our place in the universe -...

1Our Place

in the Universe

1

learning goals

1.1 Our Modern View of the Universe

� What is our place in the universe?

� How did we come to be?

� How can we know what the universe

was like in the past?

� Can we see the entire universe?

1.2 The Scale of the Universe

� How big is Earth compared to our

solar system?

� How far away are the stars?

� How big is the Milky Way Galaxy?

� How big is the universe?

� How do our lifetimes compare to the

age of the universe?

1.3 Spaceship Earth

� How is Earth moving in our solar

system?

� How is our solar system moving in

the Milky Way Galaxy?

� How do galaxies move within the

universe?

� Are we ever sitting still?

Far from city lights on a clear night, you can gaze upward at a sky

filled with stars. Lie back and watch for a few hours, and you will

observe the stars marching steadily across the sky. Confronted by the

seemingly infinite heavens, you might wonder how Earth and the uni-

verse came to be. If you do, you will be sharing an experience common

to humans around the world and in thousands of generations past.

Modern science offers answers to many of our fundamental ques-

tions about the universe and our place within it. We now know the

basic content and scale of the universe. We know the age of Earth and

the approximate age of the universe. And, although much remains to be

discovered, we are rapidly learning how the simple ingredients of the

early universe developed into the incredible diversity of life on Earth.

In this first chapter, we will survey the content and history of the

universe, the scale of the universe, and the motions of Earth. We’ll

develop a “big picture” perspective of our place in the universe that

will provide a base to build upon in the rest of the book.


If you observe the sky carefully, you can see why most of our ancestorsbelieved that the heavens revolved about a stationary Earth. The Sun,Moon, planets, and stars appear to circle around our sky each day, andwe cannot feel the constant motion of Earth as it rotates on its axis andorbits the Sun. It therefore seems quite natural to assume that we live inan Earth-centered, or geocentric, universe.

Nevertheless, we now know that Earth is a planet orbiting a ratheraverage star in a vast universe. The historical path to this knowledge waslong and complex. In later chapters, we’ll see that many ancient beliefsmade sense in their day and changed only when people were confrontedby strong evidence to the contrary. We’ll also see how the process of sci-ence enabled us to acquire this evidence and to learn that we are con-nected to the stars in ways our ancestors never imagined. First, however,it’s useful to have a general picture of the universe as we know it today.


Figure 1.1 illustrates our place in the universe with what we mightcall our “cosmic address.” Earth is a planet in our solar system,which consists of the Sun and all the objects that orbit it: the planetsand their moons, and countless smaller objects including rockyasteroids and icy comets.

Our Sun is a star, just like the stars we see in our night sky. The Sunand all the stars we can see with the naked eye make up only a small partof a huge, disk-shaped collection of stars called the Milky Way Galaxy. Agalaxy is a great island of stars in space, containing from a few hundredmillion to a trillion or more stars. The Milky Way Galaxy is relativelylarge, containing more than 100 billion stars. Our solar system is located alittle over halfway from the galactic center to the edge of the galactic disk.

1. How to Succeed in Your Astronomy Course [pp. xxii–xxiii]

2. Powers of 10 [Appendixes C.1, C.2]

3. Working with Units [Appendix C.3]

4. The Metric System (SI) [Appendix C.4]

essential preparation

2 Part 1 Developing Perspective

approx. size: 1021 km ≈ 100 million ly

approx. size: 3 x 1019 km ≈ 3 million ly

approx. size: 1018 km ≈ 100,000 ly

approx. size: 1010 km ≈ 60 AU

approx. size: 104 km

Local Supercluster

Local Group

Milky Way Galaxy

Earth

Solar System(not to scale)

Universe

Figure 1.1

Our cosmic address. These diagrams show key levels ofstructure in our universe; for a more detailed view, see the “You Are Here in Space” foldout diagram in the front of thebook.

The rate at which galaxies are movingapart suggests that the universe wasborn about 14 billion years ago, in theevent we call the Big Bang.

Some people think that our tiny physical size in the vastuniverse makes us insignificant. Others think that our ability

to learn about the wonders of the universe gives us significance despite our small size.Study the more detailed view of our cosmic address in the “You Are Here in Space”foldout diagram in the front of the book. What do you think?

Billions of other galaxies are scattered throughout space. Some gal-axies are fairly isolated, but many others are found in groups. Our MilkyWay, for example, is one of the two largest among about 40 galaxies inthe Local Group. Groups of galaxies with more than a few dozen mem-bers are often called galaxy clusters.

We live on one planet orbiting one staramong more than 100 billion stars in theMilky Way Galaxy, which in turn is one ofbillions of galaxies in the universe.

On a very large scale, observa-tions show that galaxies and galaxyclusters appear to be arranged ingiant chains and sheets with hugevoids between them. The regions in

which galaxies and galaxy clusters are most tightly packed are calledsuperclusters, which are essentially clusters of galaxy clusters. OurLocal Group is located in the outskirts of the Local Supercluster.

Together, all these structures make up our universe. In other words,the universe is the sum total of all matter and energy, encompassing thesuperclusters and voids and everything within them.


According to modern science, we humans are newcomers in an olduniverse. We’ll devote much of the rest of this textbook to studying thescientific evidence that backs up this idea. To help prepare you for thisstudy, let’s look at a quick overview of the scientific history of theuniverse, as summarized in Figure 1.2 (pages 6–7).

The Big Bang and the Expanding Universe Telescopic observa-tions of distant galaxies show that the entire universe is expanding, mean-ing that the average distances between galaxies are increasing with time.This fact implies that galaxies must have been closer together in the past,and if we go back far enough, we must reach the point at which theexpansion began. We call this beginning the Big Bang, and from theobserved rate of expansion we estimate that it occurred about 14 billionyears ago. The three cubes in the upper left corner of Figure 1.2 representthe expansion of a small piece of the universe over time.

The universe as a whole has con-tinued to expand ever since the BigBang, but on smaller scales the forceof gravity has drawn matter together.Structures such as galaxies and galaxy

clusters occupy regions where gravity has won out against the overallexpansion. That is, while the universe as a whole continues to expand,individual galaxies and galaxy clusters do not expand. This idea is alsoillustrated by the three cubes in Figure 1.2. Notice that as the region as awhole grew larger, the matter within it clumped into galaxies and galaxyclusters. Most galaxies, including our own Milky Way, probably formedwithin a few billion years after the Big Bang.

Stellar Lives and Galactic Recycling Within galaxies like theMilky Way, gravity drives the collapse of clouds of gas and dust to formstars and planets. Stars are not living organisms, but they nonetheless go


Chapter 1 Our Place in the Universe 5

through “life cycles.” A star is born when gravity compresses the materialin a cloud to the point where the center becomes dense and hot enoughto generate energy by nuclear fusion, the process in which lightweightatomic nuclei smash together and stick (or fuse) to make heavier nuclei.The star “lives” as long as it can generate energy from fusion and “dies”when it exhausts its usable fuel.

In its final death throes, a starblows much of its content back outinto space. In particular, massivestars die in titanic explosions calledsupernovae. The returned mattermixes with other matter floating be-

Stars are born in interstellar clouds,produce energy and new elementsthrough nuclear fusion, and release thosenew elements in interstellar space whenthey die.

tween the stars in the galaxy, eventually becoming part of new clouds ofgas and dust from which future generations of stars can be born. Galaxiestherefore function as cosmic recycling plants, recycling material expelledfrom dying stars into new generations of stars and planets. This cycle isillustrated in the lower right of Figure 1.2. Our own solar system is aproduct of many generations of such recycling.

basic astronomical objects, units, and motions

This box summarizes a few key astronomical definitions introduced inthis chapter and used throughout the book.

Basic Astronomical Objects

star A large, glowing ball of gas that generates heat and light throughnuclear fusion in its core. Our Sun is a star.

planet A moderately large object that orbits a star and shines pri-marily by reflecting light from its star. According to a definitionapproved in 2006, an object can be considered a planet only if it(1) orbits a star; (2) is large enough for its own gravity to makeit round; and (3) has cleared most other objects from its orbitalpath. An object that meets the first two criteria but has notcleared its orbital path, like Pluto, is designated a dwarf planet.

moon (or satellite) An object that orbits a planet. The term satellitecan refer to any object orbiting another object.

asteroid A relatively small and rocky object that orbits a star.

comet A relatively small and ice-rich object that orbits a star.

