Stellar winds are ubiquitous amongst massive stars, although the physical processes involved depend upon the location of the star within the H-R diagram. Mass-loss crucially affects the evolution and fate of a massive star (Chapter 5), while the momentum and energy expelled contribute to the dynamics and energetics of the ISM (Chapter 8). The interested reader is referred to the monograph by Lamers & Cassinelli (1999) on the topic of stellar winds, or Kudritzki & Puls (2000) for a more detailed discussion of mass-loss from OB and related stars.

The existence of winds in massive stars was first proposed by Beals (1929) to explain the emission line spectra of Wolf–Rayet stars. This gained observational support in the 1960s when the first rocket UV missions revealed the characteristic P Cygni signatures of massloss from CIV λ1550, SilV λ1400, and NV λ1240 in O stars (Morton 1967). A theoretical framework for mass-loss in hot stars was initially developed by Lucy & Solomon (1970) involving radiation pressure from lines, and refined by Castor, Abbott, & Klein (1975), thereafter known as CAK theory. The observational characteristics of stellar winds are velocity and density. The former can be directly observed, whilst the latter relies on a varying complexity of theoretical interpretation.

Radiation pressure

When a photon is absorbed or scattered by matter, it imparts its energy, hv, and momentum, hv/c, where h is Planck's constant and c is the velocity of light. Consequently, radiation is a very inefficient carrier of momentum.

The most striking recent development in the study of sunspots has been the revelation of fine structure in intensity, magnetic fields and velocity patterns in the penumbra. Within the past 15 years new observations, notably those made first with the Swedish Vacuum Solar Telescope (SVST) on La Palma, then with the aid of adaptive optics on the Dunn Telescope at Sacramento Peak and the 1-m Swedish Solar Telescope (SST) and, most recently, with the Solar Optical Telescope on the Hinode spacecraft, have resolved delicate features with unprecedented clarity – and, in so doing, have posed major problems for theory to explain (Thomas and Weiss 2004). The filamentary penumbra appears clearly in Figures 1.2 and 3.1 and is shown here in greater detail in Figure 5.1.

In this chapter we discuss this fine-scale filamentary structure and the associated interlocking-comb configuration of the penumbral magnetic field. We begin with observations and describe first the two-dimensional intensity pattern in the penumbral photosphere. Then we go on to discuss the complex three-dimensional structure that is revealed by measuring two vector fields, the magnetic field B and the velocity u. This rich and intricate magnetic geometry results from interactions between convection and the inclined magnetic fields in the outer part of a sunspot. We next outline the current theoretical understanding of this form of magnetoconvection, and attempt to interpret the observed penumbral structure in the light of available theoretical models.

Various kinds of wave motions have been observed in sunspots. These include characteristic umbral oscillations with periods around 3 minutes, umbral oscillations with periods around 5 minutes (which differ in several respects from the 5-minute p-mode oscillations in the quiet photosphere), and large-scale propagating waves in the penumbra. These oscillatory phenomena are of considerable interest because they are the most readily observable examples of magnetohydrodynamic waves under astrophysical conditions. In addition, observations of oscillations in a sunspot and its nearby surroundings can be used to probe the structure of a sunspot below the solar surface (‘sunspot seismology’).

Interest in sunspot oscillations began in 1969 with the discovery of periodic umbral flashes in the Ca II H and K lines by Beckers and Tallant (1969). These flashes were soon attributed by Havnes (1970) to the compressive effects of magneto-acoustic waves. In 1972 three other types of sunspot oscillations were discovered: running penumbral waves in Hα (Giovanelli 1972; Zirin and Stein 1972); 3-minute velocity oscillations in the umbral photosphere and chromosphere (closely connected to the umbral flashes: Giovanelli 1972; Bhatnagar and Tanaka 1972); and 5-minute velocity oscillations in the umbral photosphere (Bhatnagar, Livingston and Harvey 1972). For some time these three types of oscillations were considered as distinct phenomena, but recent work suggests that they might actually be different manifestations of the same coherent oscillations of the entire sunspot (Bogdan 2000). Here we shall follow the historical development of the subject by discussing the three types of oscillations separately before attempting to present a unified picture.

If we were to observe the Sun from the distance of α Centauri (4.3 light years), we would not be able to resolve its spots directly or to detect luminosity variations as they came and went; nor could we measure its magnetic field. Although we naturally expect that there should be analogues of solar magnetic activity on lower main-sequence stars that are similar to the Sun (Tayler 1997; Rosner 2000), we can only detect this activity through indirect measurements of other effects that are known to be associated with active regions on the Sun itself. These effects include X-ray emission from stellar coronae, optical and radio emission from flares and enhanced chromospheric emission, notably in the H and K lines of singly ionized Ca II (Wilson 1994; Schrijver and Zwaan 2000). As we shall see, there are indeed also some stars that are much more active than the Sun, whose magnetic fields can be measured directly; such stars also exhibit substantial variations in luminosity that can be ascribed to the presence of starspots on their surfaces.

