The Physics of the Interstellar Medium

J.E. Dyson, D.A. Williams

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000163155
• Edition: 3
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

This third edition of The Physics of the Interstellar Medium continues to introduce advanced undergraduates to the fundamental processes and the wide range of disciplines needed to understand observations of the interstellar medium and its role in the Milky Way galaxy. The book is suitable for undergraduate students studying physics, astronomy, and astrophysics. The book also provides concise and straightforward discussions of interstellar physics and chemistry that are useful for more experienced readers.

The book leads readers through the range of physical processes operating on both large and small scales that occur in the interstellar medium. It explores the relationship between the dusty, tenuous gas in interstellar space and the formation of stars and planets. This new edition also describes exciting developments in the field of astrochemistry and its interaction with interstellar physics, and the roles played by interstellar dust grains in interstellar physics and chemistry.

Simple models in each chapter, together with problems at the end of each chapter, encompass interdisciplinary applications in atomic, molecular, solid state, and surface physics, and gas dynamics. This popular textbook provides a useful overview and grounding in the study of the interstellar medium and brings insight into many aspects of physics.

Features

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• An authoritative textbook in the field at this academic level

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• Provides a wide introduction to the interstellar medium whilst remaining accessible and concise

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• Revised throughout, presenting a modern understanding of the interstellar medium

1 Introduction

1.1 Galaxies and the Galaxy

If we are fortunate enough to see the sky on a moonless and cloudless night, then, when our eyes have become accustomed to the dark, we can discern the prominent feature called the Milky Way. This is a band across the sky containing a large number of bright stars and a background of many fainter stars – so many, in fact, that our eyes cannot resolve them and we see their light as a diffuse (i.e. ‘milky’) luminosity. This band of stars appears across the sky because the Sun is one of a vast collection of stars, most of which are arranged in the shape of a disc. When we look in the plane of the disc, we see many stars; when we look out of the plane, we see fewer stars. This collection of stars – most of them in the plane of the disc and some out of the plane in a spherical region – is called the Milky Way galaxy. The Milky Way is only one galaxy among an enormous number of galaxies.

The shape and extent of a galaxy, seen with the eye or a telescope, are defined by the stars it contains. This book, however, is concerned with material in the Milky Way galaxy, material that our eyes do not easily see but which, nevertheless, is present and which plays an essential role in the evolution of the galaxy. This material is the interstellar medium. In this book, we’ll describe some of the interesting applications of physics to the study of the interstellar medium of the Milky Way galaxy, and at the same time, we’ll try to explore the fundamental role it plays. However, this is a book about the physics of the interstellar medium in the Milky Way and should be read as a simple account of some applications of physics. It isn’t a comprehensive description of all aspects of the interstellar medium of the Milky Way galaxy.

Before we come to describe evidence for the existence of material in the space between the stars, we should first have some idea of the dimensions of the galaxy. Our Sun appears to be a fairly typical star of the Milky Way galaxy, which is estimated to contain about 10¹¹ stars. The Sun is not located in any special position in the galaxy, such as the galactic centre, but is in the plane of the disc about two-thirds of the galactic radius from the galactic centre. We cannot see, either by eye or by optical telescope, more than a mere fraction of the galaxy, because the intensity of visible light is diminished by a general extinction as the light travels in the plane of the galaxy. Out-of-plane directions are less extinguished. We shall have more to say about that interstellar extinction, but it was the first sign that the interstellar medium wasn’t empty: evidently, it contains something that absorbs and scatters starlight. However, as we shall see, the galaxy can be investigated by other means, and we know that its many stars are distributed over a large volume. The diameter of the disc is of the order of 30 kpc (or about 100 thousand light years), and its thickness is about 2 kpc (or about 6 thousand light years). Stars are therefore about 10 light years from their nearest neighbours, on average, though the distribution of stars in space is far from uniform. Obviously, stars are very small compared with these dimensions, so that essentially all the vast space inside the galactic volume is occupied by the interstellar medium, which – as we’ll describe – is a non-uniformly distributed dusty gas. The whole galaxy is gravitationally bound; it has spiral structure within it, and it rotates. The shape and structure of the Milky Way galaxy are thought to be similar to those of some external galaxies, and an image of a galaxy believed to be similar to the Milky Way is shown in Figure 1.1.