Collections of Astronomical Objects

solar system The Sun and all the material that orbits it, includingthe planets, dwarf planets, and small solar system bodies. Al-though the term solar system technically refers only to our ownstar system (solar means “of the Sun”), it is often applied to otherstar systems as well.

star system A star (sometimes more than one star) and any planetsand other materials that orbit it.

galaxy A great island of stars in space, containing from a few hundredmillion to a trillion or more stars, all held together by gravity andorbiting a common center.

cluster (or group) of galaxies A collection of galaxies boundtogether by gravity. Small collections (up to a few dozen galaxies)are generally called groups, while larger collections are calledclusters.

supercluster A gigantic region of space where many individual gal-axies and many groups and clusters of galaxies are packed moreclosely together than elsewhere in the universe.

universe (or cosmos) The sum total of all matter and energy—thatis, all galaxies and everything between them.

observable universe The portion of the entire universe that can beseen from Earth, at least in principle. The observable universe isprobably only a tiny portion of the entire universe.

Astronomical Distance Units

astronomical unit (AU) The average distance between Earth andthe Sun, which is about 150 million kilometers. More technically,1 AU is the length of the semimajor axis of Earth’s orbit.

light-year The distance that light can travel in 1 year, which is about9.46 trillion kilometers.

Terms Relating to Motion

rotation The spinning of an object around its axis. For example,Earth rotates once each day around its axis, which is an imaginaryline connecting the North Pole to the South Pole.

orbit (revolution) The orbital motion of one object around another.For example, Earth orbits around the Sun once each year.

expansion (of the universe) The increase in the average distancebetween galaxies as time progresses. Note that while the universeas a whole is expanding, individual galaxies and galaxy clusters donot expand.

Birth of the Universe: The expansion of the universe began with the hot and dense Big Bang. The cubes show how one region of the universe has expanded with time. The universe continues to expand, but on smaller scales gravity has pulled matter together to make galaxies.

Earth and Life: By the time our solar system was born, 41/2 billion years ago, about 2% of the original hydrogen and helium had been converted into heavier elements. We are therefore “star stuff,” because we and our planet are made from elements manufactured in stars that lived and died long ago.

Throughout this book we will see that human life is intimately connected with the development of the universe as a whole. This illustration presents an overview of our cosmic origins, showing some of the crucial steps that made our existence possible.

1

4

Figure 1.2. Our Cosmic OriginscosmicContext

Galaxies as Cosmic Recycling Plants: The early universe contained only two chemical elements: hydrogen and helium. All other elements were made by stars and recycled from one stellar generation to the next within galaxies like our Milky Way.

Life Cycles of Stars: Many generations of stars have lived and died in the Milky Way.

Stars are born in clouds of gas and dust; planets may form in surrounding disks.

Massive stars explode when they die, scattering the elements they’ve produced into space.

Stars shine with energy released by nuclear fusion, which ultimately manufactures all elements heavier than hydrogen and helium.

3

2


By the time our solar systemformed, about 4 billion years ago,earlier generations of stars had con-verted about 2% of our galaxy’s

12

Stars Manufacture the Elements of Earth and Life The recyclingof stellar material is connected to our existence in an even deeper way.By studying stars of different ages, we have learned that the early uni-verse contained only the simplest chemical elements: hydrogen andhelium (and a trace of lithium). We and Earth are made primarily ofother elements, such as carbon, nitrogen, oxygen, and iron. Where didthese other elements come from? Evidence shows that these elementswere manufactured by stars—some through the nuclear fusion thatmakes stars shine, and others through nuclear reactions accompanyingthe explosions that end stellar lives.

We are “star stuff”—made of materialthat was manufactured in stars from thesimple elements born in the Big Bang.

original hydrogen and helium into heavier elements. Therefore, thecloud that gave birth to our solar system was made of about 98% hydro-gen and helium and 2% other elements. That 2% may seem a smallamount, but it was more than enough to make the small rocky planetsof our solar system, including Earth. On Earth, some of these elementsbecame the raw ingredients of simple life forms, which ultimately blos-somed into the great diversity of life on Earth today.

In summary, most of the material from which we and our planet aremade was created inside stars that lived and died before the birth of ourSun. As astronomer Carl Sagan (1934–1996) said, we are “star stuff.”

� How can we know what the universe

was like in the past?

You may wonder how we can claim to know anything about what theuniverse was like in the distant past. It’s possible because we can actuallysee into the past by studying light from distant stars and galaxies.

Light travels extremely fast by earthly standards. The speed of lightis 300,000 kilometers per second, a speed at which it would be possibleto circle Earth nearly eight times in just 1 second. Nevertheless, evenlight takes time to travel the vast distances in space. For example, lighttakes about 1 second to reach Earth from the Moon, and about 8 min-utes to reach Earth from the Sun. Light from stars takes many years toreach us, so we measure distances to stars in units called light-years.One light-year is the distance that light can travel in 1 year—about 10trillion kilometers, or 6 trillion miles (see Cosmic Calculations 1.1). Notethat a light-year is a unit of distance, not time.

Because light takes time to travel through space, we are led to a re-markable fact: The farther away we look in distance, the furtherback we look in time. For example, the brightest star in the night sky,Sirius, is about 8 light-years away, which means its light takes about 8 years to reach us. When we look at Sirius, we are seeing it not as it istoday but as it was about 8 years ago.

commonMisconceptions

The Meaning of a Light-Year

You’ve probably heard people say things like “It will take me light-years to finish this homework!” But a statement like this one doesn’tmake sense, because light-years are a unit of distance, not time. If youare unsure whether the term light-year is being used correctly, try test-ing the statement by replacing “1 light-year” with its equivalent dis-tance of “10 trillion kilometers” or “6 trillion miles.” The statementthen reads, “It will take me 6 trillion miles to finish this homework!”which clearly does not make sense.

Light takes time to travel the vastdistances in space. When we look deepinto space, we also look far into the past.

The effect is more dramatic atgreater distances. The AndromedaGalaxy (also known as M31) liesabout 2.5 million light-years from

Earth. Figure 1.3 is therefore a picture of how this galaxy lookedabout 2.5 million years ago, when early humans were first walking onEarth. We see more distant galaxies as they were even further backinto the past.

cosmicCalculations 1.1

How Far Is a Light-Year?

One light-year (ly) is defined as the distance that light cantravel in 1 year. This distance is fixed because light alwaystravels at the same speed—the speed of light, which is300,000 km/s (186,000 mi/s).

We can calculate the distance represented by a light-year by recalling that

For example, if you travel at a speed of 50 km/hr for 2hours, you will travel 100 km. To find the distance repre-sented by 1 light-year, we simply multiply the speed oflight by 1 year:

Because we are given the speed of light in units ofkilometers per second but the time as 1 year, we must carryout the multiplication while converting 1 year into seconds.You can find a review of unit conversions in Appendix C;here, we show the result for this particular case:

That is, 1 light-year is equivalent to 9.46 trillion km, whichis easier to remember as almost 10 trillion km.

= 9,460,000,000,000 km

* 24hr

day* 60

min

hr* 60

s

minb

= a300,000km

sb * a1yr * 365

day

yr

1 light-year = a300,000km

sb * 11 yr2

1 light-year = 1speed of light2 * 11 yr2

distance = speed * time


It’s also amazing to realize that any “snapshot” of a distant galaxy isa picture of both space and time. For example, because the AndromedaGalaxy is about 100,000 light-years in diameter, the light we see fromthe far side of the galaxy must have left on its journey to us 100,000years before the light from the near side. Figure 1.3 therefore showsdifferent parts of the galaxy spread over a time period of 100,000 years.When we study the universe, it is impossible to separate space andtime.

The glow from the central region of the AndromedaGalaxy is faintly visible to the naked eye and easy to

see with binoculars. Use a star chart to find it in the night sky. Contemplate the factthat you are seeing light that spent 2.5 million years in space before reaching your eyes.If students on a planet in the Andromeda Galaxy were looking at the Milky Way, whatwould they see? Could they know that we exist here on Earth?


Recall that the observed expansion of the universe implies that it is about14 billion years old. This fact, combined with the fact that looking far intospace means looking far back in time, places a limit on the portion of theuniverse that we can see, even in principle.

