Chandrasekhar and Modern Stellar Dynamics

TL;DR

This paper reviews the historical debate between Eddington and Chandrasekhar on how the galactic potential influences stellar velocity dispersion, highlighting their ideas, disputes, and modern observational efforts in stellar dynamics.

Contribution

It clarifies the historical and conceptual development of the relationship between galactic potential and velocity dispersion orientation, including Chandrasekhar's counter-example and modern measurement approaches.

Findings

Eddington's hypothesis about eigenvector integral curves

Chandrasekhar's explicit counter-example

Recent measurements of velocity dispersion in the Milky Way

Abstract

Stellar dynamics occupied Chandrasekhar's interest for a brief interlude between his more prolonged studies of stellar structure and radiative transfer. This paper traces the history of one of his ideas -- namely, that the shape of the galactic potential controls the orientation of the stellar velocity dispersion tensor. It has its roots in papers by Eddington (1915) and Chandrasekhar (1939), and provoked a fascinating dispute between these two great scientists -- less well-known than their famous controversy over the white dwarf stars. In modern language, Eddington claimed that the integral curves of the eigenvectors of the velocity dispersion tensor provide a one-dimensional foliation into mutually orthogonal surfaces. Chandrasekhar challenged this, and explicitly constructed a counter-example. In fact, the work of neither of these great scientists was without flaws, though further developments in stellar dynamics were to ultimately draw more on Eddington's insight than Chandrasekhar's. We conclude with a description of modern attempts to measure the orientation of the velocity dispersion tensor for populations in the Milky Way Galaxy, a subject that is coming into its own with the dawning of the Age of Precision Astrometry.