The Milky Way (Early Bird Astronomy)

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by gregory l. vogt

d LERner Publications Company

•

Minneapolis

The images in this book are used with the permission of: © Paschalis Bartzoudis/Dreamstime .com, p. 4; NASA/JPL Caltech/S. Stolovy (Spitzer Science Center/Caltech), p. 5; NASA/JPL, p. 6; ©Â€Photononstop/SuperStock, p. 7; © Seth Goldfarb/Photonica/Getty Images, p. 8; ©Â€Gerard Lodriguss/Photo Researchers, Inc., p. 9; © Astrofoto/Peter Arnold, Inc., p. 10; ©Â€Laura Westlund/ Independent Picture Service, pp. 11, 13; © Radius Images/Photolibrary, p. 12; ©Â€Stuart O’Sullivan/ Stone/Getty Images, p. 14; NASA, pp. 15, 18; NASA/JSC, p. 16; © SPL/Photo Researchers, Inc., p. 17; NASA/JPL-Caltech, pp. 19, 25, 38; NASA/GSFC, p. 20; NASA/JPL-Caltech/R. Hurt (SSC), p. 21; © blickwinkel/Alamy, p. 22; ESA/NASA, p. 23; © Galaxy Picture Library/Alamy, p. 24; NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst), p. 26; NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team, p. 27; © Kevin Kelley/The Image Bank/Getty Images, p. 28; NASA/CXC/MIT/F.K. Baganoff et al., p. 29; © Stockbyte/Getty Images, p.€30; © Babak Tafreshi/Photo Researchers, Inc., p. 31; NASA, ESA, and AURA/Caltech, p. 32; © Detlev van Ravenswaay/Photo Researchers, Inc., p. 33; © Jon Lomberg/Photo Researchers, Inc., p. 34; NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI, p. 35; NASA and the Hubble Heritage Team (STScl/AURA), p. 36; ESA (image by A. Fujii), p. 37; NASA, ESA and The Hubble Heritage Team (STScl/AURA), p. 39; ESA (image by R. Gendler), p. 40; NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration, p. 41; NASA and F. Summers (Space Telescope Science Institute), C. Mihos (Case Western Reserve University), L. Hernquist (Harvard University), p. 42; The Hubble Heritage Team (AURA/STScI/NASA), p. 43; © Joe Tucciarone/ Photo Researchers, Inc., p. 46; NASA, ESA, N. Smith (UC Berkley) and The Hubble Heritage Team (STScI/AURA), p. 47; NASA/JPL/California Institute of Technology, p. 48 (top); © Mark Garlick/Photo Researchers, Inc., p.€48 (bottom). Front cover: © Michael Simpson/Taxi/Getty Images. Back cover: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Copyright © 2010 by Lerner Publishing Group, Inc. All rights reserved. International copyright secured. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means—electronic, mechanical, photocopying, recording, or otherwise—without the prior written permission of Lerner Publishing Group, Inc., except for the inclusion of brief quotations in an acknowledged review. Lerner Publications Company A division of Lerner Publishing Group, Inc. 241 First Avenue North Minneapolis, MN 55401 U.S.A. Website address: www.lernerbooks.com Library of Congress Cataloging-in-Publication Data Vogt, Gregory. The Milky Way / by Gregory L. Vogt. p. cm. — (Early bird astronomy) Includes index. ISBN 978–0–7613–3875–8 (lib. bdg. : alk. paper) 1. Milky Way—Juvenile literature. I. Title. QB857.7.V645 2010 Caption to come. Caption to2009020559 come. Caption 523.1'13—dc22 Manufactured in the United States of America 1 – BP – 12/15/09

eISBN: 978-0-7613-5931-9

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contents

Be a Word Detective . . . . . . . . . . . . . . . . . .å°“ . . . 5

Chapter 1

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the milky way . . . . . . . . . . . . . . . . . .å°“ . . . . to come. Caption to6 Chapter 2

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big and far. . . . . . . . . . . . . . . . . .å°“ . . . . . . 15 Chapter 3

stars and more. . . . . . . . . . . . . . . . . .å°“ . . 22 Chapter 4

moving through space . . . . . . . . . . . . . . 30 Chapter 5

we are not alone . . . . . . . . . . . . . . . . . .å°“ 38 A Note to Adults on Sharing a Book . . . . . . . . . . . . 44 Learn More about the Milky Way . . . . . . . . . . . . . . 45 Glossary . . . . . . . . . . . . . . . . . .å°“ . . . . . . . . . . 46 Index . . . . . . . . . . . . . . . . . .å°“ . . . . . . . . . . . . 48

Caption to come. Caption to come. Caption to come. Caption to come.