Figure 1.4 shows the idea. If we look at a galaxy that is 7 billionlight-years away, we see it as it looked 7 billion years ago—whichmeans we see it as it was when the universe was half its current age.If we look at a galaxy that is 12 billion light-years away (like the mostdistant ones in the Hubble Space Telescope photo on page 1), we see itas it was 12 billion years ago, when the universe was only 2 billionyears old. And if we tried to look beyond 14 billion light-years, we’dbe looking back to a time more than 14 billion years ago—which is

before the universe existed, sothere is nothing to see. This dis-tance of 14 billion light-yearstherefore marks the boundary ofour observable universe—the

portion of the entire universe that we can potentially observe. Notethat this fact does not put any limit on the size of the entire universe,which may be far larger than our observable universe. We simplyhave no hope of seeing or studying anything beyond the bounds ofour observable universe.

Because the universe is about 14 billionyears old, we cannot observe lightcoming from anything more than 14 billion light-years away.

Cassiopeia

M31

AndromedaPegasus

Figure 1.3

The Andromeda Galaxy (M31). When we look at this galaxy, wesee light that has been traveling through space for 2.5 millionyears. The inset shows its location in the constellationAndromeda.

VIS

Far: We see a galaxy 7 billion light-years awayas it was 7 billion years ago–when the universewas half its current age of 14 billion years.

Farther: We see a galaxy 12 billion light-yearsaway as it was 12 billion years ago–when theuniverse was only about 2 billion years old.

The limit of our observable universe: Light from nearly 14 billion light-years awayshows the universe as it looked shortlyafter the Big Bang, before galaxies existed.

Beyond the observable universe:We cannot see anything fartherthan 14 billion light-years away,because light has not hadenough time to reach us.

7 billion light-years



Figure 1.4

The farther away we look in space, the further back we look intime. The age of the universe therefore puts a limit on the sizeof the observable universe—the portion of the entire universethat we could observe in principle.


Scale of the Universe Tutorial, Lessons 1–3

1.2 The Scale of the UniverseThe numbers in our description of the size and age of the universe havelittle meaning for most people—after all, they are literally astronomical—but understanding them is crucial in astronomy. In this section, we willtry to give meaning to astronomical distances and times.

� How big is Earth compared

to our solar system?

Illustrations and photo montages often make our solar system look like itis crowded with planets and moons, but the reality is far different. One ofthe best ways to develop perspective on cosmic sizes and distances is toimagine our solar system shrunk down to a scale that would allow you towalk through it. The Voyage scale model solar system in Washington,D.C., makes such a walk possible (Figure 1.5). The Voyage model showsthe Sun and the planets, and the distances between them, at one ten-billionth of their actual sizes and distances.

Figure 1.6a shows the Sun and planets at their correct sizes (but notdistances) on the Voyage scale: The model Sun is about the size of a largegrapefruit, Jupiter is about the size of a marble, and Earth is about thesize of a pinhead or the ball point in a pen. You can immediately seesome key facts about our solar system. For example, the Sun is far largerthan any of the planets; in mass, the Sun outweighs all the planets com-bined by a factor of more than 1000. The planets also vary considerablyin size: The storm on Jupiter known as the Great Red Spot (visible nearJupiter’s lower left in Figure 1.6a) could swallow up the entire Earth.

Figure 1.5

This photo shows the pedestals housing the Sun (the goldsphere on the nearest pedestal) and the inner planets in theVoyage scale model solar system (Washington, D.C.). Themodel planets are encased in the sidewalk-facing disks visibleat about eye level on the planet pedestals. The building at theleft is the National Air and Space Museum.

The scale of the solar systembecomes even more remarkablewhen you combine the sizesshown in Figure 1.6a with the dis-tances illustrated by the map of

On a scale in which the Sun is the size ofa grapefruit, Earth is the size of a ballpoint from a pen, orbiting the Sun at adistance of 15 meters.

the Voyage model in Figure 1.6b. For example, the ball point–sizedEarth is located about 15 meters (16.5 yards) from the grapefruit-sizedSun, which means you can picture Earth’s orbit by imagining a ballpoint taking a year to make a circle of radius 15 meters around agrapefruit.

Perhaps the most striking feature of our solar system when we viewit to scale is its emptiness. The Voyage model shows the planets along astraight path, so we’d need to draw each planet’s orbit around themodel Sun to show the full extent of our planetary system. Fitting allthese orbits would require an area measuring more than a kilometer ona side—an area equivalent to more than 300 football fields arranged ina grid. Spread over this large area, only the grapefruit-size Sun, theplanets, and a few moons would be big enough to notice with youreyes. The rest of it would look virtually empty (that’s why we callit space!).

Earth is the only place in our solar system—and the only placewe yet know of in the universe—with conditions suitable for

human life. How does visualizing Earth to scale affect your perspective on humanexistence? How does it affect your perspective on our planet? Explain.


Pluto

Neptune

Pluto

Neptune

Uranus

Saturn

Jupiter MarsEarth

Venus Mercury

Eris

Sun

a This painting shows the scaled sizes (but not distances) of the Sun, the planets, and the two largest known dwarf planets.

b This map shows the locations of the Sun and planets in the Voyage model; the distance from the Sun to Pluto is about 600 meters (1/3 mile). Planets are lined up in the model, but in reality each planet orbits the Sun independently and a perfect alignment never occurs.

Saturn Jupiter

Earth

MarsVenus

Mercury

SUN

toCapitolHill

UranusNeptunePluto

to WashingtonMonument

Art and IndustriesBuilding

HirshhornMuseum

National Air and Space Museum

7thSt

Figure 1.6

The Voyage scale model represents the solar system at one ten-billionth of its actualsize. Pluto is included in the Voyage model, which was built before the InternationalAstronomical Union classified Pluto as a dwarf planet.

Figure 1.7

This famous photograph from the first Moon landing (Apollo 11 in July 1969) showsastronaut Buzz Aldrin, with Neil Armstrong reflected in his visor. Armstrong was thefirst to step onto the Moon’s surface, saying, “That’s one small step for a man, onegiant leap for mankind.”

Seeing our solar system to scale also helps put space exploration intoperspective. The Moon, the only other world on which humans haveever stepped (Figure 1.7), lies only about 4 centimeters (1 inches) fromEarth in the Voyage model. On this scale, the palm of your hand cancover the entire region of the universe in which humans have so fartraveled. The trip to Mars is some 200 times as far as the trip to theMoon, even when Mars is on the same side of its orbit as Earth. Andwhile you can walk from the Sun to Pluto in a few minutes on theVoyage scale, the New Horizons spacecraft that is currently making thereal journey will have been in space nearly a decade by the time it fliespast Pluto in 2015.

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If you visit the Voyage model in Washington, D.C, you can walk theroughly 600-meter distance from the Sun to Pluto in just a few min-utes. But how far would you have to walk to reach the next star onthis scale?

Amazingly, you would need to walk to California. If this answerseems hard to believe, you can check it for yourself. A light-year is about10 trillion kilometers, which becomes 1000 kilometers on the 1-to-10-billion scale (because 10 trillion ÷ 10 billion = 1000). The nearest star sys-tem to our own, a three-star system called Alpha Centauri (Figure 1.8), isabout 4.4 light-years away. That distance becomes about 4400 kilometers(2700 miles) on the 1-to-10-billion scale, or roughly equivalent to thedistance across the United States.

specialTopic: | How Many Planets Are There in Our Solar System?

AS CHILDREN, WE were taught that there are nine planets in our solarsystem. However, in 2006 astronomers voted to demote Pluto to adwarf planet, leaving our solar system with only eight official planets.Why the change, and is this really the end for Pluto as a planet?

When Pluto was discovered in 1930, it was assumed to be similar toother planets. But as we’ll discuss in Chapter 9, we’ve since learned thatPluto is much smaller than any of the first eight planets and that itshares the outer solar system with thousands of other icy objects. Still, aslong as Pluto was the largest known of these objects, most astronomerswere content to leave the planetary status quo. Change was forced bythe 2005 discovery of an object called Eris. Because Eris is slightly largerthan Pluto, astronomers could no longer avoid the question of whatobjects should count as planets.

At a contentious meeting in 2006, members of the InternationalAstronomical Union (IAU) voted to define a planet as an object that

(1) orbits a star (but is itself neither a star nor a moon); (2) is massiveenough for its own gravity to give it a nearly round shape; and (3) hascleared the neighborhood around its orbit. Objects that meet the firsttwo criteria but that have not cleared their orbital neighborhoods—including Pluto, Eris and the asteroid Ceres—are designated dwarfplanets. The myriad objects that orbit the Sun but are too small to beround, including most asteroids and comets, make up a class calledsmall solar system bodies.

