The Solar System naturally divides into two groups of planets, separated by the asteroid belt. The four inner planets have many things in common with the Earth, whereas the next four planets present worlds of an entirely different type. (Pluto is an additional enigma.) In this chapter, we look at Mercury, Venus and Mars, comparing their properties with each other, and with the Earth.

Basic features

Mercury

Mercury is the closest planet to the Sun, and is not much larger than our Moon. There is an interesting story concerning its rotation period. Since Mercury is so close to the Sun, we never have a really good view of it, and surface features are hard to recognize. By noting the positions of large surface features, it appeared that the rotation period was 88 days, the same as the planet's orbital period. This would have meant that Mercury always keeps the same face towards the Sun (just as the Moon keeps the same face towards the Earth). Since Mercury is so close to the Sun, it seemed plausible that some tidal effect could keep its rotation period synchronized with its orbital period.

However, the situation was corrected following radar observations. Radio waves were bounced off Mercury and then detected back on Earth. The planet's rotation causes a spread in the Doppler shifts of the reflected waves. From the amount of spread, we can tell how fast the planet is rotating.

When we look at the spatial distribution of stars in our galaxy, we find that most of the light is concentrated in a thin disk. We are inside this disk, so we see it as a band of light on the sky, called the Milky Way. We will discuss this farther in Part III, but we will see in this chapter that location of stars in the galaxy can tell us something about those stars. In particular, some stars are confined to the thin disk of the Milky Way, while others form a more spherical distribution. In this chapter, we will discuss groupings of stars, called clusters, and see how they vary in size, content and galactic distribution.

Types of clusters

We distinguish between two types of star clusters – galactic clusters and globular clusters.

Galactic clusters are named for their confinement to the galactic disk. A selection of images of galactic clusters is in Fig. 13.1. A familiar galactic cluster, the Pleiades, is shown in Fig. 13.1(a). Note the open appearance in which individual stars can be seen. Because of this appearance, galactic clusters are also called open clusters. Galactic clusters typically contain <103 stars, and are less than ~10 pc across. Recent sensitive near IR surveys are showing more members than we had previously thought in many clusters. In the photograph, we see some starlight reflected from interstellar dust. Galactic clusters are sometimes associated with interstellar gas and dust.

Our study of the Milky Way has been aided greatly by studies of other galaxies. However, for a long time it wasn't clear that the spiral nebulae we see in the sky are really other galaxies. From their appearance, it might just be assumed that these nebulae are small nearby objects, just as HII regions are part of our galaxy.

The issues were crystallized in 1920 in a debate between Harlow Shapley and Heber D. Curtis. Curtis argued that spiral nebulae were really other galaxies. His argument was based on some erroneous assumptions. First, he confused novae in our galaxy with supernovae in other galaxies. Shapley thought the spiral nebulae were part of our own galaxy, partly based on an erroneous report of a measurable proper motion for some nebulae.

The issue was settled in 1924 by the observational astronomer Edwin Hubble (after whom the Space Telescope is named). Hubble studied Cepheids in three spiral nebulae (including the Andromeda Galaxy), and clearly established their distance as being large compared with the size of the Milky Way. There is some problem with Hubble's analysis, involving type I vs. type II Cepheids. However, even this factor of 2 error in the distance was not enough to alter the basic conclusion that spiral nebulae are not part of our own galaxy. Following this work, Hubble made a number of pioneering studies of other galaxies, essentially opening up the field of extragalactic astronomy.

In this chapter we look at galaxies with unusual activity within and around them. For many years astronomers thought of these various types of activity as being distinct. We now realize that many of them have similar origins, but differ in the specific conditions within the galaxy or its environment. We realize that all of this activity takes place in the nucleus of the galaxy, or is driven by activity in the nucleus. We say that these phenomena are associated with active galactic nuclei (AGN).

Starburst galaxies

Some galaxies appear to be giving out excessive amounts of radiation in the infrared. When we studied star formation in Chapter 15, we saw that regions with recent star formation give off a lot of infrared radiation. The energy comes from the newly formed stars, and heats the dust (from the parent cloud) surrounding the stars. The dust then glows in the infrared. The more energy the young stars put into the cloud, the more infrared radiation is released. The excess infrared radiation from some galaxies suggests that those galaxies have very high rates of star formation. The rate is so high that it cannot be sustained for very long, or it would use up all of the interstellar material. This leads to the idea that this excessive star formation is a short-lived phenomenon. We therefore call such galaxies starburst galaxies.

There are a vast number of smaller objects in our Solar System, not as substantial as our Moon, but which provide important clues on the history of the Solar System. These are asteroids, comets and meteoroids. We have also included the ninth planet, Pluto, in this chapter. As we will see below, recent determinations of Pluto's mass make it by far the least massive planet, and it has more properties in common with the other less massive objects in the Solar System.

