We will begin our work on stars by studying the one we know best: our Sun. The Sun is the closest star. Unlike every other star, it is so close that we can minutely observe its surface and surrounding layers.

We owe our existence to the Sun. Its great emissions warm us, and protect us from the unimaginable cold of interstellar space. Indeed most of the Universe, far from the Sun or other stars, is just a few degrees above absolute zero, which is 459 degrees below zero Fahrenheit. We lie huddled close to a warming campfire in a frigid wasteland.

As we have noted, the Sun is gigantic – more than a hundred times the size of the Earth – and immensely massive – far more massive than all the planets of the Solar System combined. And it is so hot as to vaporize every known substance. We might say that the Sun is nothing more than a ball of superheated gas. But perhaps we should put quotation marks around that “nothing more.” As we will see, this blazing sphere of gas exhibits a rich and complex behavior.

In this chapter we will discuss the outer regions of the Sun: the Sun that we can see. In Chapter 14 we will turn our attention to the Sun that we cannot see: its deep interior.

In the previous chapter we discussed our home in the Universe: the Milky Way Galaxy. But ours is not the only galaxy. Here we discuss all the others. Some are like our own: others are completely different.

The number of galaxies defies the imagination. Immense, faintly glowing, slowly rotating, they drift about, influenced by their mutual gravitational attraction. Occasionally they actually encounter one another. You might think of them as resembling snowflakes in a blizzard.

Our own Galaxy is disk-shaped, with a central bar and winding spiral arms. Other galaxies lack the central bar; yet others are more nearly elliptical and also lack the spiral arms. Yet still others are completely irregular in shape. Some have lots of interstellar gas; some hardly any.

And some are the seat of violent, hugely powerful explosions. These objects emit vast quantities of energy as optical light, radio waves and other forms of radiation. These emissions probably arise from matter falling into giant black holes lying at their cores.

Galaxies lie all about us: indeed, some are so close they can be seen with the naked eye. But for years nobody knew what they were. We begin with the story of how astronomers came to discover their true nature.

In the previous chapter we studied one particular star: the nearest one, our Sun. We now transfer attention to all the others.

To the naked eye, stars look like nothing more than tiny points of light. That's what they look like through a telescope, too. Nothing in their appearance, even through our most powerful of telescopes, tells us much about them. Indeed, just as we saw in our study of the Sun, it is not easy to find out things about the stars. Our first task in this chapter will be to find techniques for answering questions about them – questions like “how far away is that star?” or “how bright is it?”

Our second task will be to use these techniques to conduct a census of stars. Just as pollsters do not interview each and every person in America, so it would be impossible to study each and every star in the sky. Accordingly, much of our time will be spent in developing ways to identify a representative sample of stars with which to conduct our census.

When we finally do so, a remarkable pattern will emerge, a pattern summarized in the so-called “Hertzsprung–Russell diagram.” It will take us two full chapters to reach an understanding of this pattern.

We now turn to a more detailed tour of the Solar System.

In this chapter we will visit our Moon, and then tour those planets lying closest to the Sun. Our own home world is one of them: the others are Mercury, Venus and Mars. These planets constitute what might be called our immediate vicinity, the portions of the Solar System that lie closest to us. But the real reason to treat all these bodies at once is that, taken together, they constitute the inner Solar System, all of whose planets are relatively small and dense and are composed of rock. In the next chapter we will encounter the outer planets, which are entirely different: far larger, and composed primarily of gas.

The farther out we will look, the more alien will the worlds become: in this chapter we’ll start with something more or less familiar. But even these, our nearest-neighbor worlds, will turn out to be full of surprises. And perhaps this is the most important lesson of all: the Universe continually surprises us, and it is endlessly fascinating.

The Moon

Observing the Moon

The first stop on our journey will be our own Moon (Figure 7.1). To the naked eye the Moon consists of light areas and irregular dark splotches. These dark regions were termed maria (“MAH-ria”) by the ancients, from the Latin mare (“MAH-ray”) for “sea.” Although we now know they are ancient lava flows, not oceans, the name has stuck. The view through even a small telescope or pair of binoculars is far richer. In lighter areas can be seen mountains together with innumerable craters, from the largest of which great white rays are seen to emanate. The maria too have craters, though far fewer of them.

