Introduction

The theory of stellar structure and evolution is one of the most exact of the astrophysical sciences. It is inextricably involved in many of the topics needed to understand the role which high energy astrophysical processes play in the origin and evolution of stars and galaxies, providing, for example, evidence on their chemical abundances, the ages of the systems, and so on. The objective of this chapter is to provide a succinct summary of a number of the key results needed in the subsequent development of the story. Many of the equations and concepts will recur in different guises in the course of the exposition. There are many excellent books on these vast topics, my personal favourites being the books by Tayler, Karttunen and his colleagues, and by Kippenhahn and Weigert (Tayler, 1994; Karttunen et al., 2007; Kippenhahn and Weigert, 1990). The last volume is a classic and is particularly strong on the physics of the stars.

Basic observations

It is necessary to become familiar with some of the vocabulary of the study of the stars and the basic results of observation. These studies begin with measurements of the total amount of radiation emitted by a star, its luminosity L, and its surface temperature T. The spectra of stars are not black-bodies and so the effective temperature Teff is introduced. It is defined to be the temperature of a black-body of the same radius as the star which would emit the same luminosity.

The second part of this book is concerned with elementary physical processes involved in studies of high energy phenomena in the Universe. There are many excellent books which discuss this material at various levels of sophistication. Those which I have found most helpful are Jackson's Classical Electrodynamics (Jackson, 1999), Radiation Processes in Astrophysics by Rybicki and Lightman (1979) and Electromagnetic Processes by Gould (2005). Zombeck's Handbook of Space Astronomy and Astrophysics (Zombeck, 2006) contains a very useful compendium of relevant data.

My intention is to emphasise the underlying physical principles involved in these processes so that the functional forms of the equations have an intuitive significance. I will build up each discussion gently, often deriving approximate results which give physical insight before deriving, or quoting, the results of more complete calculations. I will treat the key processes of synchrotron radiation and inverse Compton scattering in some detail.

In the various calculations and derivations, I use Système International (SI) units, which have been officially adopted by almost all countries in the world. According to the Wikipedia web site (2008), ‘Three nations have not officially adopted the International System of Units as their primary or sole system of measurement: Liberia, the Union of Myanmar (Burma) and the United States.’

Galactic coordinates and projections of the celestial sphere onto a plane

The complexities of defining the celestial system of coordinates go far beyond what is needed in this text. These arise because the Earth does not move in a perfectly elliptical orbit about the Sun but is subject to wobbles and precessions because of the perturbing influence of the Moon and planets. These issues are dealt with in the textbooks by Smart and Murray (Smart, 1977; Murray, 1983).

The positions of celestial objects are described by a fixed set of spherical polar coordinates on the sky known as right ascension (RA or α) and declination (Dec or δ). The north celestial pole (NCP) is defined to be the mean direction of the rotation axis of the Earth and declination is the polar angle measured from the equator (δ = 0°) towards the north celestial pole (δ = 90°) (Fig. A.1). The south celestial pole (SCP) has declination δ = −90°. In the year 2000.0, the Earth's rotation axis was tilted at an angle of 23° 26′ 21.448″ with respect to the direction perpendicular to the plane of the ecliptic, which is the plane of the Earth's orbit about the Sun (Fig. A.1). The coordinates of right ascension and declination are referred to the reference epoch 2000.0 which is known as the 2000.0 coordinate system. The Earth's rotation axis points more or less in the same direction as the Earth moves round the Sun and this gives rise to the seasonal changes of climate.

Introduction

Galaxies are complex, many-body systems. Typically, a galaxy can consist of hundreds of millions or billions of stars, it can contain considerable quantities of interstellar gas and dust and can be subject to environmental influences through interactions with other galaxies and with the intergalactic gas. Star formation takes place in dense regions of the interstellar gas. To complicate matters further, it is certain that dark matter is present in galaxies and in clusters of galaxies and that its mass is considerably greater than the mass in baryonic matter. Consequently, the dynamics of galaxies are dominated by this invisible dark component, the nature of which is unknown.