Figure 1.1 Image of spiral galaxy M74. This galaxy is seen by face-on by observers on Earth. It is about 10 Mpc distant, contains about 10¹¹ stars, is slightly smaller than the Milky Way, and shares similar spiral structure. The arms contain many bright blue stars and regions of red emission from ionized hydrogen. (Image credit: NASA, ESA, and the Hubble Heritage (STSC/AURA)-ESA/Hubble Collaboration; Acknowledgment: R Chandar (University of Toledo) and J Miller (University of Michigan).)

The Milky Way galaxy is merely one of an enormous number, perhaps ~10¹¹, of galaxies in the Universe, and the dimensions of the visible Universe are about a factor of about 10⁵ larger than those of the Galaxy. We can sometimes investigate the properties of our own Galaxy by studying other galaxies that we can see more readily. The distant galaxies appear to be receding from our own Galaxy, with velocities increasing (and apparently accelerating) with distance. Distances between neighbouring galaxies are very much greater than the dimensions of galaxies themselves, and are typically measured in Mpc (or millions of light years). Therefore, the light gathered by our telescopes and focused to form an image of distant galaxies must have been travelling on its journey long before human beings evolved on Earth. The space between the galaxies – intergalactic space – is not our concern in this book, but, in passing, we note that any gas in intergalactic space must be much less dense than the gas in interstellar space. Studies show that galaxies exist in various forms. Some are irregular, some show a more well-defined disc shape, while some of these have spiral structure (spiral arms) within them. Some are classified as elliptical. Our own Galaxy is known to have spiral structure similar to M74. Figure 1.2 shows photographs of several types of galaxy. All galaxies have interstellar matter; they may be poorer or richer in interstellar matter than the Milky Way. Our discussion in this book is directed towards interstellar matter in the Milky Way galaxy, but the ideas expressed have general application to interstellar matter in all galaxies.

Figure 1.2 Variety in galaxies. (a) Elliptical galaxies, of which ESO 325-G004 is one example, have a smooth profile and an ellipsoidal shape. They have relatively few high-mass bright stars and consist mainly of low mass stars. (Credit: NASA, ESA, and The Hubble Heritage team and J Blakeslee.) (b) Starburst galaxies have a very high rate of star formation which can be stimulated by a close collision between galaxies. This is the case for the Antenna Galaxies, a merger between two galaxies – NGC 4038 and 4039. (Credit: ESA/Hubble and NASA.) (c) Irregular galaxies are often small and their shapes are a result of near collisions with other massive galaxies. The example shown is NGC 1427A. (Credit: NASA, ESA, and the Hubble Heritage Team.) (d) Some galaxies are very bright, especially in the infrared, and are known as luminous infrared galaxies, or ultraluminous infrared galaxies (ULIRG). IRAS 1927–0406 shown here is an example of a ULIRG. (Credit: NASA).

When we look at the sky, it is obvious that some stars appear much brighter than others. This is often because such stars are relatively nearby, but some stars appear bright because they are intrinsically powerful sources of radiation. Astronomers can, by a variety of means, deduce the masses and intrinsic luminosities of stars, and these results agree well with theories of stellar evolution. These theories tell us that stars have masses within a range of about 0.1 to about 100 times the mass of the Sun (M_ʘ), and that luminosities corresponding to these masses may range from about 10⁻³ to 10⁶ times the luminosity of the Sun. The more massive the star, the greater is its luminosity. Therefore, the brightest stars, containing nearly 100 times the amount of fuel as the Sun, are squandering it at a rate 10⁶ times faster. We therefore expect that they can exist only for 10⁻⁴ times the life of the Sun, that is, for about a few million years. This seems a long time for us on Earth, but for the galaxy, these bright stars are transient objects, like candles which soon burn down. We are forced to conclude that such stars have formed in the recent past, and – by implication – that they are certainly forming now. The galaxy evolves; it was not formed in the state in which we see it now.