Not all astronomers were happy with the new definitions, but fornow they seem likely to hold. Of course, some people are likely to keepthinking of Pluto as a planet regardless of what professional astrono-mers say, much as many people still talk of Europe and Asia as separatecontinents even though both belong to the same land mass (Eurasia).So if you’re a Pluto fan, don’t despair: It’s good to know the officialdefinitions, but it’s better to understand the science behind them.

AlphaCentauri

Centaurus

BetaCentauri

GammaCentauri

OmegaCentauri(globularcluster)

Crux(Southern

Cross)

Figure 1.8

This photograph and diagram show the constellation Centaurus,visible from tropical and southern latitudes. Alpha Centauri’sreal distance of 4.4 light-years becomes 4400 kilometers (2700 miles) on the 1-to-10-billion Voyage scale.

The tremendous distances tothe stars give us some perspectiveon the technological challenge ofastronomy. For example, becausethe largest star of the Alpha Cen-tauri system is roughly the same size

and brightness as our Sun, viewing it in the night sky is somewhat likebeing in Washington, D.C., and seeing a very bright grapefruit in San Fran-cisco (neglecting the problems introduced by the curvature of the Earth). Itmay seem remarkable that we can see this star at all, but the blackness ofthe night sky allows the naked eye to see it as a faint dot of light. It looksmuch brighter through powerful telescopes, but we still cannot see anyfeatures of the star’s surface.

Now, consider the difficulty of detecting planets orbiting nearby stars.It is equivalent to looking from Washington, D.C., and trying to find ballpoints or marbles orbiting grapefruits in California or beyond. When youconsider this challenge, it is remarkable to realize that we have alreadydetected hundreds of such planets [Section 6.5].

The vast distances to the stars also offer a sobering lesson aboutinterstellar travel. Although science fiction shows and movies like StarTrek and Star Wars make such travel look easy, the reality is far differ-ent. Consider the Voyager 2 spacecraft. Launched in 1977, Voyager 2flew by Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune

On the same scale on which Pluto is justa few minutes’ walk from the Sun orEarth, the distance to the nearest starsis equivalent to the distance across theUnited States.

VIS


in 1989. It is now bound for the stars at a speed of close to 50,000kilometers per hour—about 100 times as fast as a speeding bullet. Buteven at this speed, Voyager 2 would take about 100,000 years to reachAlpha Centauri if it were headed in that direction (which it’s not).Convenient interstellar travel remains well beyond our presenttechnology.


The 1-to-10-billion scale is useless for thinking about distances beyondthe nearest stars, because more distant stars would not fit on Earth onthis scale. Visualizing the entire galaxy requires a new scale.

Let’s reduce our solar system scale by another factor of 1 billion(making it a scale of 1 to 1019). On this new scale, each light-year be-comes 1 millimeter, and the 100,000-light-year diameter of the MilkyWay Galaxy becomes 100 meters, or about the length of a football field.Visualize a football field with a scale model of our galaxy centered overmidfield. Our entire solar system is a microscopic dot located around the20-yard line. The 4.4-light-year separation between our solar systemand Alpha Centauri becomes just 4.4 millimeters on this scale—smallerthan the width of your little finger. If you stood at the position of oursolar system in this model, millions of star systems would lie withinreach of your arms.

Another way to put the galaxy into perspective is to consider itsnumber of stars—more than 100 billion. Imagine that tonight you arehaving difficulty falling asleep (perhaps because you are contemplatingthe scale of the universe). Instead of counting sheep, you decide to countstars. If you are able to count about one star each second, on average,how long would it take you to count 100 billion stars in the Milky Way?Clearly, the answer is 100 billion (1011) seconds, but how long is that?

Amazingly, 100 billion secondsturns out to be more than 3000years. (You can confirm this by di-viding 100 billion by the number of

seconds in 1 year.) You would need thousands of years just to count thestars in the Milky Way Galaxy, and this assumes you never take abreak—no sleeping, no eating, and absolutely no dying!


As incredible as the scale of our galaxy may seem, the Milky Way is onlyone of roughly 100 billion galaxies in the observable universe. Just as itwould take thousands of years to count the stars in the Milky Way, itwould take thousands of years to count all the galaxies.

Think for a moment about the total number of stars in all these gal-axies. If we assume 100 billion stars per galaxy, the total number of starsin the observable universe is roughly 100 billion × 100 billion, or10,000,000,000,000,000,000,000 (1022).

It would take thousands of years just tocount out loud the number of stars inthe Milky Way Galaxy.

How big is this number? Visit abeach. Run your hands through thefine-grained sand. Imagine count-ing each tiny grain of sand as it slips

through your fingers. Then imagine counting every grain of sand on thebeach and continuing on to count every grain of dry sand on every beachon Earth. If you could actually complete this task, you would find that,

Roughly speaking, there are as many starsin the observable universe as there aregrains of sand on all the beaches on Earth.

commonMisconceptions

Confusing Very Different Things

Most people are familiar with the terms solar system and galaxy,but some people mix them up. Remember that our solar system isa single star system, while our galaxy is a collection of more than100 billion star systems—so many that it would take thousands ofyears just to count them. Confusing the terms solar system andgalaxy therefore means making a mistake by a factor of 100 billion—a fairly big mistake!


Figure 1.9

The number of stars in the observable universe is compara-ble to the number of grains of dry sand on all the beacheson Earth.

Contemplate the fact that there are as many stars in the observ-able universe as grains of sand on all the beaches on Earth and

that each star is a potential sun for a system of planets. With so many possible homes forlife, do you think it is conceivable that life exists only on Earth? Why or why not?

� How do our lifetimes compare to the age

of the universe?

Now that we have developed some perspective on the scale of space, wecan do the same for the scale of time. Imagine the entire history of theuniverse, from the Big Bang to the present, compressed into a single year.We can represent this history with a cosmic calendar, on which the BigBang takes place at the first instant of January 1 and the present day isthe stroke of midnight on December 31 (Figure 1.10). For a universe thatis about 14 billion years old, each month on the cosmic calendar repre-sents a little more than 1 billion years.

On this time scale, the Milky Way Galaxy probably formed sometimein February. Many generations of stars lived and died in the subsequentcosmic months, enriching the galaxy with the “star stuff” from which weand our planet are made.

Our solar system and our planet did not form until early Septemberon this scale, or 4 billion years ago in real time. By late September, lifeon Earth was flourishing. However, for most of Earth’s history, livingorganisms remained relatively primitive and microscopic. On the scale ofthe cosmic calendar, recognizable animals became prominent only inmid-December. Early dinosaurs appeared on the day after Christmas.Then, in a cosmic instant, the dinosaurs disappeared forever—probablydue to the impact of an asteroid or a comet [Section 9.4]. In real time, thedeath of the dinosaurs occurred some 65 million years ago, but on thecosmic calendar it was only yesterday. With the dinosaurs gone, small

12

January 1:The Big Bang

February:The Milky Way forms

September 3:Earth forms

September 22:Early life on Earth

December 17:Cambrian explosion

December 26:Rise of the dinosaurs

December 30:Extinction ofthe dinosaurs

THE HISTORY OF THE UNIVERSE IN 1 YEAR

FEBRUARY S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

APRIL S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

JULY S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

SEPTEMBER S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

OCTOBER S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

NOVEMBER S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

MARCH S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 2324

31 25 26 27 28 29 30

JUNE S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

30 24 25 26 27 28 29

JANUARY S M T W T F S

1 2 3 4 5 6

7 8 9 10 11 12 13

14 15 16 17 18 19 20

21 22 23 24 25 26 27

28 29 30 31

AUGUST S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2425 26 27 28 29 30 31

DECEMBER S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

MAY S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

DECEMBER

30 (7:00 A.M.)Dinosaurs

extinct

S M T W T F S1 2 3 4 5 6 7

8 9 10 11 12

15 16 18 19 20 21

22 23 24

1413

29 31

27 28 26Rise of the dinosaurs

25

TheCambrianexplosion

17

Figure 1.10

The cosmic calendar compresses the 14-billion-year history ofthe universe into 1 year, so that each month represents a littlemore than 1 billion years. This cosmic calendar is adaptedfrom a version created by Carl Sagan. (For a more detailedversion, see the “You Are Here in Time” foldout diagram in thefront of the book.)

roughly speaking, the number of grains of sand is comparable to the num-ber of stars in the observable universe (Figure 1.9).


