Of all the scientific insights attained to date, one stands out boldly: Earth is neither central nor special. We inhabit no unique place in the universe. Astronomical research, especially within the past few decades, strongly suggests that we live on what seems to be an ordinary rocky planet called Earth, which is one of nine known planets orbiting an average star called the Sun, which is one star near the edge of a huge collection of stars called the Milky Way Galaxy, which is one galaxy among countless billions of others spread throughout the observable universe. To begin to get a feel for the relationships among these very different objects, consult Figures 1.1 through 1.4; put them in perspective by studying Figure 1.5.

Figure 1.1 Earth is a planet, a mostly solid object, though it has some liquid in its oceans and its core, and gas in its atmosphere. (In this view, you can clearly see the North and South American continents.)

Figure 1.2 The Sun is a star, a very hot ball of gas. Much bigger than Earth, the Sun is held together by its own gravity.

Figure 1.3 A typical galaxy is a collection of a hundred billion stars, each separated by vast regions of nearly empty space. This galaxy is the 83d entry in the catalog compiled by the eighteenth-century French astronomer Charles Messier—M83 for short. Our Sun is a rather undistinguished star near the edge of another such galaxy, called the Milky Way.

Figure 1.4 This photograph shows a typical cluster of galaxies, roughly a billion light years from Earth. Each galaxy contains hundreds of billions of stars, probably planets, and possibly living creatures.

We are connected to the most distant realms of space and time not only by our imaginations, but also through a common cosmic heritage: most of the chemical elements in our bodies were created billions of years ago in the hot centers of long-vanished stars. Their fuel supply spent, these giant stars died in huge explosions, scattering afar the elements created deep within their cores. Eventually, this matter collected into clouds of gas that slowly collapsed to give birth to a new generation of stars. In this way, the Sun and its family of planets formed nearly 5 billion years ago. Everything on Earth embodies atoms from other parts of the universe and from a past far more remote than the beginning of human evolution. Elsewhere, other beings, perhaps with intelligence much greater than our own, may at this very moment be gazing in wonder at their own night sky. Our own Sun may be nothing more than an insignificant point of light to them, if it is visible at all. Yet if such beings exist, then they must share our cosmic origin.

Simply put, the universe is the totality of all space, time, matter, and energy. Astronomy is the study of the universe. It is a subject unlike any other, for it requires us to change profoundly our view of the cosmos and to consider matter on scales totally unfamiliar from everyday experience. Look again at the galaxy in Figure 1.3. This galaxy, whose catalog name is M83, is a swarm of about a hundred billion stars—more stars than the number of people who have ever lived on Earth. The entire assemblage is spread across a vast expanse of space some 100,000 light years in diameter. A light year is the distance traveled by light, at a speed of about 300,000 kilometers per second, in a year. It equals about 10 trillion kilometers (or around 6 trillion miles). Typical galactic systems are truly "astronomical" in size! The light year is a convenient unit introduced by astronomers to help them describe immense distances. We will encounter many such "custom" units in our studies (see Appendix 2).

A thousand (1000), a million (1,000,000), a billion (1,000,000,000), and even a trillion (1,000,000,000,000)—these words occur regularly in everyday speech. But let's take a moment to understand the magnitude of these numbers and to appreciate the differences among them. One thousand is easy enough to understand; at the rate of one number per second, you could count to a thousand in about 16 minutes. However, if you wanted to count to a million, you would need more than 2 weeks of counting at the rate of one number per second, 16 hours per day (allowing 8 hours per day for sleep). To count from 1 to a billion at the same rate of one number per second and 16 hours per day would take nearly 50 years—the better part of an entire human lifetime.

In this text we consider spatial domains spanning not just billions of kilometers but billions of light years, objects containing not just trillions of atoms but trillions of stars, time intervals of not just billions of seconds or hours but billions of years. You will need to become familiar with—and comfortable with—such enormous numbers. A good way to begin is to try and recognize just how much larger than a thousand is a million, and how much larger still is a billion. (Appendix 1 explains the convenient method used by scientists for writing and manipulating very large and very small numbers.)