparent magnitude [m.sub.V] = 11.1. The faintest stars visible to the naked eye have [m.sub.V] [??] 6, and the limiting magnitude of the deepest astronomical images at this time is [m.sub.V] [??] 29. The apparent magnitudes [m.sub.V] and [m.sub.R] are often abbreviated simply as V and R.
The distance modulus m - M = 5 [log.sub.10](d/10 pc) is often used as a measure of distance.
The color of a star is measured by the ratio of the luminosity in two wavelength bands, for example by [L.sub.R]/[L.sub.V] or equivalently by [M.sub.V] - [M.sub.R] = [m.sub.V] - [m.sub.R] = V - R. Sirius has color V - R = -0.01 and Proxima Centauri has V - R = 1.67. Stellar spectra are approximately black-body and hence the color is a measure of the temperature at the surface of the star.
A more precise measure of the surface temperature is the effective temperature [T.sub.eff], defined as the temperature of the black body with the same radius and bolometric luminosity as the star in question. If the stellar radius is R, then the Stefan-Boltzmann law implies that the bolometric luminosity is
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.6)
where [sigma] = 5.670x[10.sup.-8] W [m.sup.-2] [K.sup.-4]. The relation between color and effective temperature is tabulated in BM 3.4 and shown in Figure 1.1.
(Continues...)
Excerpted from Galactic Dynamicsby James Binney Scott Tremaine Copyright © 2008 by Princeton University Press. Excerpted by permission.
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