DECEMBER 31

9:00 pm:Early hominids evolve

11:58 pm:Modern humans evolve

December 31:

25 seconds ago:Agriculture arises

11 seconds ago:Pyramids built

1 second ago:Kepler and Galileoshow that Earthorbits the Sun Now

8:00 pm9:00 pm

10:00 pm11:00 pm

7:00 pm6:00 pm5:00 pm4:00 pm3:00 pm2:00 pm1:00 pm

12:00 noonMorning...

11:58 pm

12:00 midnight11:59 pm

furry mammals inherited Earth. Some 60 million years later, or around 9 P.M. on December 31 of the cosmic calendar, early hominids (humanancestors) began to walk upright.

Perhaps the most astonishingthing about the cosmic calendar isthat the entire history of humancivilization falls into just the lasthalf-minute. The ancient Egyptians

built the pyramids only about 11 seconds ago on this scale. About 1 sec-ond ago, Kepler and Galileo proved that Earth orbits the Sun rather thanvice versa. The average college student was born about 0.05 second ago,around 11:59:59.95 P.M. on the cosmic calendar. On the scale of cosmictime, the human species is the youngest of infants, and a human lifetimeis a mere blink of an eye.

1.3 Spaceship EarthWherever you are as you read this book, you probably have the feelingthat you’re “just sitting here.” Nothing could be further from the truth. Infact, you are being spun in circles as Earth rotates, you are racing aroundthe Sun in Earth’s orbit, and you are careening through the cosmos inthe Milky Way Galaxy. In the words of noted inventor and philosopherR. Buckminster Fuller (1895–1983), you are a traveler on spaceship Earth.In this section, we’ll take a brief look at the motion of spaceship Earththrough the universe.

� How is Earth moving in our solar system?

The most basic motions of Earth are its daily rotation (spin) and itsyearly orbit (or revolution) around the Sun.

Earth rotates once each day around its axis, which is the imaginaryline connecting the North Pole to the South Pole. Earth rotates from westto east—counterclockwise as viewed from above the North Pole—whichis why the Sun and stars appear to rise in the east and set in the west

If we imagine the 14-billion-year historyof the universe compressed into 1 year, ahuman lifetime lasts only a fraction of asecond.


At the same time Earth is rotat-ing, it is also orbiting the Sun, com-pleting one orbit each year (Figure1.12). Earth’s average orbital dis-tance is called an astronomical

unit, or AU, equivalent to about 150 million kilometers (93 millionmiles). Again, even though we don’t feel the effects of this motion, thespeed is impressive: At all times we are racing around the Sun at a speedin excess of 100,000 kilometers per hour (60,000 miles per hour), fasterthan any spacecraft yet launched.

As you study Figure 1.12, notice that Earth’s orbital path definesa flat plane that we call the ecliptic plane. Earth’s axis is tilted by23 from a line perpendicular to the ecliptic plane. This axis tilt hap-pens to be oriented so that it points almost directly at a star calledPolaris, or the North Star. Keep in mind that the idea of axis tilt makessense only in relation to the ecliptic plane. That is, the idea of “tilt”by itself has no meaning in space, where there is no absolute up ordown. In space, “up” and “down” mean only “away from the centerof Earth (or another planet)” and “toward the center of Earth,”respectively.

12°

Earth rotates once each day and orbitsthe Sun once each year. Its average or-bital distance, called an astronomical unit(AU), is about 150 million kilometers.

If there is no up or down in space, why do you think mostglobes have the North Pole on top? Would it be equally

correct to have the South Pole on top or to turn the globe sideways? Explain.

0 km�hr

1275 km�hr

1670 km�hr

1275 km�hr

Figure 1.11

As Earth rotates, your speed around Earth’s axis depends onyour location: The closer you are to the equator, the fasteryou travel with rotation. Notice that Earth rotates from west toeast, which is why the Sun appears to rise in the east and setin the west.

to Polaristo Polaris

1 AU

Earth’s axis remains pointedin the same direction(toward Polaris)throughoutthe year.

The average Earth–Sun distanceis 1 AU, or about 150 million km.

Earth takes 1 year to orbit the Sun atan average speed of 107,000 km/hr.

axis

ecliptic plane

(not to scale!)

23 °12

Figure 1.12

Earth takes a year to complete an orbit of the Sun, but itsorbital speed is still surprisingly fast. Notice that Earth bothrotates and orbits counterclockwise as viewed from above theNorth Pole.

each day. Although we do not feel any obvious effects from Earth’s rota-tion, the speed of rotation is substantial (Figure 1.11). Unless you livevery near the North or South Poles, you are whirling around Earth’s axisat a speed of more than 1000 kilometers per hour (600 miles per hour)—faster than most airplanes travel.

Notice also that Earth orbits the Sun in the same direction that itrotates on its axis: counterclockwise as viewed from above the NorthPole. This is not a coincidence but a consequence of the way our planetwas born. As we’ll discuss in Chapter 6, Earth and the other planetswere born in a spinning disk of gas that surrounded our Sun when itwas young, and Earth rotates and orbits in the same direction as thedisk was spinning.

� How is our solar system moving

in the Milky Way Galaxy?

Rotation and orbit are only part of the travels of spaceship Earth. Our en-tire solar system is on a great journey within the Milky Way Galaxy.

Our Local Solar Neighborhood Let’s begin with the motion ofour solar system relative to nearby stars in what we call our local solarneighborhood, the region of the Sun and nearby stars. The small box inFigure 1.13 shows that stars within the local solar neighborhood (likethe stars of any other small region of the galaxy) move essentially atrandom relative to one another. They also generally move quite fast.For example, we are moving relative to nearby stars at an averagespeed of about 70,000 kilometers per hour (40,000 miles per hour),about three times as fast as the Space Station orbits Earth.


Stars in the localsolar neighborhoodmove randomlyrelative to one another. . .

Sun

27,000

light-years

230-million-year orbit

. . . while the galaxy'srotation carries themaround the galacticcenter at even higherspeed.

Figure 1.13

This painting illustrates the motion of our solar system withinour local solar neighborhood and around the center of theMilky Way Galaxy.

Given these high speeds, why don’t we see nearby stars racingaround the sky? The answer lies in their vast distances from us. You’veprobably noticed that a distant airplane appears to move through the sky

more slowly than one flying closeoverhead. Stars are so far away thateven at speeds of 70,000 kilometersper hour, their motions would be

noticeable to the naked eye only if we watched them for thousands ofyears. That is why the patterns in the constellations seem to remainfixed. Nevertheless, in 10,000 years the constellations will be noticeablydifferent from those we see today. In 500,000 years they will be unrecog-nizable. If you could watch a time-lapse movie made over millions ofyears, you would see stars racing across the sky.

Stars in our local solar neighborhoodmove in essentially random directionsrelative to each other.

Despite the chaos of motion in the local solar neighborhoodover millions and billions of years, collisions between star

systems are extremely rare. Explain why. (Hint: Consider the sizes of star systems, suchas the solar system, relative to the distances between them.)

Galactic Rotation If you look closely at leaves floating in a stream,their motions relative to one another might appear random, just likethe motions of stars in the local solar neighborhood. As you widen yourview, you see that all the leaves are being carried in the same generaldirection by the current. In the same way, as we widen our view be-yond the local solar neighborhood, the seemingly random motions ofits stars give way to a simpler and even faster motion: rotation of theMilky Way Galaxy. Our solar system, located about 27,000 light-yearsfrom the galactic center, completes one orbit of the galaxy in about 230million years. Even if you could watch from outside our galaxy, thismotion would be unnoticeable to your naked eye. However, if youcalculate the speed of our solar system as we orbit the center of the gal-axy, you will find that it is close to 800,000 kilometers per hour(500,000 miles per hour).


Careful study of the galaxy’s rotation reveals one of the greatestmysteries in science. Stars at different distances from the galactic centerorbit at different speeds, and we can learn how mass is distributed in thegalaxy by measuring these speeds. Such studies indicate that the stars inthe disk of the galaxy represent only the “tip of the iceberg” compared to

the mass of the entire galaxy(Figure 1.14). Most of the mass ofthe galaxy seems to be located out-side the visible disk, in what wecall the halo. We don’t know the

nature of this mass, but we call it dark matter because we have notdetected any light coming from it. Studies of other galaxies suggest thatthey also are made mostly of dark matter, which means this mysteriousmatter must significantly outweigh the ordinary matter that makes upplanets and stars. An even more mysterious dark energy seems to makeup much of the total energy content of the universe. We’ll discuss themysteries of dark matter and dark energy in Chapter 16.

