One of the thrills and challenges of studying astronomy is becoming familiar and comfortable with the vast range of sizes that occurs in it. In our everyday lives we typically deal with distances ranging from millimeters to thousands of kilometers. (The metric or International System (SI) of units is standard in science and will be used throughout this book; however, we will often provide the equivalent in U.S. customary units. Appendix K: Common Conversions lists conversion factors between these two sets of units.)

A hundredth of a meter or a thousand kilometers are numbers that are easy to visualize and write. In astronomy, we deal with particles as small as a millionth of a billionth of a meter and systems of stars as large as a thousand billion billion kilometers across. Similarly, the speeds of some things, like light, are so large as to be cumbersome if you have to write them out in words each time. Scientific notation (Appendix A: Powers-of-Ten Notation) makes comparisons easy, telling us how many factors of 10 in size, mass, brightness, distance, and other parameters one object is compared to another.

The size of the observable universe and the range of sizes of the objects in it are truly staggering. Figure 1-29 summarizes the array of sizes from atomic particles up to the diameter of the entire universe visible to us. Unlike linear intervals measured on a ruler, moving up 0.5 × 10⁻² (0.5) cm along the arc of this figure brings you to objects 10 times larger. Because of this, going from the size of a proton (roughly) up to the size of an atom (roughly) takes about the same space along the arc as going from the distance between Earth and the Sun to the distance between Earth and the nearby stars.

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This wide range of sizes underscores the fact that astronomy synthesizes or brings together information from many other fields of science. While we cannot go to the ends of the universe to examine all its components, the light from the universe coming to us, combined with our understanding of the laws of nature, provides invaluable insights into how the various components of the cosmos work and how they interact with each other. We will discuss some of the underlying principles of science as we need them.

What, then, have astronomers seen of the universe? Figure 1-30 presents examples of the types of objects we will explore in this text. An increasing number of planets like Jupiter, rich in hydrogen and helium (Figure 1-30a), as well as rocky planets similar to Earth, are being discovered orbiting other stars. Much smaller pieces of space debris—some of rock and metal called asteroids or meteoroids (Figure 1-30b), and others of rock and ice called comets (Figure 1-30c)—orbit the Sun (Figure 1-30d) and other stars. Vast stores of interstellar gas and dust are found in many galaxies; these “clouds” are often the incubators of new generations of stars (Figure 1-30e). Stars by the millions, billions, or even trillions are held together in galaxies by the force of gravity (Figure 1-30f). Most galaxies contain black holes, objects with such strong gravitational attraction that nothing can escape from them (Figure 1-30g). Groups of galaxies, called clusters, are held together by gravity (Figure 1-30h), and clusters of galaxies are grouped together in superclusters. Huge quantities of intergalactic gas are often found between galaxies (Figure 1-30i).

Every object in astronomy is constantly changing—each has an origin, an active period you might consider as its “life,” and each will have an end. In addition to examining the objects that fill the universe, we will also study the processes that cause them to change. After all is said and done, you will discover that all the matter and energy that astronomers have detected are but the tip of the cosmic iceberg—there is much more in the universe, but astronomers do not yet know its nature.

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