Traditionally, galaxies have been classified by meticulous morphological studies of samples of bright galaxies. These morphological classification schemes had to encompass a vast amount of detail and this was reflected in Hubble's pioneering studies, as elaborated by de Vaucouleurs, Kormendy, Sandage, van den Bergh and others. The Hubble sequence of galaxies has real astrophysical significance because a number of physical properties are correlated with Hubble type. While the detailed study of individual galaxies was feasible for reasonably large samples, a different approach had to be adopted for massive surveys of galaxies such as the Anglo-Australian 2dF survey (AAT 2dF) and the Sloan Digital Sky Survey (SDSS) which have provided enormous quantitative databases for the studies of galaxies.

The interstellar medium in the life cycle of stars

The understanding of the nature and physical properties of the interstellar medium is of the first importance astrophysically since new stars are formed in dense regions of the interstellar gas and the medium is continually replenished by mass loss from stars and by metal-rich material processed in supernova explosions. Thus, the interstellar medium plays a key role in the birth-to-death cycle of stars. The same diagnostic tools are applicable to the study of diffuse gas and magnetic fields anywhere in the Universe, be they galaxies, the intergalactic gas or the environs of active galactic nuclei. Furthermore, interstellar gas will prove to be an essential ingredient in the fuelling of active galactic nuclei.

The mass of the interstellar gas amounts to about 5% of the visible mass of our Galaxy. In the Galactic plane close to the Sun, the overall gas density is to about 106 particles m−3, but there are very wide variations in density and temperature from place to place throughout the interstellar medium.

Diagnostic tools – neutral interstellar gas

Neutral hydrogen: 21-cm line emission and absorption

Neutral hydrogen emits line radiation at a frequency ν0 = 1420.4058 MHz (λ0 = 21.1 cm) through an almost totally forbidden hyperfine transition in which the spins of the electron and proton change from being parallel to antiparallel.

Associations of galaxies range from pairs and small groups, through giant clusters containing over a thousand galaxies, to the vast structures on scales much greater than clusters such as the vast ‘walls’ and voids observed in the distribution of galaxies. Clustering occurs on all scales and very few galaxies can be considered truly isolated. Rich clusters of galaxies are of particular interest because they are the largest gravitationally bound systems in the Universe. The gravitational potential of the cluster is defined by the distribution of dark matter, the mass of which greatly exceeds that of the baryonic matter, such as that contained in the stars in galaxies and the associated interstellar gas and the intracluster gas. The deep gravitational potential wells of clusters can be observed directly through the bremsstrahlung X-ray emission of hot intracluster gas which forms a hydrostatic atmosphere within the cluster. The hot gas can also be detected through the decrements which it causes in the Cosmic Microwave Background Radiation as a result of the Sunyaev–Zeldovich effect. Gravitational lensing has proved to be a very powerful tool for defining the large scale distribution of dark matter in clusters, as well as in individual galaxies within them. Interactions of galaxies with each other and with the intergalactic medium in the cluster can be studied and radio source events can strongly perturb the distribution of hot gas.

Introduction

In Sect. 8.9, a convincing case was made that the high energy electrons observed at the top of the atmosphere are a representative sample of those present throughout the interstellar medium and are responsible for the diffuse Galactic synchrotron radio emission. In Sect. 15.4, a similar exercise was carried out for cosmic ray protons. The spectrum and properties of the γ-ray emission of the Galaxy provide compelling evidence that a flux of cosmic ray protons, of similar properties to those observed in our vicinity in the Galaxy, permeates the plane of the Galaxy. In this chapter, these observations are interpreted in terms of the propagation of these particles from their sources through the interstellar medium and the energetics of potential sources in the Galaxy. Key diagnostic tools are provided by the aging processes which can result in observable features in the synchrotron spectra of relativistic electrons and by the energy requirements of sources of synchrotron radiation. The TeV γ-ray emission of supernova remnants is direct evidence for the presence of large fluxes of particles with cosmic ray energies in supernova remnants, although it is not yet clear if these are associated with high energy electrons or protons (Sect. 16.4.2). The tools are developed in the context of the origin of cosmic rays in supernovae explosions and are of applicability to the whole of high energy astrophysics.