� How do galaxies move within

the universe?

The billions of galaxies in the universe also move relative to one another.Within the Local Group (see Figure 1.1), some of the galaxies move to-ward us, some move away from us, and at least two small galaxies(known as the Large and Small Magellanic Clouds) apparently orbit ourMilky Way Galaxy. Again, the speeds are enormous by earthly standards.For example, the Milky Way is moving toward the Andromeda Galaxy atabout 300,000 kilometers per hour (180,000 miles per hour). Despite thishigh speed, we needn’t worry about a collision anytime soon. Even if theMilky Way and Andromeda Galaxies are approaching each other head-on, it will be billions of years before any collision begins.

Most of the galaxy’slight comes from starsand gas in the galacticdisk and central bulge . . .

. . . but measurements suggestthat most of the mass lies unseenin the spherical halo that surroundsthe entire disk.

Figure 1.14

This painting shows an edge-on view of the Milky Way Galaxy.Study of galactic rotation shows that although most visiblestars lie in the disk and central bulge, most of the mass lies inthe halo that surrounds and encompasses the disk. Becausethis mass emits no light that we have detected, we call it darkmatter.

The Sun and other stars in our neighbor-hood orbit the center of the galaxy every230 million years, because the entiregalaxy is rotating.


When we look outside the Local Group, however, we find two aston-ishing facts recognized in the 1920s by Edwin Hubble, for whom theHubble Space Telescope was named:

1. Virtually every galaxy outside the Local Group is moving awayfrom us.

2. The more distant the galaxy, the faster it appears to be racing away.

These facts might make it sound like we suffer from a cosmic case ofchicken pox, but there is a much more natural explanation: The entireuniverse is expanding. We’ll save the details for later in the book (Chapter15), but you can understand the basic idea by thinking about a raisincake baking in an oven.

Imagine that you make a raisin cake in which the distance be-tween adjacent raisins is 1 centimeter. You place the cake into theoven, where it expands as it bakes. After 1 hour, you remove the cake,which has expanded so that the distance between adjacent raisins hasincreased to 3 centimeters (Figure 1.15). The expansion of the cakeseems fairly obvious. But what would you see if you lived in the cake,as we live in the universe?

Pick any raisin (it doesn’t matter which one) and call it the LocalRaisin. Figure 1.15 shows one possible choice, with three nearby raisinsalso labeled. The accompanying table summarizes what you would seeif you lived within the Local Raisin. Notice, for example, that Raisin 1starts out at a distance of 1 centimeter before baking and ends up at adistance of 3 centimeters after baking, which means it moves a distanceof 2 centimeters away from the Local Raisin during the hour of baking.Hence, its speed as seen from the Local Raisin is 2 centimeters per hour.Raisin 2 moves from a distance of 2 centimeters before baking to a dis-tance of 6 centimeters after baking, which means it moves a distance of4 centimeters away from the Local Raisin during the hour. Hence, itsspeed is 4 centimeters per hour, or twice as fast as the speed of Raisin 1.Generalizing, the fact that the cake is expanding means that all theraisins are moving away from the Local Raisin, with more distantraisins moving away faster.

12 3

12

3

Local Raisin

LocalRaisin

3 cm3 cm

3 cm

1cm

1cm

1cm

1hr

Before baking: raisinsare all 1 cm apart.

After baking:raisins are all3 cm apart.

From an outside perspective, the cake expands uniformly as it bakes . . .

. . . but from the point of view of the Local Raisin,all other raisins move farther away during baking,with more distant raisins moving faster.

Figure 1.15

An expanding raisin cake offers an analogy to the expandinguniverse. Someone living in one of the raisins inside the cakecould figure out that the cake is expanding by noticing thatall other raisins are moving away, with more distant raisinsmoving away faster. In the same way, we know that we livein an expanding universe because all galaxies outside ourLocal Group are moving away from us, with more distantones moving faster.

Distances and Speeds as Seen from the Local Raisin

RaisinNumber

Distance BeforeBaking

Distance After Baking (1 hour later) Speed

1 1 cm 3 cm 2 cm/hr

2 2 cm 6 cm 4 cm/hr

3 3 cm 9 cm 6 cm/hr

o o o o

Hubble’s discovery that galaxiesare moving in much the same wayas the raisins in the cake, with mostmoving away from us and moredistant ones moving away faster,

implies that our universe is expanding much like the raisin cake. If younow imagine the Local Raisin as representing our Local Group of galaxiesand the other raisins as representing more distant galaxies or clusters ofgalaxies, you have a basic picture of the expansion of the universe. Likethe expanding batter between the raisins in the cake, space itself is growingbetween galaxies. More distant galaxies move away from us faster be-cause they are carried along with this expansion like the raisins in theexpanding cake. Many billions of light-years away, we see galaxiesmoving away from us at speeds approaching the speed of light.

There’s one important distinction between the raisin cake and theuniverse: A cake has a center and edges, but we do not think the same istrue of the entire universe. Anyone living in any galaxy in an expandinguniverse sees just what we see—other galaxies moving away, with moredistant ones moving away faster. Because the view from each point inthe universe is about the same, no place can claim to be any more “cen-tral” than any other place.

Distant galaxies are all moving away fromus, with more distant ones moving faster,indicating that we live in an expandinguniverse.


Earth rotates around its axis once each day, carrying peoplein most parts of the world around the axis at more than 1000 km/hr.

Earth orbits the Sun once each year, moving at more than 100,000 km/hr.

The Solar System moves relative to nearby stars, typically at a speed of 70,000 km/hr.

The Milky Way Galaxy rotates, carrying our Sun around its centeronce every 230 million years, at a speed of about 800,000 km/hr.

Our galaxy moves relative to others in theLocal Group; we are traveling toward theAndromeda Galaxy at about 300,000 km/hr.

The universe expands. Themore distant an object, thefaster it moves away from us;the most distant galaxies arereceding from us at speedsclose to the speed of light.

Figure 1.16

This figure summarizes the basic motions of Earth in theuniverse, along with their associated speeds.

It’s also important to realize that, unlike the case with a raisincake, we can’t actually see galaxies moving apart with time—the dis-tances are too vast for any motion to be noticeable on the time scale ofa human life. Instead, we measure the speeds of galaxies by spreadingtheir light into spectra and observing what we call Doppler shifts[Section 5.2]. This illustrates how modern astronomy depends both oncareful observations and on using current understanding of the laws ofnature to explain what we see.


As we have seen, we are never truly sitting still. Figure 1.16 summa-rizes the motions we have covered. We spin around Earth’s axis at morethan 1000 km/hr, while our planet orbits the Sun at more than 100,000

km/hr. Our solar system movesamong the stars of the local solarneighborhood at typical speeds of70,000 km/hr, while also orbiting

the center of the Milky Way Galaxy at a speed of about 800,000 km/hr.Our galaxy moves among the other galaxies of the Local Group, whileall other galaxies move away from us at speeds that increase with dis-tance in our expanding universe. Spaceship Earth is carrying us on a re-markable journey.

We and our planet are constantly on themove through the universe, and at sur-prisingly high speeds.


the big picturePutting Chapter 1

into Perspective

In this first chapter, we developed a broad overview of our place in theuniverse. As we consider the universe in more depth in the rest of thebook, remember the following “big picture” ideas:

• Earth is not the center of the universe but instead is a planet orbitinga rather ordinary star in the Milky Way Galaxy. The Milky Way Gal-axy, in turn, is one of billions of galaxies in our observable universe.

• We are “star stuff.” The atoms from which we are made began ashydrogen and helium in the Big Bang and were later fused intoheavier elements by massive stars. Stellar deaths released theseatoms into space, where our galaxy recycled them into new starsand planets. Our solar system formed from such recycled mattersome 4 billion years ago.

• Cosmic distances are literally astronomical, but we can put them inperspective with the aid of scale models and other scaling techniques.When you think about these enormous scales, don’t forget that everystar is a sun and every planet is a unique world.

• We are latecomers on the scale of cosmic time. The universe wasalready more than half its current age when our solar systemformed, and it took billions of years more before humans arrivedon the scene.

• All of us are being carried through the cosmos on spaceship Earth.Although we cannot feel this motion, the associated speeds are sur-prisingly high. Learning about the motions of spaceship Earth givesus a new perspective on the cosmos and helps us understand its na-ture and history.

