A Modern View of 1 the Universe LEARNING GOALS 1.1 The Scale of the Universe 1.3 Spaceship Earth ■ What is our place in the universe? ■ How is Earth moving through space? ■ How big is the universe? ■ How do galaxies move within the universe? 1.2 The History of the Universe 1.4 The Human Adventure of Astronomy ■ How did we come to be? ■ How has the study of astronomy affected human ■ How do our lifetimes compare to the age of the history? universe? ▲ About the photo: This Hubble Space Telescope photo shows thousands of galaxies in a region of the sky so small you could cover it with a grain of sand at arm’s length. 1 M01_BENN9068_08_SE_C01_001-023.indd 1 03/11/15 1:11 PM It suddenly struck me that that tiny pea, pretty Our Cosmic Address The galaxies that we see in the Hub- and blue, was the Earth. I put up my thumb and ble Space Telescope photo are just one of several key levels Our Cosmic Address of structure in our universe, all illustrated as our “cosmic shut one eye, and my thumb blotted out the planet address” in FIGURE 1.1. Earth. I didn’t feel like a giant. I felt very, very small. Earth is a planet in our solar system, which consists of —Neil Armstrong on looking back at the Earth from the Sun, the planets and their moons, and countless smaller Universe the Moon, July 1969 objects that include rocky asteroids and icy comets. Keep in mind that our Sun is a star, just like the stars we see in our 21 approx. size: 10 km ≈ 100 million ly night sky. Our solar system belongs to the huge, disk-shaped ar from city lights on a clear night, you can gaze upward Local Supercluster collection of stars called the Milky Way Galaxy. A galaxy 19 at a sky filled with stars. Lie back and watch for a few approx. size: 3 x 10 km ≈ 3 million ly F is a great island of stars in space, all held together by grav- hours, and you will observe the stars marching steadily across ity and orbiting a common center. The Milky Way is a rela- the sky. Confronted by the seemingly infinite heavens, you might Local Group tively large galaxy, containing more than 100 billion stars, wonder how Earth and the universe came to be. If you do, you will be sharing an experience common to humans around the world and many of these stars are orbited by planets. Our solar and in thousands of generations past. system is located a little over halfway from the galactic Modern science offers answers to many of our fundamental center to the edge of the galactic disk. questions about the universe and our place within it. We now know Billions of other galaxies are scattered throughout space. approx. size: the basic content and scale of the universe. We know the ages of Some galaxies are fairly isolated, but most are found in 1018 km ≈ 100,000 ly Earth and the universe. And, although much remains to be discov- groups. Our Milky Way, for example, is one of the two larg- ered, we are rapidly learning how the simple ingredients of the early est among more than 70 galaxies, most relatively small, universe developed into the incredible diversity of life on Earth. that make up the Local Group. Groups of galaxies with Milky Way Galaxy In this first chapter, we will survey the scale, history, and many more large members are often called galaxy clusters. motion of the universe. This “big picture” perspective on our On a very large scale, galaxies and galaxy clusters appear universe will provide a base on which you’ll be able to build a to be arranged in giant chains and sheets with huge voids deeper understanding in the rest of the book. between them; the background of Figure 1.1 represents this large-scale structure. The regions in which galax- ies and galaxy clusters are most tightly packed are called 1.1 The Scale of the Universe superclusters, which are essentially clusters of galaxy clusters. Our Local Group is located in the outskirts of the For most of human history, our ancestors imagined Earth Local Supercluster (which was recently named Laniakea, to be stationary at the center of a relatively small universe. Hawaiian for “immense heaven”). These ideas made sense at a time when understanding was Together, all these structures make up our universe. built upon everyday experience. After all, we cannot feel the In other words, the universe is the sum total of all matter constant motion of Earth as it rotates on its axis and orbits the and energy, encompassing the superclusters and voids and Solar System Sun, and if you observe the sky you’ll see that the Sun, Moon, everything within them. (not to scale) planets, and stars all appear to revolve around us each day. Nevertheless, we now know that Earth is a planet orbiting a Think about it Some people think that our tiny rather average star in a rather typical galaxy in a vast universe. physical size in the vast universe makes us insignificant. Earth The historical path to this knowledge was long and Others think that our ability to learn about the wonders of complex. In later chapters, we’ll see that the ancient belief the universe gives us significance despite our small size. in an Earth-centered (or geocentric) universe changed only What do you think? when people were confronted by strong evidence to the contrary, and we’ll explore how the method of learning that we call science enabled us to acquire this evidence. Astronomical Distance Measurements The labels in First, however, it’s useful to have a general picture of the Figure 1.1 give an approximate size for each structure in universe as we know it today. kilometers (recall that 1 kilometer ≈ 0.6 mile), but many distances in astronomy are so large that kilometers are not the What is our place in the universe? most convenient unit. Instead, we often use two other units: Take a look at the remarkable photo that opens this chap- ■ One astronomical unit (AU) is Earth’s average distance approx. size: 1010 km ≈ 60 AU ter (on page 1). This photo, taken by the Hubble Space from the Sun, which is about 150 million kilometers (93 Telescope, shows a piece of the sky so small that you could million miles). We commonly describe distances within block your view of it with a grain of sand held at arm’s our solar system in AU. length. Yet it encompasses an almost unimaginable expanse ■ One light-year (ly) is the distance that light can travel of both space and time. Nearly every object within it is a in 1 year, which is about 10 trillion kilometers (6 tril- galaxy filled with billions of stars, and some of the smaller lion miles). We generally use light-years to describe the approx. size: 104 km smudges are galaxies so far away that their light has taken distances of stars and galaxies. billions of years to reach us. Let’s begin our study of astron- omy by exploring what a photo like this one tells us about Be sure to note that a light-year is a unit of distance, our own place in the universe. not of time. Light travels at the speed of light, which is 2 PART I DEVELOPING PERSPECTIVE M01_BENN9068_08_SE_C01_001-023.indd 2 03/11/15 1:11 PM Our Cosmic Address FIGURE 1.1 Our cosmic address. These diagrams show key levels of structure in our universe. For a more detailed view, see the “You Are Universe Here in Space” foldout diagram in the front of the book. approx. size: 1021 km ≈ 100 million ly Local Supercluster approx. size: 3 x 1019 km ≈ 3 million ly Local Group approx. size: 1018 km ≈ 100,000 ly Milky Way Galaxy Solar System (not to scale) Earth approx. size: 1010 km ≈ 60 AU approx. size: 104 km M01_BENN9068_08_SE_C01_001-023.indd 3 03/11/15 1:11 PM 300,000 kilometers per second. We therefore say that Orion Nebula since that time, we cannot yet know about them one light-second is about 300,000 kilometers, because because the light from these events has not yet reached us. that is the distance light travels in one second. Similarly, The general idea that light takes time to travel through one light-minute is the distance that light travels in one space leads to a remarkable fact: minute, one light-hour is the distance that light travels in The farther away we look in distance, the further one hour, and so on. Mathematical Insight 1.1 (page 6) back we look in time. shows that light travels about 10 trillion kilometers in one year, so that distance represents a light-year. The Andromeda Galaxy (FIGURE 1.3) is about 2.5 million light-years away, which means we see it as it looked about Looking Back in Time The speed of light is extremely fast 2.5 million years ago. We see more distant galaxies as they by earthly standards. It is so fast that if you could make were even further in the past. Some of the galaxies in the light go in circles, it could circle Earth nearly eight times Hubble Space Telescope photo that opens the chapter are in a single second. Nevertheless, even light takes time to more than 12 billion light-years away, meaning we see travel the vast distances in space.