Pluto

Pluto was discovered in 1930, following an extensive search, by Clyde Tombaugh. The search was initiated by Percival Lowell after it was thought that a planet beyond Neptune might be perturbing Neptune's orbit. Calculations narrowed the range of possible locations on the sky, and a search was carried out. As Fig. 26.1 shows, Pluto doesn't stand out very well against the background of stars. It is detectable as a planet only by its very slow motion with respect to the stars.

For Pluto to have a perturbing effect on other planets, its mass must be greater than that of the Earth. For this reason, since its discovery, Pluto's mass has been overestimated. We now know that its mass is much less than previously thought, and that it has no measurable effect on other planets. In a sense, Pluto's discovery was accidental. It was a result of an extensive search of a particular region in the sky.

Overview

When we look at photographs of the Milky Way (see Fig. 16.1), we note large regions where no light is seen. We think that these are due to dust blocking the light between us and the stars. We can see the same effect on a smaller scale (Fig. 14.1). Note that there is a high density of stars near the edges of the image. As one moves close to the center, the density of stars declines sharply. Near the center, no stars can be seen. This apparent hole in the distribution of stars is really caused by a small dust cloud, called a globule. The more dust there is in the globule, the fewer background stars we can see through the globule. We can use images like this to trace out the interstellar dust. We find that it is not uniformly distributed. Rather, it is mostly confined to concentrations or interstellar clouds.

We detect the presence of the gas by observing absorption or emission lines from the gas. By tracing these lines, we find that the gas also has an irregular distribution. Often the gas appears along the same lines of sight as the dust clouds. From this apparent coincidence we form the idea that the gas and dust are generally well mixed, with the gas having about 99% of the mass in a given cloud. In this chapter, we will see how the masses of different types of clouds are determined.

The past decades have seen dramatic improvements in our observing capabilities. There have been improvements in our ability to detect visible radiation, and there have also been exciting extensions to other parts of the spectrum. These improved observing capabilities have had a major impact on astronomy and astrophysics. In this chapter we will first discuss the basic concepts behind optical observations. We will then discuss observations in other parts of the spectrum.

What a telescope does

An optical telescope provides two important capabilities:

1. (1) It provides us with light-gathering power. This means that we can see fainter objects with a telescope than we can see with our naked eye.

2. (2) It provides us with angular resolution. This means that we can see greater detail with a telescope than without.

For ground-based optical telescopes, light-gathering power is usually the most important feature.

Light gathering

We can think of light from a star as a steady stream of photons striking the ground with a certain number of photons per unit area per second. If we look straight at a star, we will see only the photons that directly strike our eyes. If we can somehow collect photons over an area much larger than our eye, and concentrate them on the eye, then the eye will receive more photons per second than the unaided eye. A telescope provides us with a large collecting area to intercept as much of the beam of incoming photons as possible, and then has the optics to focus those photons on the eye, or a camera, or onto some detector.

Overview

Throughout this book we have discussed the components of our galaxy: stars, clusters of stars, interstellar gas and dust. We now look at how these components are arranged in the galaxy. The study of the large scale structure of our galaxy is difficult from our particular viewing point. We are in the plane of the galaxy, so all we see is a band of light (Fig. 16.1). The interstellar dust prevents us from seeing very far into the galaxy. We see a distorted view.

The first evidence on our true position in the galaxy came from the work of Harlow Shapley, who studied the distribution of globular clusters (Fig. 16.2). He found the distances to the clusters from observations of Cepheids and RR Lyrae stars. Shapley found that the globular clusters form a spherical distribution. The center of this distribution is some 10 kpc from the Sun. Presumably, the center of the globular cluster distribution is the center of the galaxy. This means that we are about 10 kpc from the galactic center.

In Chapter 13, when we studied HR diagrams for clusters, we introduced the concept of stellar populations I and II. The distribution of these populations in the galaxy can help us understand how the galaxy has evolved. Population I material is loosely thought of as being the young material in the galaxy.

Now that we know the basic properties of stars, we look at how the laws of physics determine those properties, and then how stars change with time – how they evolve. Stars go through a recurring full life cycle. They are born, they live through middle age, and they die. In their death, they distribute material into interstellar space to be incorporated into the next generation of stars.

In describing the life cycle, we can start anywhere in the process. In Chapter 9, we discuss the most stable part of their life cycle, life on the main sequence – stellar middle age. In Chapters 10, 11 and 12 we will look at the deaths of different types of stars. After discussing the interstellar medium in Chapter 14, we will look at star formation in Chapter 15.

Brightness of starlight

When we look at the sky, we note that some stars appear brighter than others. At this point we are not concerned with what causes these brightness differences. (They may result from stars actually having different power outputs, or from stars being at different distances.) All we know at first glance is that stars appear to have different brightnesses.