12

� How can we know what the universe was like

in the past?

Light takes time to travel through space, so the farther awaywe look in distance, the further back we look in time. Whenwe look billions of light-years away, we see pieces of theuniverse as they were billions of years ago.


No. The age of the universelimits the extent of ourobservable universe. Be-

cause the universe is about 14 billion years old, our observableuniverse extends to a distance of about 14 billion light-years.If we tried to look beyond that distance, we’d be trying to lookto a time before the universe existed.

summary of key concepts



Earth is a planet orbiting the Sun. Our Sun isone of more than 100 billion stars in theMilky Way Galaxy. Our galaxy is one ofabout 40 galaxies in the Local Group. TheLocal Group is one small part of the LocalSupercluster, which is one small part ofthe universe.


The universe began in the Big Bangand has been expanding ever since,except in localized regions where grav-ity has caused matter to collapse intogalaxies and stars. The Big Bang essen-tially produced only two chemicalelements: hydrogen and helium. Therest have been produced by stars,which is why we are “star stuff.”

1.2 The Scale of the Universe

� How big is Earth compared to our solar system?

On a scale of 1 to 10 billion, the Sun isabout the size of a grapefruit. Planetsare much smaller, with Earth the sizeof a ball point and Jupiter the size of amarble on this scale. The distances be-tween planets are huge compared totheir sizes, with Earth orbiting 15meters from the Sun on this scale.


On the 1-to-10-billion scale, it is possible to walk from the Sunto Pluto in just a few minutes. On the same scale, the neareststars besides the Sun are thousands of kilometers away.


Using a scale on which the Milky Way galaxy is the size of afootball field, the distance to the nearest star would be onlyabout 4 millimeters. There are so many stars in our galaxythat it would take thousands of years just to count them.


The observable universe contains roughly 100 billion galaxies,and the total number of stars is comparable to the number ofgrains of dry sand on all the beaches on Earth.

� How do our lifetimes compare to the age

of the universe?

On a cosmic calendar that compressesthe history of the universe into 1 year,human civilization is just a few secondsold, and a human lifetime lasts only afraction of a second.

1.3 Spaceship Earth

� How is Earth moving in our solar system?

Earth rotates on its axis once eachday and orbits the Sun once each year.Earth orbits at an average distance fromthe Sun of 1 AU and with an axis tilt

of 23 to a line perpendicular to the ecliptic plane.

� How is our solar system moving in the Milky Way

Galaxy?

We move randomly relative to otherstars in our local solar neighborhood.The speeds are substantial by earthlystandards, but stars are so far awaythat their motion is undetectable tothe naked eye. Our Sun and other

stars in our neighborhood orbit the center of the galaxyevery 230 million years, because the entire galaxy is rotating.

� How do galaxies move within the universe?

Galaxies move essentially at random within the Local Group,but all galaxies beyond the Local Group are moving away fromus. More distant galaxies are moving faster, which tells us thatwe live in an expanding universe.


We are never truly sitting still. We spinaround Earth’s axis and orbit the Sun.Our solar system moves among the starsof the local solar neighborhood whileorbiting the center of the Milky WayGalaxy. Our galaxy moves among the

other galaxies of the Local Group, while all other galaxiesmove away from us in our expanding universe.

12°

DECEMBERS M T W T F S

1 2 3 4 5 6 7

8 9 10 11 12

15 16 18 19

22 23 24

1413

29 31

27 28

20 21

25

S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

S M T W T F S

1 2 3 4 5 6

7 8 9 10 11 12 13

14 15 16 17 18 19 20

21 22 23 24 25 26 27

28 29 30 31

FEBRUARY

SEPTEMBER

JANUARY

(7:00 A.M.)Dinosaurs

extinct

26

TheCambrianexplosion

Rise of the dinosaurs

30

17DECEMBER

S M T W T F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

to Polaristo Polaris

axis

ecliptic plane

(not to scale!) 23 °12

1 AU

visual skills check

Use the following questions to check your understanding of some of the many types of visual information usedin astronomy. Answers are provided in Appendix J. For additional practice, try the Chapter 1 Visual Quiz at www.masteringastronomy.com.

The figure below shows the sizes of Earth and the Moon to scale; the scale used is 1 cm = 4000 km. Usingwhat you’ve learned about astronomical scale in this chapter, answer the following questions. Hint: If you areunsure of the answers, you can calculate them using the following real values:

Earth–Sun distance = 150,000,000 km Diameter of Sun = 1,400,000 km

Earth–Moon distance = 384,000 km Diameter of Earth = 12,800 km


www.masteringastronomy.com


1. If you wanted to show the distance between Earth and theMoon on the same scale, about how far apart would you need toplace the two photos on page 22?a. 10 centimeters (about the width of your hand)b. 1 meter (about the length of your arm)c. 100 meters (about the length of a football field)d. 1 kilometer (a little more than a half mile)

2. Suppose you wanted to show the Sun on the same scale. Abouthow big would it need to be?a. 2.5 centimeters in diameter (the size of a golf ball)b. 25 centimeters in diameter (the size of a basketball)c. 2.5 meters in diameter (about 8 feet across)d. 2.5 kilometers in diameter (the size of a small town)

3. About how far away from Earth would the Sun be located onthis scale?a. 3.75 meters (about 12 feet)b. 37.5 meters (about the height of a 12-story building)c. 375 meters (about the length of four football fields)d. 37.5 kilometers (the size of a large city)

4. Could you use the same scale to represent the distances tonearby stars? Why or why not?

exercises and problems

For instructor-assigned homework go to www.masteringastronomy.com.

Review Questions

1. What do we mean by a geocentric universe? Contrast a geocentricview with our modern view of the universe.

2. Briefly describe the major levels of structure (such as planet, star,galaxy) in the universe.

3. What do we mean when we say that the universe is expanding?How does expansion lead to the idea of the Big Bang?

4. What did Carl Sagan mean when he said that we are “star stuff”?5. How fast does light travel? What is a light-year?6. Explain the statement The farther away we look in distance, the

further back we look in time.7. What do we mean by the observable universe? Is it the same thing

as the entire universe?8. Describe the solar system as it looks on the 1-to-10-billion scale

used in the text. How far away are other stars on this same scale?9. Describe at least one way to put the scale of the Milky Way

Galaxy into perspective and at least one way to put the size ofthe observable universe into perspective.

10. Use the cosmic calendar to describe how the human race fits intothe scale of time.

11. Define astronomical unit, ecliptic plane, and axis tilt. Explain howeach is related to Earth’s rotation and/or orbit.

12. What is the shape of the Milky Way Galaxy? Describe our solarsystem’s location and motion.

13. Distinguish between our galaxy’s disk and halo. Where does themysterious dark matter seem to reside?

14. What key observations lead us to conclude that the universe isexpanding? Use the raisin cake model to explain how theseobservations imply expansion.

Test Your Understanding

Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or doesnot make sense (or is clearly false). Explain clearly; not all of these havedefinitive answers, so your explanation is more important than yourchosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has nomeaning at the North Pole—all directions are south from the North Pole.

15. Our solar system is bigger than some galaxies.16. The universe is billions of light-years in age.17. It will take me light-years to complete this homework

assignment!18. Someday we may build spaceships capable of traveling a light-

year in only a decade.19. Astronomers discovered a moon that does not orbit a planet.20. NASA plans soon to launch a spaceship that will photograph our

Milky Way Galaxy from beyond its halo.21. The observable universe is the same size today as it was a few

billion years ago.22. Photographs of distant galaxies show them as they were when

they were much younger than they are today.23. At a nearby park, I built a scale model of our solar system in

which I used a basketball to represent Earth.24. Because nearly all galaxies are moving away from us, we must

be located at the center of the universe.

Quick Quiz

Choose the best answer to each of the following. Explain your reasoningwith one or more complete sentences.

25. Which of the following correctly lists our “cosmic address” fromsmall to large? (a) Earth, solar system, Milky Way Galaxy, LocalGroup, Local Supercluster, universe (b) Earth, solar system,Local Group, Local Supercluster, Milky Way Galaxy, universe (c) Earth, Milky Way Galaxy, solar system, Local Group, LocalSupercluster, universe

26. When we say the universe is expanding, we mean that (a) every-thing in the universe is growing in size. (b) the average distancebetween galaxies is growing with time. (c) the universe is gettingolder.