At the end of its life, a sufficiently massive star explodes violently in a dramatic event called a supernova. These explosions are so powerful that we can observe their effects even when they occur in distant galaxies; the exploding star may become (very briefly) as bright as its host galaxy. The last one known to occur in the Milky Way occurred more than 300 years ago, but on average the Milky Way may be host to two supernovae per century. Perhaps the most famous supernova that occurred in our own galaxy is the one that was observed in 1054 AD, and which caused the Crab nebula, an extended supernova remnant that we can still observe (Figure 1.3(a)). A more evolved supernova remnant is shown in Figure 1.3(b). It is in the Large Magellanic Cloud, a neighbouring galaxy to the Milky Way. In 1987, another supernova occurred in the Large Magellanic Cloud. It (SN1987A) has become the best studied of all supernovae.

Figure 1.3 (a) The supernova remnant known as the Crab nebula. (Credit: NASA, ESA, J Hester, and A Loll (Arizona State University).) (b) An image of the supernova remnant SNR 0519690 which represents X-ray emission from very hot gas in the nebula as blue (Chandra Observatory) and optical emission in red (Hubble Space Telescope) from the outer boundaries of the nebula. (Credit: X-ray; NASA/CXC/Rutgers/J Hughes: Optical; NASA/STScI.)

Supernova explosions eject material in large amounts, comparable to the mass of our Sun, from the interiors of stars into interstellar space. This ejected material is rich in the ‘ashes’ of the thermonuclear processes which power the stars, and these ‘ashes’ are the elements heavier than hydrogen. Less-massive stars also contribute to the enrichment of the interstellar medium with the ‘ashes’ of thermonuclear burning, albeit in a less dramatic way. The fact that we and our Earth are made predominantly from these elements, and that the Sun also contains them, indicates that the Solar System is made from ‘recycled’ material, that is, it was formed from interstellar gases containing the ‘ashes’ of a previous generation of stars. The picture that emerges from these general considerations is that as the galaxy evolves, stars are formed, and they ‘age’ and ‘die’, sometimes explosively. The ejected material enriches the interstellar medium with atoms of elements other than hydrogen, and the gases in interstellar space somehow condense to form a new generation of stars, and the cycle is repeated. In this scenario, we see that the interstellar medium plays a crucial role: it is the reservoir of mass for forming new stars and planets in any galaxy, even though it may at first seem a minor component of a galaxy, and it is continually enriched in the heavy elements that are the ashes of thermonuclear processes in stars.

1.2 Evidence for Matter between the Stars

Naked-eye observation of the Galaxy tells us nothing about interstellar space – except that it appears to be empty. However, this is easily seen to be an incorrect conclusion. Photographs from even low-powered telescopes provide the most striking and visually beautiful evidence of matter in interstellar space. Where interstellar gas is so near to a star that it is hot, it will radiate, and we can detect this radiation and identify it. We show later in this book images of some of these radiating regions.

The main evidence for the existence of interstellar matter rests, as we shall see in the succeeding chapters, on spectroscopy. The atoms, ions, and molecules in the interstellar gas may emit or absorb radiation corresponding to transitions between their various energy levels. A well-known transition is that of hydrogen atoms giving rise to the 21 cm radio line. Hydrogen atoms may also give rise to many other lines; the one giving the red colour prominent in many photographs of nebulae corresponds to the transition H(3p) → H(2s) at 656.3 nm. There are also other ways in which emission of radiation can occur, in particular over a continuum of wavelengths. All such radiation carries with it information concerning the interstellar medium, as we shall discuss in the following chapter.

Gravitational effects within the Galaxy also give indirect evidence for the existence of interstellar matter, and allow us to deduce an upper limit to the interstellar density. The argument goes as follows: a star situated at a distance z above or below the galactic plane experiences an acceleration, g_z, towards the p...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface to the Third Edition
• Some Relevant Physical and Astronomical Information
• Authors
• Chapter 1. Introduction
• Chapter 2. How We Obtain Information about the Interstellar Medium
• Chapter 3. Microscopic Processes in the Interstellar Medium
• Chapter 4. Interstellar Grains
• Chapter 5. Radiatively Excited Regions
• Chapter 6. Introduction to Gas Dynamics
• Chapter 7. Gas Dynamical Effects of Stars on the Interstellar Medium
• Chapter 8. Star Formation and Star-Forming Regions
• Answers to Problems
• Index