To the instructor

The philosophy behind this book

When I was in college studying science, I found the experience fundamentally unsatisfying. I was continually oppressed by the feeling that my only role was to “shut up and learn.” I felt there was nothing I could say to my instructors that they would find interesting. Nor did I feel that there was anything I could tell my fellow-students that they would find interesting. As I sat in the science lecture hall, I was utterly silent. That's not a good state to be in when you are 19 years old.

Doubly galling was the fact that at the same time my roommate was taking a history course. One day he came back to our dorm room filled with excitement over a class discussion. (The question was whether President Truman was right to have dropped the atom bomb on Hiroshima.) Another friend at the time was taking a literature course, and he mentioned to me that, during a class discussion, he had made a point the instructor himself had found striking.

Meanwhile, I was busy with Ampère’s law. We never had any fascinating class discussions about this law. No one, teacher or student, ever asked me what I thought about it.

We professors have a tendency to think that independent, creative thinking cannot be done by non-science students, and that only advanced science majors have learned enough of the material to think critically about it. I believe this attitude is false. This book is designed to move beyond a “shut up and learn” format, and to challenge students to think for themselves – even at the beginning level. It asks students to use their native intelligence to actually confront subtle scientific issues.

In the previous chapter we studied the inner Solar System, whose planets are relatively small and dense, and are composed of rock. Now, proceeding farther outward away from the Sun, we skip over the asteroid belt to reach the outer Solar System. This is the domain of the mighty Jupiter, Saturn, Uranus and Neptune; and then tiny Pluto. (As we have mentioned, it was recently decided that Pluto is not really a planet: we will defer this question to Chapter 9, and for now we will simply call it not a planet but a “planet.”) The giant planets are immensely larger than our own, and are composed almost entirely of gas. They are utterly unlike the Earth: indeed one of them, Jupiter, can be thought of as almost a star. They are graced with lovely rings.

Like stars, these outer planets have miniature “solar systems” of their own: great numbers of moons revolving about them, just as planets revolve about the Sun. Before the advent of the space age, Earth-bound telescopes were unable to resolve these satellites, and we knew them as little more than tiny points of light. But recent space missions have revealed these satellites as being worlds in their own right, utterly unlike our own Moon – and indeed utterly unlike anything we have seen before.

The Solar System

The Solar System is our first step outward into the cosmos. It consists of the Sun and the family of bodies orbiting about it. Largest among this family are the planets. Until recently there were nine in all, but in 2006 Pluto was redefined to be no longer a planet. So now there are eight. Mercury is the innermost, and Neptune the outermost planet: our Earth is the third.

Solar System

• Sun

• Mercury

• Venus

• Earth

• Mars

• (Asteroid Belt)

• Jupiter

• Saturn

• Uranus

• Neptune

• (Pluto)

Orbiting about the planets are the moons. We have one, some planets have none – Mercury and Venus, for example – while others have many – Saturn had more than 60 at last count.

Lying between the orbits of Mars and Jupiter are the asteroids: small, rocky bodies ranging in size from tiny bits of rubble to that of a small moon. Orbiting through the Solar System in highly inclined, highly elliptical orbits are the comets, “dirty snowballs” that vaporize when near the Sun, the vapor streaming away in graceful tails. Also orbiting through the Solar System are the meteoroids, small bodies that occasionally slam into us producing fiery trails in the sky. Some are sufficiently large to survive their passage through the atmosphere and fall to Earth, there to dig craters. When we find them strewn about the ground we have been granted free samples of the cosmos.

So far in this book we have been concerned with individual things – with moons and planets, with stars and nebulae and galaxies. Now we move on to study, not Things, but Everything. What is the nature of the Universe as a whole?

Our answers to this question have continually evolved. When we were babies “the world” consisted of little more than our homes and families. As we grew older, our worlds expanded to include other families, school and friends, our home town. This kind of expansion is also true historically. Primitive peoples regarded their immediate vicinity to be “the world.” Early societies, such as the ancient Greeks, drew larger maps, and as we see in Figure 18.1 their maps grew ever more comprehensive. With the scientific revolution (Figure 18.2) the view expanded immensely. But even here the view was strictly limited: in Figure 18.2 the entire Universe beyond the orbit of Saturn is represented merely as a single sphere of “fixed stars.”

This book has followed the same path. We began with a study of the relatively nearby – the Solar System – and progressively moved to more and more distant objects. Perhaps we are ready for yet another giant expansion. It is the greatest of them all – out into what may well be infinite.