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27. If stars existed but galaxies did not, (a) we would probably existanyway. (b) we would not exist because life on Earth dependson the light of galaxies. (c) we would not exist because we aremade of material that was recycled in galaxies.

28. Could we see a galaxy that is 20 billion light-years away? (a) Yes,if we had a big enough telescope. (b) No, because it would bebeyond the bounds of our observable universe. (c) No, because agalaxy could not possibly be that far away.

29. The star Betelgeuse is about 425 light-years away. If it explodestonight, (a) we’ll know because it will be brighter than the fullMoon in the sky. (b) we’ll know because debris from the explo-sion will rain down on us from space. (c) we won’t know aboutit until 425 years from now.

30. If we represented the solar system on a scale that allowed us towalk from the Sun to Pluto in a few minutes, then (a) the plan-ets would be the size of basketballs and the nearest stars wouldbe a few miles away. (b) the planets would all be marble-size orsmaller and the nearest stars would be thousands of miles away.(c) the planets would be microscopic and the stars would belight-years away.

31. The total number of stars in the observable universe is roughlyequivalent to (a) the number of grains of sand on all the beacheson Earth. (b) the number of grains of sand on Miami Beach. (c) infinity.

32. The age of our solar system is about (a) one-third of the age ofthe universe. (b) three-fourths of the age of the universe. (c) twobillion years less than the age of the universe.

33. As astronomical unit is (a) any planet’s average distance fromthe Sun. (b) Earth’s average distance from the Sun. (c) any largeastronomical distance.

34. The fact that nearly all galaxies are moving away from us, withmore distant ones moving faster, tells us that (a) the universe isexpanding. (b) galaxies repel each other like magnets. (c) ourgalaxy lies near the center of the universe.

Process of Science

35. Earth as a Planet. For most of human history, scholars assumedEarth was the center of the universe. Today, we know that Earthis just one planet orbiting the Sun, and the Sun is just one star ina vast universe. How did science make it possible for us to learnthese facts about Earth?

36. Thinking About Scale. One key to success in science is finding asimple way to evaluate new ideas, and making a simple scalemodel is often helpful. Suppose someone tells you that the rea-son it is warmer during the day than at night is that the day sideof Earth is closer to the Sun than the night side. Evaluate thisidea by thinking about the size of Earth and its distance fromthe Sun in a scale model of the solar system.

37. Looking for Evidence. In this first chapter, we have discussed thescientific story of the universe but have not yet discussed mostof the evidence that backs it up. Choose one idea presented inthis chapter—such as the idea that there are billions of galaxiesin the universe, or that the universe was born in the Big Bang,or that the galaxy contains more dark matter than ordinarymatter—and briefly discuss the type of evidence you wouldwant to see before accepting the idea. (Hint: It’s okay to lookahead in the book to see the evidence presented in laterchapters.)

Group Work Exercise

38. Counting the Milky Way’s Stars. In this exercise, you will first makean estimate of the number of stars in the Milky Way, and thenapply some scientific thinking to your estimation method. Be-fore you begin, assign the following roles to the people in yourgroup: Scribe (takes notes on the group’s activities), Proposer (pro-poses explanations to the group), Skeptic (points out weaknessesin proposed explanations), and Moderator (leads group discussionand makes sure everyone contributes).a. Estimate the number of stars in the Milky Way as follows.

First, count the number of stars within 12 light-years ofthe Sun, which are listed in Appendix F. Assuming that theMilky Way’s disk is 100,000 light-years across and 1000light-years thick, its volume is about 1 billion times the vol-ume of the region of your star count. You should thereforemultiply your count by 1 billion to get an estimate of thetotal number of stars in the Milky Way.

b. Your estimate from part (a) is based on the number of starsnear the Sun. Compare it to the value given in this chapterand determine whether your estimate is an underestimateor an overestimate of the total number of stars in the MilkyWay. Write down a list of possible reasons why your tech-nique gave you an under/overestimate.

Investigate Further

Short-Answer/Essay Questions

39. Our Cosmic Origins. Write one to three paragraphs summarizingwhy we could not be here if the universe did not contain bothstars and galaxies.

40. Alien Technology. Some people believe that Earth is regularly vis-ited by aliens who travel here from other star systems. For thisto be true, how much more advanced than our own technologywould the aliens’ technology have to be? Write one to two para-graphs to give a sense of the technological difference. (Hint: Theideas of scale in this chapter can help you contrast the distancethe aliens would have to travel with the distances we are nowcapable of traveling.)

41. Stellar Collisions. Is there any danger that another star will comecrashing through our solar system in the near future? Explain.

42. Raisin Cake Universe. Suppose that all the raisins in a cake are 1 cen-timeter apart before baking and 4 centimeters apart after baking.a. Draw diagrams to represent the cake before and after baking.b. Identify one raisin as the Local Raisin on your diagrams.

Construct a table showing the distances and speeds of otherraisins as seen from the Local Raisin.

c. Briefly explain how your expanding cake is similar to theexpansion of the universe.

43. The Cosmic Perspective. Write a short essay describing how theideas presented in this chapter affect your perspectives on yourown life and on human civilization.

Quantitative Problems

Be sure to show all calculations clearly and state your final answers incomplete sentences.

44. Distances by Light. Just as a light-year is the distance that lightcan travel in 1 year, we define a light-second as the distance thatlight can travel in 1 second, a light-minute as the distance that


light can travel in 1 minute, and so on. Calculate the distance inboth kilometers and miles represented by each of the following:a. 1 light-second b. 1 light-minutec. 1 light-hour d. 1 light-day

45. Moonlight and Sunlight. How long does it take light to travel froma. the Moon to Earth?b. the Sun to Earth?

46. Saturn vs. the Milky Way. Photos of Saturn and photos of galaxiescan look so similar that children often think the photos showsimilar objects. In reality, a galaxy is far larger than any planet.About how many times larger is the diameter of the Milky WayGalaxy than the diameter of Saturn’s rings? (Data: Saturn’s ringsare about 270,000 km in diameter; the Milky Way is 100,000light-years in diameter.)

47. Driving Trips. Imagine that you could drive your car at a constantspeed of 100 km/hr (62 mi/hr), even across oceans and in space.How long would it take to drivea. around Earth’s equator? (Hint: Use Earth’s circumference of

about 40,000 km.)b. from the Sun to Earth?c. from the Sun to Pluto? (Hint: You can find Pluto’s distance in

Appendix E.)d. to Alpha Centauri (4.4 light-years away)?

48. Faster Trip. Suppose you wanted to reach Alpha Centauri in100 years.a. How fast would you have to go, in km/hr?b. How many times faster is the speed you found in (a) than the

speeds of our fastest current spacecraft (around 50,000 km/hr)?

Discussion Questions

49. Vast Orbs. Dutch astronomer Christiaan Huygens may havebeen the first person to truly understand both the large sizes ofother planets and the great distances to other stars. In 1690, hewrote: “How vast those Orbs must be, and how inconsiderable

this Earth, the Theatre upon which all our mighty Designs, allour Navigations, and all our Wars are transacted, is when com-pared to them. A very fit consideration, and matter of Reflec-tion, for those Kings and Princes who sacrifice the Lives of somany People, only to flatter their Ambition in being Mastersof some pitiful corner of this small Spot.” What do you thinkhe meant? Explain.

50. Infant Species. In the last few tenths of a second before midnighton December 31 of the cosmic calendar, we have developed anincredible civilization and learned a great deal about the uni-verse, but we also have developed technology through whichwe could destroy ourselves. The midnight bell is striking, and thechoice for the future is ours. How far into the next cosmic yeardo you think our civilization will survive? Defend your opinion.

51. A Human Adventure. Astronomical discoveries clearly are impor-tant to science, but are they also important to our personal lives?Defend your opinion.

Web Projects

52. Astronomy on the Web. The Web contains a vast amount of astro-nomical information. Spend at least an hour exploring astronomyon the Web. Write two or three paragraphs summarizing what youlearned from your search. What was your favorite astronomicalWeb site, and why?

53. NASA Missions. Visit the NASA Web site to learn about upcomingastronomy missions. Write a one-page summary of the missionyou feel is most likely to provide new astronomical informationduring the time you are enrolled in this astronomy course.

54. The Hubble Ultra Deep Field. The photo that opens this chapter iscalled the Hubble Ultra Deep Field. Find the photo on theHubble Space Telescope Web site. Learn how it was taken, whatit shows, and what we’ve learned from it. Write a short sum-mary of your findings.

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