Stellar Ca II emission

The most widely used indicators of stellar magnetic activity are the emission cores of the Ca II H and K absorption lines. On the Sun, Ca II emission forms a chromospheric network, first observed by Hale (Bray and Loughhead 1974), which corresponds to the magnetic network that outlines supergranules in the photosphere. This emission could be detected if the Sun were viewed from a nearby star, and the cyclic variation of solar activity would also be apparent, as can be seen from Figure 8.1.

We turn now to a discussion of the fine structure of a sunspot, beginning here with features in the umbra and continuing in the next chapter with features in the penumbra. Our knowledge of this fine structure has been transformed in recent years due to remarkable improvements in high-resolution observations. We review the results of these observations and theoretical interpretations of them. Our enhanced knowledge of the fine structure of sunspots has not only provided us with a far larger collection of details; it has also stimulated new insights that allow us to start assembling a coherent picture of the formation and maintenance of a sunspot, with its dark umbra and its puzzling filamentary penumbra.

Umbral dots

In many images, sunspot umbrae – like pores, which are just isolated umbrae – appear uniformly dark. When such images are appropriately exposed, however, as in Figure 4.1, it becomes apparent that there is an intensity pattern in sunspot umbrae, composed of many small, isolated, bright features embedded in a darker, smoothly varying background. These features are called umbral dots and they are found in essentially all sunspots and also in pores (Sobotka 1997, 2002). Earlier observations of an intensity pattern in umbrae, with a resolution of about 1″, had failed to resolve the umbral dots and instead showed a pattern that looked more like a weaker version of the photospheric granulation (Chevalier 1916; Bray and Loughhead 1964).

In this chapter we offer a brief historical introduction to sunspots, from the earliest naked-eye observations up to the remarkable advances in high-resolution observations and numerical simulations of recent years. We also discuss early speculations about starspots and their first observational detections. While we aim to give a balanced account, covering both observations and theory, our presentation does not follow a strict chronological path, but is instead arranged by topic. We do, however, provide a list of major advances in strict chronological order at the end of this chapter.

Early observations of sunspots

Seasonal changes were all-important to early agrarian societies and so they naturally worshipped the Sun. Indeed, the sun-god headed the pantheon in many cultures, ranging from Egypt to Peru. Since astronomy was also practised in these cultures, it seems likely that sunspots must occasionally have been detected with the naked eye, which is possible when the Sun is low on the horizon and partially obscured by dust storms, volcanic dust or smoke.

Giant HII regions are extensive regions of ionized hydrogen (and other elements) powered by hot stars. Their high concentration of very massive stars often makes them appear as the optically most luminous structures in galaxies. Nearby giant HII regions allow us to study star formation and evolution in great detail and help us understand unresolved giant HII regions in distant galaxies.

Giant HII regions: definition and structural parameters

In this section we will concentrate on the properties of giant HII regions, such as the famous 30 Doradus nebula in the LMC. The focus of this book is on hot, massive stars, and we are interested in HII regions because their properties allow us to learn about the stars powering them. As opposed to smaller, Orion-like HII regions, giant HII regions are sufficiently rich in O stars that the entire upper end of the mass spectrum is sampled and an unbiased view of the hot-star population can be obtained.

Giant HII regions are among the most conspicuous objects in nearby late-type galaxies, in particular when observed in narrow emission lines, such as Hα. Systematic studies of nearby giant HII regions such as that by Kennicutt (1984) have established their fundamental properties. These HII regions typically have diameters of 100 pc or larger, densities of a few particles per cm3, and ionized gas masses of order 105M⊙. In Fig. 9.1 we show a well-known example, NGC 604 in the Triangle galaxy M33.

In 1858, Richard Carrington wrote, “Our knowledge of the Sun's action is but fragmentary, and the publication of speculations on the nature of his spots would be a very precarious venture.” Fifty years later, George Ellery Hale's discovery of the magnetic field in a sunspot ushered in the modern era of research into solar, stellar and cosmical magnetic fields. This book, coincidentally, marks the hundredth anniversary of his discovery. The past century has seen enormous and rapidly accelerating progress in our understanding not only of sunspots but also of starspots and the whole solar–stellar connection. Our purpose here is to bring these advances together and to offer a unified account of sunspots and starspots in the context of solar and stellar magnetic activity.

Our own collaboration goes back more than 40 years, to the academic year 1966–67 when JHT was a NATO postdoctoral fellow in the Department of Applied Mathematics and Theoretical Physics at Cambridge, where NOW was a recently appointed lecturer. In 1991 we organized a NATO Advanced Research Workshop on Sunspots: Theory and Observations, which produced an edited volume (Thomas and Weiss 1992a) designed to serve as a monograph on the subject. Progress on sunspots has been very rapid since then, especially in high-resolution observations (both ground-based and from space) and in numerical modelling; meanwhile, with new techniques such as Doppler and Zeeman–Doppler imaging, the study of starspots (treated briefly in the 1992 volume) has emerged as a fully fledged subject of its own. Hence it seems to us that the time has come for a new, comprehensive book on sunspots and starspots that emphasizes recent developments.