Olbers’ Paradox: why is it dark at night?

Perhaps the most profound question that we can ask about the Universe is whether it is infinite. Does the cosmos extend endlessly far into the depths? Or does it have an edge?

There is a fascinating paradox related to this question. If the Universe is infinite it would contain an infinite number of stars, and taken together they would emit an infinite amount of light. So why don’t we receive an infinite amount of light? Why is it dark at night?

So far in this book we have surveyed what might be called our corner of the Universe: a region of space extending outward several thousand light years. An important result of that survey was that there was no overall pattern to the distribution of stars in space. Stars were found to scatter essentially randomly about us.

In this chapter we extend our survey much farther – out to roughly one hundred thousand light years' distance. In doing so we will reach a dramatically different conclusion. In a survey over such gigantic distances, a pattern emerges. We will find that everything we have so far studied – the Earth, the Sun and all the Solar System; the stars visible to the naked eye and the far more distant stars that telescopes reveal – all these are part of an enormous structure: the Milky Way Galaxy.

The discovery of our Galaxy is one of the triumphs of twentieth-century astronomy. But why was the Galaxy so difficult to discover? After all, we live within it! One answer is that much of the Galaxy is hidden from our view by interstellar clouds. But another reason is simply that it is so big. The Milky Way Galaxy is too big to see. Until recently we had been something like a race of intelligent ants, crawling across the face of a mountain but completely unaware of its existence.

Constituents of the Galaxy

Before recounting the story of this discovery it is worthwhile to pause a moment, and review the state of our knowledge at the time of the discovery of the Milky Way Galaxy. Of what did the astronomical Universe, as understood at the time, consist?

So far in this book we have encountered:

• planets, moons, etc.;

• stars;

• clusters of stars – two types are known: so-called “open clusters” such as the Pleiades (Figure III.2) and “globular clusters” (Figure III.3);

• interstellar clouds (e.g. Figure 13.1);

• remnants of stellar explosions: planetary nebulae (Figure 15.5) and supernova remnants (Figure 15.21).

Introducing steps to astronomy

Astronomy was the first science. Indeed, it is older than science. Thousands of years before the scientific revolution, thousands of years before telescopes and modern chemistry, geology and physics, people gazed at the sky and realized there was a lot going on up there to think about.

We begin our study of astronomy by considering what you can see with your naked eye. The daily passage of the Sun across the sky, the phases of the Moon, eclipses and the migration of the Sun across the constellations – all these regularities cry aloud for explanation, and they hint of a great cosmic structure. Early ideas of this structure – we now call it the Solar System – were formulated by ancient peoples, and they persisted for millennia.

Eventually these ideas were overthrown in the scientific revolution. We will trace briefly the course of this revolution, but in doing so our concern is not really historical. Our actual concern is to illuminate the nature of science through a study of its origins. Science is a way of thinking, a way of looking at the world, that was unique in the history of thought. Nothing more vividly illustrates the remarkable nature of science than a study of how it differs from what came before.

With the work of Isaac Newton the scientific revolution reached its climax. In his magisterial Mathematical Principles of Natural Philosophy this extraordinary genius set forth principles that govern the workings of the cosmos. We will devote an entire chapter to Newton’s laws of motion and of gravitation, the single most important force operating in the astronomical universe.

Before turning to a study of the structure of the Sun and other stars, we discuss how they were formed. As we will see, the processes that form stars also form planets orbiting about them. So we are really discussing the origin of the Solar System as a whole.

Before the advent of modern science, such a study was really the province of myths and religion: ancient peoples made up stories about the creation of the world. It is important to emphasize that these stories had no basis in the kind of observation and deduction that forms the essential basis of scientific reasoning. Not so very long ago, however, the situation changed.

It changed for a variety of reasons. In the first place, the studies we have described of the structure of the Solar System showed that our Earth is but one among a whole panoply of moons, asteroids and planets – and that the configuration of this vast system gives us clues as to its origin. In the second place, the development of modern physics gives us the tools to reach an understanding of this origin. And in the third place, recently we have been able to actually observe other stars and planetary systems in the midst of their formation. These observations have added new and totally unexpected elements to our understanding.

Most importantly of all, though, is that our thinking about the origin of the world has totally changed. No longer does this origin lie in the realm of the magical or the supernatural. Now it is an empirical matter, one that we can study with the tools of science, and one that we are capable of understanding.