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Be a Word Detective

Can you find these words as you read about the Milky Way? Be a detective and try to figure out what they mean. You can turn to the glossary on page 46 for help. astronomers

dark matter

orbits

axis

galaxy

rotates

black hole

gravity

solar system

cluster

light-year

telescope

constellations

nebulas 5

Earth and the Moon are part of the solar system. What larger group does the solar system belong to?

Chapter 1

The Milky Way

We live on Earth. Earth is a small planet that orbits the Sun. This means that our planet circles around the Sun. Other planets and moons also orbit the Sun. Together, the Sun, planets, and moons make up our solar system. 6

Our Sun and the solar system are part of a larger family. It is called a galaxy. The galaxy is filled with billions of swirling stars.

On a clear night, you can see the stars in our galaxy. They appear as a white band across the sky. This photo has been changed to make the sky look brighter than it actually is.

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People discovered our galaxy thousands of years ago. At that time, the night sky was very dark. People could see thousands of stars sparkling in the sky. There were no city lights or pollution then. Light and pollution can make it difficult to see stars in the sky.

Bright lights from streets and buildings make it difficult to see light from stars.

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This photo shows the constellation known as Orion the Hunter. The lines and labels show the outline of the hunter.

People imagined that the stars formed groups. If people connected the stars in a group, they could see patterns. These patterns of stars are called constellations (cahn-stuh-LAYshunz). People named the constellations after gods, animals, or objects. 9

This band of light and gases is the Milky Way. The Milky Way has other names too. In China, it is called the Silver River. In Sweden, it is the Winter Street.

But people could see something bigger than constellations. A whitish band of light crossed the sky. It looked like smoke blowing away from a campfire. Many people thought the band of light looked like a river of milk. So they called it the Milky Way. 10

Every night, the stars in the Milky Way appear to cross the sky together. That is because Earth rotates (ROH-tayts). To rotate means “to spin.” Earth rotates around an imaginary line, called an axis (AK-suhs). As Earth rotates, objects in the sky seem to rise in the east and set in the west. But it is really Earth that is moving.

It takes 24 hours for Earth to rotate once. In this illustration, the Sun appears much closer to Earth than in real life.

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The best time to see the Milky Way is in the summer. You need to travel away from city lights and pollution. Take a star map with you. Look south for the constellation called Sagittarius. The center of the Milky Way is found there.

This photo of the Milky Way shows the stars in the constellation Sagittarius. Lines have been added to show the outline of the constellation.

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Earth orbits the Sun. Part of the year, Earth is on one side of the Sun. And part of the year, Earth is on the other side. So we see different stars at different times of the year.

You can also see the Milky Way in the winter. But it is much fainter then. You can see only the outer rim where there are fewer stars. 13

To see more stars, look at the Milky Way through binoculars or a telescope (TEH-luhskohp). These tools make far-off objects seem closer and larger. You still won’t see all the Milky Way’s stars. But you will see hundreds or thousands of them.

Kids take turns looking through a telescope to view the night sky.

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The Hubble Space Telescope gathers information about the Milky Way. Who studies this information?

Chapter 2

Big and Far

Astronomers (uh-STRAH-nuh-muhrz) are scientists who study objects in space. It is a tough job because the Milky Way is not just something in space. Our solar system is a part of the Milky Way. We are inside it. Imagine standing on a street corner in a strange city. Try to picture what the entire city looks like from that spot. Studying the Milky Way is like that.

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Both Earth and the Moon are in the Milky Way.

The Milky Way is about the biggest and farthest thing you will ever see with your eyes. How big and far takes a bit of explaining. On Earth, we measure distances in miles or kilometers. If you fly a rocket to the Moon, you will travel 238,000 miles (384,400 km). The Sun is 93,000,000 (93 million) miles (150 million km) away. 16

The next closest star from Earth is Alpha Centauri. It is about 24,000,000,000,000 (24 trillion) miles (39 trillion km) away. For an object in space, that’s still pretty close. The rest of the Milky Way is much farther still!