This monograph had its origins about a decade ago when it became apparent that the field of luminous hot stars was rapidly expanding in extent and depth and connections to extragalactic astrophysics, in particular starburst galaxies, were first recognized. At that time a decade had passed since the 1988 O stars and Wolf–Rayet stars NASA monograph of Conti and Underhill. Since then, there have been far reaching advances in the astrophysics of these stars and of star-forming galaxies, both locally and at high-redshift, together with the way each affect their surroundings. On the observational side, progress interpreting the spectra of luminous hot stars allows their physical parameters to be derived with unprecedented accuracy. Studies of their ubiquitous stellar winds plus their dependence upon the element abundances has provided the impetus for revised stellar evolutionary model calculations. For the first time, additional physics, such as rotational mixing and magnetic fields, is being considered.

The advent of the Hubble Space Telescope in 1990 provided us with a UV wavelength range of greatly increased sensitivity just where most of the energy of hot stars is emitted. The use of newly commissioned 8-m ground-based telescopes has opened the door to studies of more distant hot stars in external galaxies, with different initial abundances and star formation histories compared to the Milky Way. In addition, techniques for the identification of starforming galaxies at an epoch when the Universe was as little as one tenth of its present age have been developed, and exploited with these instruments.

Our Sun is a typical, middle-aged star, but it occupies a special place in astronomy as the only star that we can observe in great detail. Conversely, it is only by studying other stars with different properties, whether of age, mass or angular momentum, that we can fully explain the behaviour of the Sun. This book is concerned with dark spots on the surfaces of the Sun and other stars, which result from the interplay between magnetic fields and convection. In this opening chapter we provide a brief introduction to the properties of these spots, a summary of the important overall properties of the Sun and other stars, and an overview of the topics that will be covered in the remainder of the book.

Sunspots and solar magnetic activity

In this section we introduce a variety of features and phenomena associated with sunspots and solar activity, all of which will be discussed in greater detail in later chapters.

In images of the full solar disc, such as that shown in Figure 1.1, sunspots appear as dark patches at low latitudes. The fact that sunspots are associated with strong magnetic fields emerging through the solar surface is readily apparent in the accompanying magnetogram in Figure 1.1, which shows the strength and polarity of the longitudinal (line-of-sight) magnetic field.

In a close-up image, such as the one in Figure 1.2, a typical sunspot is seen to consist of a dark central region called the umbra surrounded by a less dark, annular region called the penumbra. Some sunspots are remarkably circular and axisymmetric (favourites of theoreticians), while others have very irregular shapes with perhaps only partial penumbrae.

In this chapter we describe the gross features of sunspots, including their shapes and sizes and their overall thermal and magnetic structure. We also discuss those models of a sunspot that ignore its fine structure and make the simplifying assumption that the spot can be treated as an axisymmetric magnetic flux tube. This is a reasonable approximation, for although most spots are irregularly shaped, as can be seen from Figure 1.2, there are still many examples that are approximately circular, like that in Figure 3.1.

Sunspots are dark because they contain strong magnetic fields that partially inhibit the normal transport of energy by convection at, or just below, the solar photosphere. A welldeveloped spot may have a radius of 10,000–20,000 km, with a dark central nucleus (the umbra), surrounded by a less dark, filamentary penumbra. Such sunspots have approximately similar structures. The umbra occupies about 18% of the area of the spot, corresponding to an umbral radius that is about 40% of that of the spot. The umbra radiates energy at only 20% of the normal photospheric rate, corresponding to a temperature deficit of 2000 K, while the average penumbral intensity is about 75% of that outside the spot, corresponding to a deficit of only 400 K (Bray and Loughhead 1964; Thomas and Weiss 1992a; Stix 2002), so the total ‘missing energy’ is about 35% of that from a corresponding field-free area.

The phrase “starburst” was used before in this volume, referring to a region with intense star formation. In this chapter we will define and discuss starbursts in more rigorous terms and relate them to the properties of the host galaxies. While luminous HII regions are often considered starbursts as well, starburst phenomena are more general and have profound cosmological implications. A major theme of this chapter, in contrast to the local focus of Chapter 9, will be the global interrelation between a starburst region and its environment. The starburst is nourished by the gas supply of the host, and it energizes the galaxy ISM with stellar winds and supernovae.

Definition of a starburst

One of the earliest uses of a term similar to “starburst” goes back to a seminal publication by Searle, Sargent, & Bagnuolo (1973). These authors added data points of many late-type galaxies to a theoretical two-color diagram which plots (B–V) against (U–B) color. Surprisingly, the galaxies had bluer colors than expected for a stellar population either evolving with steady, continuous star formation or with no star formation at all after the formation of the first stellar generation. This immediately suggested that some galaxies are observed in a special epoch: they are currently forming stars at a higher rate than previously, otherwise the bluer colors, indicative of an excess of hot stars, would not be understandable.