1-1 WHERE ARE YOU? - you a preview of what that means. The planet yo...

C H A P T E R 1

HERE AND NOW

GUIDEPOST

As you study astronomy, you will learn about yourself. You are a planet-walker, and this chapter will give you a preview of what that means. The planet you live on whirls around a star that moves through a Universe filled with other stars and galaxies which are all results of billions of years of history and evolution. You owe it to yourself to know where you are in the Universe and also to know when you are in its history because those are important steps toward knowing what you are. In this chapter, you will consider three important

questions about astronomy:

►Where is Earth in the Universe?►How does human history fit into the history of the Universe?►Why study astronomy?

1-1 WHERE ARE YOU?

Where is Earth in the Universe?You surveyed the Universe by taking a cosmic zoom in which each field of view was 100 times wider than the previous field of view.Astronomers use the metric system because it simplifies calculations and scientific notation for very large or very small numbers.You live on a planet, Earth, which orbits our star, the Sun, once a year. As Earth rotates once a day, you see the Sun rise and set.The Moon is only one-fourth the diameter of Earth, but the Sun is 100 times larger in diameter than Earth—a typical size for a star.The Solar System includes the Sun at the center, all of the major planets that orbit around it— Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—plus the moons of the planets and other objects such as asteroids, comets, and dwarf planets like Pluto, bound to the Sun by its gravity.The astronomical unit (AU) is the average distance from Earth to the Sun. Mars, for example, orbits 1.5 AU from the Sun. The light-year (ly) is the distance light can travel in one year. The nearest star is 4.2 ly from the Sun.Astronomers have found more than a thousand extrasolar planets orbiting stars other than our Sun, even though such distant and small bodies are very difficult to detect. So far only a few extrasolar planets are known to be Earth-like in size and temperature.The Milky Way, the hazy band of light that encircles the sky, is the Milky Way Galaxy seen from inside. The Sun is just one out of the billions of stars that fill the Milky Way Galaxy.

ANSWERS TO REVIEW QUESTIONS

(Foundations of Astronomy, 13e Michael Seeds, Dana Backman) 1 / 4

Seeds’ FOUNDATIONS OF ASTRONOMY Instructors Manual 13e

Galaxies contain many billions of stars. The Milky Way Galaxy is about 80,000 ly in diameter and contains over 100 billion stars.Some galaxies, including our own, have graceful spiral arms, which are bright with stars. Many other galaxies are plain globes of stars without spiral arms, and a few galaxies have irregular shapes.Our galaxy is just one of billions of galaxies that fill the Universe in great clusters, clouds, filaments, and walls—the largest structures in the Universe.

1-2 WHEN IS NOW?

How does human history fit into the history of the Universe?Astronomers have evidence that the Universe began about 14 billion years ago in an event called the big bang, which filled the Universe with hot gas.The hot gas cooled, the first galaxies began to form, and stars began to shine about 400 million years after the big bang.The Sun and planets of our Solar System formed about 4 .6 billion years ago.Life began in Earth’s oceans soon after Earth formed but did not emerge onto land until 400 million years ago, less than 1 / 30 of the age of the Universe. Dinosaurs evolved relatively soon after that and went extinct just 65 million years ago.Humanlike creatures developed on Earth only about 4 million years ago, less than 1 / 3000 of the age of the Universe, and human civilizations developed just 10,000 years ago.

1-3 WHY STUDY ASTRONOMY ?

Why study astronomy?Although astronomy seems to be about stars and planets, it describes the Universe in which you live, so it is really about you. Astronomy helps you answer the question, “What are we?” As you study astronomy, you should ask, “How do we know?” and that will help you understand how science gives us a way to understand nature.In its simplest outline, science follows the scientific method, in which scientists test hypotheses against evidence from experiments and observations. This method is a powerful way to learn about nature.

CHAPTER OUTLINE

1-1 Where Are You?1-2 When Is Now?1-3 Why Study Astronomy?

How Do We Know? 1-1: The Scientific Method

What Are We? Participants

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Seeds’ FOUNDATIONS OF ASTRONOMY Instructors Manual 13e

KEY TERMS

astronomical unit (AU) dwarf planet extrasolar planets field of view galaxy light-year (ly) Milky Way Milky Way Galaxy planets scientific method scientific notation Solar System spiral arms star

RESOURCES AND TIPS

• Ask students to watch this cosmic zoom sequence, which is similar to the one in the chapter and a

second version of a famous earlier film.Powers of Ten ™ (1977) by Charles and Ray Eames: http://www.youtube.com/watch?v=0fKBhvDjuy0 Which power of ten is most surprising to them and why?