Alpha Centauri is part of a group of three stars. This image shows two of them.

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Astronomers use a different measurement for the distances of stars. It is called the lightyear. A light-year is the distance light travels in one year. In a year, light travels about 6 trillion miles (10 trillion km). So we can say that Alpha Centauri is 4 light-years away instead of 24 trillion miles (39 trillion km) away.

This giant cloud of gas and dust is part of the Milky Way. It is about 8,000 light-years from Earth.

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An artist created this image of the Milky Way. It shows how the Milky Way might look from far outside the galaxy.

Imagine flying 1 million light-years from Earth. From there, we could see the whole Milky Way at one time. It would look like a pinwheel. The center would be a round cluster of billions of stars. A wide, straight arm called a bar would run through the center. From the ends of the bar, there would be four or more big and small spiral arms. The bar and the arms are also made of stars. 19

This is what the Milky Way looks like from the side. A special camera in space took this image of the gases in its center.

The Milky Way is about 100,000 light-years across the spiral arms. It’s about 1,000 light-years thick. It appears thicker because a halo of gas surrounds it. The glowing halo is about 12,000 light-years thick. 20

Our Sun is in a spiral arm about 26,000 light-years from the center of the Milky Way. But you couldn’t see the Sun from way out in space. It would be lost in all the stars.

The Sun is labeled in this artist’s image of the Milky Way.

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The Milky Way stretches across the night sky. How many stars are in the Milky Way?

Chapter 3

Stars and More No one has ever counted all the stars in the

Milky Way. Most of its stars are hidden behind other stars. Astronomers have to estimate the number. Their best guess is that the Milky Way has 200 to 400 billion stars! 22 22

A star is a giant ball of hot gases. Gravity holds the gases together. Gravity is a force that attracts things to one another. In the center of a star, heat and light are made. The heat and light escape into space.

This is a photo of the Sun. The Sun is a star. Stars are giant balls of hot gases.

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The stars in the Milky Way are different sizes and colors. Our Sun is a small yellow star. There are many others like it in the Milky Way. The biggest stars are reddish. Medium-sized stars are bluish. Very white stars are the smallest. They are about the size of a small moon.

This image shows the many different colors of stars in the Milky Way.

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This photo shows what is left of the star Cassiopeia A. It exploded more than three hundred years ago. We can see the light and heat from the star as different colors.

The color of stars tells astronomers about their temperatures. Reddish stars are about half as warm as our Sun. Whitish and bluish stars are five to ten times hotter than the Sun. Some stars have used up all of their heat and light. They are much cooler and dark. 25

This image shows new stars being born in the center of the Milky Way.

Our Sun is in an arm of the Milky Way where stars are far apart. The stars are much closer together in the center of the galaxy. As many as 10 million stars fit into a space just 1 light-year across. If our solar system were in that part of the galaxy, we would never have night. The sky would glow with the light from millions of stars! 26

Stars are only one part of the Milky Way. There are also giant clouds of gas and dust. These clouds are called nebulas (NEH-byuhluhs). The gases in nebulas collect into stars. Nebulas glow from the light of the stars inside.

This nebula shows colorful gases and new stars forming.

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The Milky Way is also home to things astronomers can’t see. This is called dark matter. Dark matter is material that doesn’t glow. It’s dark and can’t be seen. It is hard to know what dark matter is and how much there is. But dark matter’s gravity helps hold the galaxy together. Astronomers believe that dark matter keeps the stars in the Milky Way from flying apart.

Dark matter can’t be seen. But it helps hold the Milky Way and this star cluster together.

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The Milky Way’s black hole is located at Sagittarius A. This image shows clouds of gas and light that have been pulled toward it.

Another large mass of dark material is in the center of the Milky Way. Astronomers call it a black hole. They think the black hole weighs more than 3 million suns. Black holes have superstrong gravity. Nearby stars have to orbit them very fast. If they don’t, the stars will be pulled into the black holes. The gravity of black holes is so powerful that even light is drawn into them. 29

This photograph of the Sun and Earth was taken from space. Does Earth stay in the same place in space?

