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Physics and Astronomy Department

Pre-lab 6: Sizes and Scales in the Universe

Prologue: Charting the Heavens

Of all the scientific insights achieved to date, one stands out boldly: Earth is neither central nor special. We inhabit no unique place in the universe. We live on an ordinary rocky planet called Earth, one of nine known planets orbiting an average star called the Sun, a middle-aged star near the edge of a huge collection of stars called the Milky Way galaxy, one galaxy among countless billions of others spread throughout the observable universe.

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. Consider, for example, the galaxy shown above. It 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 across. What is a light-year? It is the distance traveled by light, moving at a speed of about 300,000 kilometers per second, in a year. One light-year equals about 10 trillion kilometers (around 6 trillion miles)—astronomical systems are truly “astronomical” in size!

A thousand (1000, or 103 in scientific notation), a million 1,000,000 = 106 a billion 1,000,000,000 = 109 a trillion 1,000,000,000,000 = 1012—let’s take a moment to understand the magnitude of these numbers. One thousand is easy enough to understand: At the rate of one number per second, you could count to a thousand in a little over 16 minutes. However, if you wanted to count to a million, you would need more than two weeks of counting at the rate of one number per second, 16 hours per day (allowing eight hours per day for sleep). To count from one to a billion at the same rate of one number per second for 16 hours per day would take nearly 50 years. 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 start is to recognize just how much larger than a thousand is a million and how much larger still is a billion.

The universe is a very big place—so vast, in fact, that most people have difficulty comprehending the distances and true sizes of the things we see, from stars to galaxies to groups of galaxies. Lacking any understanding of the astronomical objects they observed, early skywatchers made up elaborate stories to explain them. They told of the Sun being pulled across the sky by a chariot drawn by winged horses, and of patterns of stars tracing heroes and animals placed in the sky by the gods. These stories featured familiar Earthly creatures—bears, hunters, swans—made larger to fit the sky.

Today we realize that the stars we see in the night sky are each hundreds of times larger than our entire planet, and the patterns they form span tens or even hundreds of light years. These facts radically change our conception of the heavens. How have we reached this level of understanding? In Part I of this book we present the basic methods used by astronomers to chart the space around us. We will describe the slow progress of scientific knowledge, from stories of chariots and gods to today’s well-tested theories of planetary motion and the laws of physics, and discuss why we now rely on science rather than myth to help us explain the universe.

Nor will our studies be confined to astronomical scales. We will also delve into the microscopic realm of atoms and molecules, whose properties hold the key to understanding the cosmos on macroscopic scales. The figures on this page illustrate the range of scales we will encounter in Part I, from atoms to Earth. In later parts of this book we will talk about much larger objects, lying at incredible distances from us. Our ability to see them, study them, and fathom their behavior is one of the great achievements of modern science.

For more on the notation used by astronomers to represent very large and very small numbers, see Appendix 1. For more on units, see Appendix 2. For more on astronomy and the study of the universe, read on!

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Department of Physics & Astronomy
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