Turbulence in the Interstellar Medium: a Retrospective Review

Summary

Introduction

I first heard about the Kolmogorov law (Kolmogorov 1941) for the velocity structure of turbulent fluids, in a lecture given by Chandrasekhar on the theory of the origin of the solar system proposed by C.F. von Weizsäcker (1943), when I was at the Yerkes Observatory as a Junior staff member. I could not imagine then that 50 years later I still would be talking about the matter. Shortly afterwards, when I joined the Mt. Wilson and Palomar Observatories, the work of von Hörner (1951) on the gas motions in the Orion Nebula, based on sixty measured radial velocities (Campbell & Moore 1918), became known and led to observations of the nebula using the coudé spectrograph of the 5 meter Hale telescope, with the highest angular and spectral resolution then possible. A few years later Wilson et al (1959) published about 10,000 radial velocities of the nebula, in the [OIII], [OII] and Hβ lines, besides a sample of line profiles from photographic plates. Their study (Münch 1958) essentially confirmed von Hörner's result, in the sense that the rms difference in radial velocity of two points on the nebula separated in the sky by a distance r, varies nearly as r^0.4, somewhat more steeply than the r^1/3 predicted by Kolmogorov law. This rather unexpected agreement was difficult to explain at the time, but it clearly implied that the formation path of the [OIII] line analyzed, determined by extinction and scattering by dust along the line of sight, is not large enough to smooth out the velocity variations of mass motions along the line of sight.