James Binneyis professor of physics at the University of Oxford. His books include Galactic Astronomy. Scott Tremaineis 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.

Galactic Dynamics

By James Binney Scott Tremaine

Princeton University Press

Copyright © 2008Princeton University Press
All right reserved.
ISBN: 9780-691-13027-9

Chapter One

Introduction

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…