Chapter 4

Moving Through Space You are probably sitting down as you read

about the Milky Way. You may feel as if you are resting. But you are actually moving very fast. That’s because the Milky Way is moving. 30 30

We travel with it through space at about 366 miles (589 km) every second. That’s more than 2,000 times faster than a jet plane. In a year, our galaxy moves about 11 billion miles (18 billion km).

We are moving with the Milky Way through space.

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These stars are part of a cluster called Pleiades. The stars in the cluster move together through space.

The stars within the Milky Way are moving too. But they move at different speeds and in different directions. It is kind of like people walking all around a ship that 32

is crossing the ocean. Our Sun is moving at about 12 miles (19 km) per second toward the constellation Lyra. The Sun’s gravity drags along the solar system. So we are moving too.

The planets and moons in our solar system orbit the Sun. As they orbit, the Sun and the solar system also move with the Milky Way through space.

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The whole galaxy rotates too. All the Milky Way’s stars orbit its center. Each star travels at its own speed. The Sun orbits at a speed of about 150 miles (240 km) per second. So we orbit the Milky Way every 220 million years.

As stars orbit the center of the galaxy, they also move with it across space.

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Many galaxies have a spiral shape like the Milky Way. This image shows the Pinwheel galaxy (M101).

Astronomers used to worry about the spiral arms of the Milky Way. They thought that the galaxy’s arms would wind up like a ribbon around a spool. As stars moved toward the center, the arms would become very thin and disappear. But that’s not happening. The Milky Way is keeping its spiral shape. 35

This photo shows one spiral galaxy lined up in front of another. The dark areas in the front galaxy are dark matter.

Astronomers think the spiral remains because of dark matter. Gravity from the dark matter pulls on stars from all directions. This helps the Milky Way keep its shape. 36

The Milky Way and its stars move in so many different ways at once. It is hard to know which way we are moving. But we know that we are getting there fast!

The stars in the Milky Way are moving in many directions at once.

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This is the Triangulum galaxy (M33). It and the Milky Way are part of a group of nearby galaxies. What is this group of galaxies called?

Chapter 5

We Are Not Alone

The Milky Way is part of a family. It is called the local group. It is made of 50 or more nearby galaxies. 38 38

Most galaxies in the local group are small. The smallest ones are called dwarfs. They are only about 500 light-years across. The Milky Way is about 200 times bigger than the dwarfs.

This is the Sagittarius dwarf galaxy. It is about 1,500 light-years across. It lies 3.4 million light-years away.

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The Andromeda galaxy is the biggest in the local group. Andromeda is 2.5 times larger than the Milky Way. This huge galaxy may be home to 1 trillion stars! It lies about 2.5 million light-years from the Milky Way.

Andromeda is a spiral galaxy like the Milky Way.

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These two spiral galaxies started mixing together millions of years ago. They are the closest colliding galaxies to our own.

The Andromeda galaxy is moving toward the Milky Way. It is moving 60 to 85 miles (85 to 137 km) per second. In 3 to 4 billion years, the two galaxies will collide. This may sound bad. But the stars in each galaxy are very far apart. So the stars will miss one another. 41

When Andromeda and the Milky Way collide, they will lose their spiral shapes. They will become one big, shapeless galaxy.

When these two galaxies collide, their stars will join. Our night sky will look very different. There will be many more stars to see. People will have to find new constellations. 42

Of course, we won’t be around for this event. But with big telescopes, we can see other galaxies colliding. There are galaxies in every direction in space. Many are like the Milky Way. By looking at them, we learn about our own galaxy.

NGC 4414 is a spiral galaxy. It is about 60 million light-years away. Astronomers can study it to learn more about the Milky Way and faraway galaxies.

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A NOTE TO ADULTS

On Sharing a Book When you share a book with a child, you show that reading is important. To get the most out of the experience, read in a comfortable, quiet place. Turn off the television and limit other distractions, such as telephone calls. Be prepared to start slowly. Take turns reading parts of this book. Stop occasionally and discuss what you’re reading. Talk about the photographs. If the child begins to lose interest, stop reading. When you pick up the book again, revisit the parts you have already read.

Be a Vocabulary Detective The word list on page 5 contains words that are important in understanding the topic of this book. Be word detectives and search for the words as you read the book together. Talk about what the words mean and how they are used in the sentence. Do any of these words have more than one meaning? You will find the words defined in a glossary on page 46.