We would like to have some way of quantifying the observed brightnesses of stars. When we speak loosely of brightness, we are really talking about the energy flux, f, which is the energy per unit area per unit time received from the star. This can be measured with current instruments (as we will discuss in Chapter 4). However, the study of stellar brightness started long before such instruments, or even telescopes, were available. Ancient astronomers made naked eye estimates of brightness. Hipparchus, the Greek astronomer, and later Ptolemy, a Greek living in Alexandria, Egypt, around 150 BC, divided stars into six classes of brightness. These classes were called magnitudes. This was an ordinal arrangement, with first-magnitude stars being the brightest and sixth-magnitude stars being the faintest.

When quantitative measurements were made, it was found that each jump of one magnitude corresponded to a fixed flux ratio, not a flux difference. Because of this, the magnitude scale is essentially a logarithmic one. This is not too surprising, since the eye is approximately logarithmic in its response to light.

Some readers will take exception to the use of the term “medieval” to describe a phase of Islamic history. The term is borrowed from European history, where it signifies a period, the “Middle Ages,” distinguished from the “classical” one that preceded it and the “Renaissance” by which it was followed. In European history the term originally had something of a pejorative connotation – that the Middle Ages constituted a sort of valley between the peaks of classical and Renaissance culture and learning – although most historians would today describe the Middle Ages as considerably less “dark” than was earlier thought. The risks of abstracting the term from the European context that produced it, and applying it to the wholly different circumstances of the Islamic Near East, are obvious.

On the other hand, there were peculiar characteristics of the Islamic society and its religious institutions that took shape in the period between the beginning of the eleventh and the end of the fifteenth centuries. In the “Islam” which emerged over the course of these centuries are to be found various patterns of religious authority, affiliation, and relationship which distinguish it from what came before, which laid the foundation for the Islamic societies (particularly in the form of the Ottoman and Safavid empires) that followed, and which shaped the Islamic identities of those Muslims who suddenly found themselves faced with the changed circumstances of the modern period.

There were two basic patterns in medieval political life which had a profound impact on the religious life of the Muslim communities of the Near East. The first was a persistent and constantly shifting diffusion of power away from the center and towards more local and limited regimes. The central fact here was the decline in the power and authority of the cAbbasid caliphs. So much of early Islamic discourse and conflict had focused on the institution of the caliphate, yet in the Middle Period, despite moments of resurgence, its authority flickered and finally died. The jurists held a deep attachment to the office of the caliph as an integral part of the sharica; nonetheless they too were forced to confront the political realities. Toward the end of the Buyid period, the Baghdadi Shafici qadi al-Mawardi (d. 1058) wrote a treatise on the law of government, in the first chapter of which he outlined the position and powers of the caliph. His famous description is a classic treatment of the caliph as an active centerpiece of the unity of the Islamic umma, as the cornerstone of the administration of God's law, in the face of the growing political fragmentation of the medieval period. There is a certain irony here, as al-Mawardi wrote his treatise at a time when the caliphate had ceased to wield effective authority.

The master narrative of the two and a half centuries which followed the cAbbasid Revolution might be characterized as one that took the institution of the caliphate from revolution to autocracy, and thence to disintegration and the concomitant fragmentation of the umma – that, at least, was the political framework within which radical transformations in the society and religious identity of Muslims transpired. What follows is a very brief sketch of some of the political highlights of the period, from the accession of al-Saffah, the first cAbbasid caliph, to the end of the tenth century.

In 762, al-Mansur, the second cAbbasid caliph, established a new capital for the empire in Iraq. The foundation of Baghdad, which al-Mansur actually called the “City of Peace,” reflected the growing tensions between the cAbbasids and the supporters of cAli's family, who were especially strong in Kufa, the principal Muslim settlement in Iraq which had served as the cAbbasid caliphs’ first capital. In many ways the city can stand as a metaphor for the character of the Islamic empire in this period, and for its greatness. The city, like the state of which it was the capital, was an ambitious enterprise. Much of it was occupied by and organized around explicitly imperial structures – palaces, gardens, vast reception halls – with a domed room housing the caliph's throne at the very center.

There is an old and well-developed heresiographical tradition in Islam. In response to reported dicta of the Prophet to the effect that his community would ultimately fracture into seventy-three (or seventy-two – the precise number varies with the reports) sects, theologians composed extensive treatises identifying the different religious groups and their relation to what the authors identified as legitimate Islam. Of course, as Islam has generally lacked an institutional authority constituted to make definitive pronouncements about matters of religious interest, what exactly the parameters of Islamic legitimacy embrace has tended to shift with the perspective of the viewer. The very terms “heresy” and “orthodoxy” (since the former cannot exist without the latter) as they are used in the Western Christian tradition are in some ways misleading in an Islamic context. On the other hand, the flexibility of the Islamic tradition has not prevented Muslims from fervent denunciations of those who (in their view) falsely claim the mantle of Islam. Indeed, the very lack of an authoritative institutional structure has probably helped to make conflicts over issues of religious identity sharper and more intense. Those conflicts were central to the history of Islam in the first century and a half of the new religion when, as we have seen, it only gradually carved out for itself an identity distinct from those of the earlier Near Eastern monotheisms.