Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

EDDINGTON'S “Internal Constitution of the Stars” was published in 1926 and gives what now ranks as a classical account of his own researches and of the general state of the theory at that time. Since then, a tremendous amount of work has appeared. Much of it has to do with the construction of stellar models with different equations of state applying in different zones. Other parts deal with the effects of varying chemical composition, with pulsation and tidal and rotational distortion of stars, and with the precise relations between the interior and the atmosphere of a star. The striking feature of all this work is that so much can be done without assuming any particular mechanism of stellar energy-generation. Only such very comprehensive assumptions are made about the distribution and behaviour of the energy sources that we may expect future knowledge of their mechanism to lead mainly to more detailed results within the framework of the existing general theory. An Introduction to the Study of Stellar Structure By S. Chandrasekhar. (Astrophysical Monographs sponsored by The Astrophysical Journal.) Pp. ix+509. (Chicago: University of Chicago Press; London: Cambridge University Press, 1939.) 50s. net.

An Introduction to the Study of Stellar Structure

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Control of Star Formation by Supersonic Turbulence

We present the largest galaxies as seen in the near-infrared (1–2 μm), imaged with the Two Micron All Sky Survey (2MASS), ranging in angular size from 1' to 15. We highlight the 100 largest in the sample. The galaxies span all Hubble morphological types, including elliptical galaxies, normal and barred spirals, and dwarf and peculiar classes. The 2MASS Large Galaxy Atlas provides the necessary sensitivity and angular resolution to examine in detail morphologies in the near-infrared, which may be radically different from those in the optical. Internal structures such as spirals, bulges, warps, rings, bars, and star formation regions are resolved by 2MASS. In addition to large mosaic images, the atlas includes astrometric, photometric, and shape global measurements for each galaxy. A comparison of fundamental measures (e.g., surface brightness, Hubble type) is carried out for the sample and compared with the Third Reference Catalogue. We further showcase NGC 253 and M51 (NGC 5194/5195) to demonstrate the quality and depth of the data. The atlas represents the first uniform, all-sky, dust-penetrated view of galaxies of every type, as seen in the near-infrared wavelength window that is most sensitive to the dominant mass component of galaxies. The images and catalogs are available through the NASA/IPAC Extragalactic Database and Infrared Science Archive and are part of the 2MASS Extended Source Catalog.

https://iopscience.iop.org/article/10.1086/345794/pdf

The 2MASS Large Galaxy Atlas

2MASS large galaxy atlas

Density wave theory of galactic spirals, discussing hydrogen distribution, young star distribution, stellar migration, magnetic field structure, density waves in stellar formation, etc

On the spiral structure of disk galaxies. iii. comparison with observations.

Quasi-steady flows of interstellar gas in a spiral gravitational field are followed for the purpose of investigating galactic shocks and the resultant processes of the formation of stars and interstellar clouds. We model the interstellar medium with two stable phases in which thermal balance is maintained through heating by low-energy cosmic rays. The problem, including transitions between the two phases, is given a general formulation but is solved in an approximation which ignores the difference in fluid velocities of the two phases. We also assume that the cosmic-ray flux is uniform in circles about the center of the Galaxy and that the relative abundances of the chemical elements are “normal.” For a spiral gravitational field with strength equal to 5 percent that of the axisymmetric field at 10 kpc from the galactic center, the density ratio at maximum and minimum compressions is 9:1 for the intercloud medium while it is 40:1 for the gas in a typical cloud. During the decompression phase of the flow, a small percentage of the mass of the clouds evaporates to become intercloud material, but this small amount is recovered in the shock. As a by-product of phase transitions, the properties of the clouds in the regions between spiral arms are such as to make their detection in 21-cm absorption very difficult. In the absence of the cloud phase, we determine the thickness of the shock layer in the intercloud medium to be typically 50 pc. An interstellar cloud immersed as a test particle in the intercloud medium experiences a dynamic rather than a quasi-static compression as it passes through the shock layer. The critical mass for the gravitational collapse of a cloud is reduced by a large factor because of the compression in the shock.

Galactic Shocks in an Interstellar Medium with Two Stable Phases

One of the most prolific applied mathematicians of the mid-twentieth century, Prof Lin is a highly respected professor at MIT. These volumes, a collection of Prof Lin's papers from 1943 to 1986, is an attempt to exhibit a historical perception of the development of ideas in the following areas of research: Stability of Parallel FlowsTurbulenceSpiral Structure of GalaxiesProf Lin has written short comments and personal recollections on the development of thinking in these subjects. In addition to research papers, there are two essays dealing with the basic thinking that underlies the development of applied mathematics as an academic discipline in USA. Other topics of interest are Aerodynamics, Liquid Helium, Solid State Physics, Plasticity and Magnetohydrodynamics. About 51 papers are included in these two volumes.

Selected papers of C.C. Lin

Applied mathematics, fluid mechanics, astrophysics : a symposium to honor C.C. Lin, 22-24 June 1987, Massachusetts Institute of Technology, Cambridge, USA

C. Yuan

Papers

On the spiral structure of disk galaxies. iii. comparison with observations.

Galactic Shocks in an Interstellar Medium with Two Stable Phases

Selected papers of C.C. Lin

Applied mathematics, fluid mechanics, astrophysics : a symposium to honor C.C. Lin, 22-24 June 1987, Massachusetts Institute of Technology, Cambridge, USA