What about Questions?

Use questions to make sure the child understands the information in this book. Here are some suggestions: What did this paragraph tell us? What does this picture show? How did the Milky Way get its name? What is the Milky Way made of? How many stars are in the Milky Way? How big is the Milky Way? What is your favorite part of the book? Why? If the child has questions, don’t hesitate to respond with questions of your own, such as What do you think? Why? What is it that you don’t know? If the child can’t remember certain facts, turn to the index.

Introducing the Index The index helps readers find information without searching through the whole book. Turn to the index on page 48. Choose an entry such as size and ask the child to use the index to find out how big the Milky Way is. Repeat with as many entries as you like. Ask the child to point out the differences between an index and a glossary. (The index helps readers find information, while the glossary tells readers what words mean.)

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learn more about

the MILKY milky WAY way SATURN BOOKS Asimov, Isaac, and Richard Hantula. The Milky Way and Other Galaxies. Strongsville, OH: Gareth Stevens Publishing, 2005. Learn how the Milky Way is similar and different to other galaxies. Elish, Dan. Galaxies. New York: Benchmark Books, 2007. Learn about different kinds of galaxies and how they formed. Jefferis, David. Galaxies: Immense Star Islands. New York: Crabtree Publishing Company, 2009. Take a quick tour of different galaxies and the objects found there. Vogt, Gregory L. Stars. Minneapolis: Lerner Publications Company, 2010. Find out more about stars, including the life and death of stars.

WEBSITES All about the Milky Way http://www.space.com/milkyway/ Visit this site for a collection of articles about the Milky Way. Ask an Astrophysicist: Milky Way and Other Galaxies http://imagine.gsfc.nasa.gov/docs/ask_astro/galaxies.html Read questions and answers about the Milky Way and other galaxies. Spitzer Space Telescope http://www.nasa.gov/mission_pages/spitzer/main/index.html This NASA site has stories on recent discoveries about the shape of the Milky Way. StarChild—A Learning Center for Young Astronomers http://starchild.gsfc.nasa.gov/docs/StarChild/StarChild.html Learn about the universe and all the things found in it on this NASA astronomy site.

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Glossary astronomers (uh-STRAH-nuh-muhrz): scientists who study planets, stars, and other things in space axis (AK-suhs): an imaginary line that goes through a planet from top to bottom. Earth spins around its axis. black hole: a mass of material so great and so compact that its gravity pulls in nearby objects and even light cluster: a close group of hundreds, thousands, or even millions of stars constellations (cahn-stuh-LAY-shunz): groups of stars in the sky that represent people, things, or animals dark matter: unknown dark material throughout the Milky Way. Its gravity helps hold the Milky Way together. galaxy: a group of billions of stars that swirl about one another and are held together by their combined gravity gravity: a force that causes all objects to be attracted to one another light-year: the distance light travels in one year (about 6 trillion miles, or 10 trillion km)

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nebulas (NEH-byuh-luhs): giant clouds of gas and dust in space that glow from the light of stars inside them orbits: travels in a circle around another object in space. Orbit can also mean the path of a star, planet, moon, or other object as it travels around a sun or planet. rotates (ROH-tayts): spins around an axis solar system: the collection of planets, moons, and other objects that orbit the Sun telescope (TEH-luh-skohp): an instrument with lenses or mirrors that collects light and makes far-off objects seem larger

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Index index

Pages listed in bold type refer to photographs.

arms of MIlky Way, 19, 20, 21, 35 center of Milky Way, 12, 19, 20, 26, 29, 34, 35 colors, 7, 10, 24, 25 constellations, 9, 12, 33, 42 dark matter, 28, 34 Earth, 6, 11, 13, 30 movement, 11, 30–35, 37, 41 names for Milky Way, 10 shape of Milky Way, 7, 10, 35–36, 42 size of Milky Way, 16, 20, 22, 39 solar system, 6–7, 15, 33 stargazing, 7, 8–10, 12–14 stars, 7, 11, 17, 18, 22–26, 27, 32–33, 34 study of Milky Way, 15, 22, 43 Sun, 6, 7, 11, 13, 16, 21, 23, 24, 25, 26, 33, 34 telescopes, 14, 15, 43 types of galaxies, 38–40, 43

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