• A good explanation of the Milky Way can be found here: http://en.wikipedia.org/wiki/Milky_Way. This

may provide extra help when providing directions (Review Question #12) and give additional context to where we are in the Universe.

ANSWERS TO REVIEW QUESTIONS

• The width increases by a factor of 100 (from 16m across to 1.6 kilometers, i.e., 1,600m across), as does

the length in any direction.

• (Answers will vary.) For a distance traveled by one’s own power, most people will have covered a

distance of a few miles at some time and a few may well have covered more on an extended backpacking or bicycling trip. A smaller number may have driven across a continent or flown to the opposite side of the world.

• Our solar system contains the Sun and objects that orbit it, including the eight planets, comets, and

asteroids. Our galaxy contains approximately 100 billion stars, of which the Sun is one, as well as large clouds of gas and dust. The Universe contains all physical matter and energy. Its basic constituents are the clusters of galaxies.

• The Moon refers to the rocky body orbiting the Earth, whereas “moon” refers to any satellite of any

planet.

• Although Pluto does have some of the characteristics required to be defined as a planet (e.g., spherical,

non-luminous, orbits a star), its small size and the fact that it is not alone in its orbit mean that it is better defined as a dwarf planet.

• We use light years instead of astronomical units or kilometers to measure the distances to stars and

galaxies for the same reason that we measure the distances between cities in miles and not inches. When measuring distance between stars or galaxies, the distances are very large. The average distance between stars in the Milky Way galaxy is about 5 light-years. That’s a number relatively easy to remember. 3 / 4

Seeds’ FOUNDATIONS OF ASTRONOMY Instructors Manual 13e

Distances to stars and galaxies given in astronomical units or kilometers would require very large numbers (e.g., the distance to the nearest star is about 300,000 AU or 4.5  10 16 km). Additionally, light years are used when discussing the distances to galaxies because the distance in light years tells us how far back in time we are viewing the galaxy.

• Seeing planets around other star systems is very difficult for three reasons. First, they are small and so

will appear as small points from Earth. Second, planets are much fainter than the stars that they orbit.This is because planets are not perfect reflectors, so only a portion of the light from their Sun is reflected away. Third, planets are too close to their (larger, brighter) Sun, making the planet even more difficult to notice.

• On photographs, the diameter of a star is related to its apparent intensity or apparent brightness. Bright

stars make larger spots on a photograph than do faint stars. Therefore, the size of a star in an image tells you how bright it appears at Earth, not the actual size of the star.

• The Milky Way is a fairly narrow band of faint diffuse light around the celestial sphere. The Milky Way

Galaxy is a spiral galaxy of about 100 billion stars. All of the objects visible with the unaided eye in the night sky, except the Andromeda galaxy, Large Magellanic Cloud, and Small Magellanic Cloud, are part of the Milky Way Galaxy.

• When looking at the Milky Way in the night sky, are you seeing spiral arms of the Milky Way Galaxy?

How do you know? Because the Milky Way is a narrow band of light, it is likely that we are looking through the spiral arms of the Milky Way. In fact, the brightest portion of the Milky Way, located in the direction of Sagittarius, is the center of our galaxy, around which all the “arms” of the galaxy rotate.

• Filaments and walls (i.e., large strings of superclusters of galaxies) appear to be the largest structures in

the Universe.

• I would direct the person to first find the Laniakea supercluster of galaxies within the Universe. Within

the Laniakea supercluster, find the Virgo supercluster of galaxies. Within the Virgo supercluster, locate the Local Group of galaxies, of which one is the Milky Way galaxy. Within the Milky Way galaxy, our solar system is located ~ 27,000 light-years away from the center of the galaxy, on the inner edge of the Orion Arm. Within our solar system, we are the third planet away from the Sun.

• The age of the Solar System is 4.6 billion years, so a life span of 75 years is ~0.000002% of the age of

the solar system. Compared to the age of the Universe (14 billion), the percentage shrinks to ~0.0000005% of the age of the Universe.

• (Answers may vary.) Studying astronomy allows us to understand what we are and how we fit into the

history of the Universe. The atoms that make up our bodies were born in the big bang that created the Universe. Studying astronomy also allows us to understand more fully how we learn about the natural world through science, preparing one to critically examine statements about the world around us in the future. I may encounter astronomy in the future, whether it is considering funding a space program or through future discoveries made about our Universe.

• Astronomy helps answer the question “What are we?” in many ways. Astronomy tells us how particles

that make up the atoms in our bodies were created during the big bang, when the Universe began, and

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