We are very pleased to present what is the first major star atlas devoted to the observation of the “Herschel objects” – some 5,000 star clusters, nebulae, and galaxies collectively discovered by Sir William Herschel, his sister Caroline, and son Sir John. With the widespread growing popularity of viewing these wonders of the heavens by amateur astronomers today, the need for such a work clearly exists. The one classic atlas that identified some of those objects found by William Herschel, using his designations (329 of them), was Norton's Star Atlas in all of its first 17 editions. Sadly, all later revised and redrawn versions – initially re-titled Norton's 2000.0 and currently back to the original Norton's Star Atlas – dropped these labels, to the dismay of observers. While this new atlas is primarily designed with observation of star clusters, nebulae, and galaxies in mind, it also serves as a general purpose guide for exploring all types of deep-sky objects, showing as it does many prominent double and multiple stars, variable stars, asterisms, and the majestic Milky Way itself. Additionally, it may be viewed as a companion volume to our previous work, The Cambridge Double Star Atlas, first published in 2009. Between these two publications, the long-standing lack of recognition accorded the discoveries of the Herschels, and those of the classic double star observers, by celestial cartographers has finally been rectified.

Who were the Herschels?

Sir William Herschel

William Herschel was without question the greatest visual observer who ever lived.

The aim of this contribution is to underline some problems related to what is called the cosmological anthropic principle. There are several statements (weaker or stronger) of this ‘principle’, which was initially introduced by Brandon Carter (cf. Demaret and Barbier, 1981; Barrow and Tipler, 1986; Demaret, 1991; Demaret and Lambert, 1996). Today, there are a huge number of references defining and discussing these statements and we do not want to enter such a discussion here. In fact, for our purpose, we can simply say that the ‘weak’ version expresses simply the causality principle: if human life exists in the Universe, then there exist precise constraints that render the emergence of such life possible. This can also be presented as an observational constraint. If, as human beings, we are observing the Universe now, the latter cannot be arbitrary. It has to be such that human life is possible. These ‘weak principles’ are in fact a translation of the fact that each empirical event or each phenomenon can be characterized by a set of necessary conditions. And the weak versions of what one called the anthropic principle are then nothing more than a logical implication: human life (H) implies necessary conditions for human life to exist (NCH). Or, if we are considering the observational constraint approach: the existence of human observers implies necessary constraints on the Universe that render this existence possible.

Preamble: the vertical inheritance of genetic material

Living systems are ephemeral vehicles of their immortal germlines. In the selfish-gene (Dawkins, 1976) or disposable-soma (Kirkwood, 1977) perspectives, genes do have a greater interest in regularly ‘buying a new vehicle and throwing away the old one’. Rather than being forever stuck in the same organism genes can reassort themselves with random samples of the genetic-material pool through sexual reproduction. As human beings we are thus familiar with the widespread biparental reproduction procedure. Two individuals – the parents – spawn reproductive cells – the gametes – which merge to produce offspring – the descendants. In diploid populations, each descendant receives a paternal copy and a maternal copy of the genome (Figure 19.1). Because of the recombination occurring in the germline each parental copy is itself a reassortment of the grandmaternal and grandpaternal genomes. From generation to generation gene copies are replicated. If they are transmitted to at least two descendants (Figure 19.1: stars), this will result in branching nodes in what we call ‘gene trees’ (Maddison, 1997).

This transmission of the genetic material is called ‘vertical’: DNA of an organism is inherited from its forebears. Vertical gene transfer implies that there is a tree structure describing the history of descent of the genetic material. By contrast, there is horizontal gene transfer when genetic material is passed on from donor organisms to receptors belonging to different species (see Chapter 20).