studying the universe - denton independent school district ... · in the 1920s, astronomer edwin...

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Astronomy—The OriginalScience . . . . . . . . . . . . 594

Self-Check . . . . 596Internet Connect . . . . . 597

Telescopes . . . . . . . . . 598QuickLab . . . . . 599Activity . . . . . . . 601Internet Connect . . . . 603

Galaxies . . . . . . . . . . . 604Activity . . . . . . . 605Internet Connect . . . . 607

Formation of the Universe . . . . . . . . 608

Apply . . . . . . . . 609Activity . . . . . . . 609Internet Connect . . . . . 611

Chapter Lab . . . . . . . . 612

Chapter Review . . . . . 615

TEKS/TAKSPractice Tests . . . . . . . 617, 618

Feature Article . . . . . . 619

LabBook . . . . . . . . . . 694

Studying theUniverseStudying theUniverse

Galaxies GaloreIf you had a telescope, what would you look for? In the 1920s,astronomer Edwin Hubble chose to look for galaxies much like the M81 galaxy shown on the next page. In 1995 the Hubble Space Telescope was used to develop the single imagecalled the Hubble Deep Field, part of which is shown below.The segment of the sky in that image contains nearly 2,000galaxies! In this chapter, you willlearn about the differenttypes of galaxies andthe science ofastronomy.

Pre-ReadingQuestions

1. What is a galaxy?2. According to the big bang

theory, how did the uni-verse begin?

3. How do astronomersstudy space? 1995 Hubble Deep Field image

Copyright © by Holt, Rinehart and Winston. All rights reserved.

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EXPLORING GALAXIES IN THE UNIVERSEGalaxies are large groupings of millions of stars. But not all galaxies are the same. In this activity, you will explore some of these differences.

Procedure

1. Look at the different galaxies inthe Hubble Deep Field image onpage 592. (The two bright spotswith spikes are not galaxies. Theyare stars, so ignore them as youdo this activity.)

2. Can you find different types ofgalaxies? In your ScienceLog, make sketches of at least three different types. Make up a namethat describes each type of galaxy.

3. In your ScienceLog, construct atable to classify, compare, anddescribe the different characteris-tics you see in these galaxies.

Analysis

4. Why did you classify the galaxiesthe way you did?

5. Compare your types of galaxieswith those of your classmates. Are there similarities?

6. What conclusions can you drawabout galaxies from the com-parisons you made in step 3?

Studying the UniverseCopyright © by Holt, Rinehart and Winston. All rights reserved.

Astronomy—The Original ScienceHow many planets are in the universe? How far away isthe farthest star?

Unfortunately, neither of these questions can be answered yet.The universe is huge! The universe consists of all space andthe matter space contains. We have barely scratched the sur-face of exploring the universe. But this isn’t because peoplehaven’t been trying. Ancient cultures discovered cycles inmovements of the moon, sun, and stars. Their observationsmay have been the beginnings of astronomy, the study of allphysical objects beyond Earth.

Astronomy and TimeAncient cultures probably did not know very much about themoon, sun, and stars. However, they did notice that these objectsmove in cycles. Careful observations of these cycles led to cal-endars. One early version of a calendar is shown in Figure 1.

Toward a Modern Calendar Modern calendars organize time into years, months, and days.Although these units are familiar in common usage, they havespecific definitions based on astronomy. A year is the timerequired for the Earth to revolve once around the sun. A monthis roughly the amount of time required for the moon to revolveonce around the Earth. A day is the time required for the Earthto rotate once on its axis.

In the early Roman calendar, a year had exactly365 days. The calendar worked well until peoplerealized that the seasons were beginning and end-ing later each year. To fix this problem, JuliusCaesar developed the Julian calendar based on a365.25-day calendar year. He added 90 days to the year 46 BCE and added an extra day every 4 years. A year in which an extra day is added tothe calendar is called a leap year.

In the mid-1500s, astronomers determinedthat there are actually 365.242 days in a year, soPope Gregory XIII developed the Gregorian calen-dar. He dropped 10 days from the year 1582 andrestricted leap years to years that are divisible by4 but not by 100 (except for years that are divis-ible by 400). Today, most countries use theGregorian calendar.

Terms to Learn

universeastronomyyearmonthdayleap year

What You’ll Do

● Explain how our modern calendar developed.

● Connect the changing con-cepts about the universewith the contributions ofearly astronomers.

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Figure 1 This stone is a calendarused by the Aztecs in precolonialAmerica.


The Who’s Who of Early AstronomyThe science of astronomy has come a long way since the earlydays. The earliest astronomers had no history to learn from.Almost everything they knew about the universe came fromwhat they could discover with their own eyes and minds. Notsurprisingly, most early astronomers thought that the universeconsisted of only the sun, the moon, and the planets. Theyalso thought that the stars occupied the edge of the universe.

Ptolemy In 140 CE, a Greek astronomer namedClaudius Ptolemy (KLAW dee uhs TAHLuh mee) took information from exist-ing theories and developed a theory of his own. His theory was called the Ptolemaic theory. As illustrated inFigure 2, Ptolemy thought that the Earthwas at the center of the universe andthat the sun and the other planetsrevolved around the Earth.

Even though the Ptolemaic theorywas incorrect, it predicted the motionsof the planets better than any knownmethod at that time. For more than1,500 years in Europe, the Ptolemaic theory was the most popular theory forthe structure of the universe.

Copernicus In 1543, a Polish astronomer namedNicolaus Copernicus (NIK uh LAY uhskoh PUHR ni kuhs) published a new theory that would eventually changeastronomy. According to his theory,which is illustrated in Figure 3, the sunis at the center of the universe and the planets revolve around the sun incircular paths. While Copernicus wascorrect that all the planets orbit the sun,his theory did not immediately replacePtolemy’s theory.

Studying the Universe 595

Figure 2 According to thePtolemaic theory, the Earth isat the center of the universe.

Figure 3 According to Copernicus’s theory, the sun is at the center of the universe.


Tycho Brahe In the late-1500s Danish astronomer Tycho Brahe (TIE kohBRAH uh) used several large tools, such as the one illustratedin Figure 4, to study the sky. Tycho favored a theory of an Earth-centered universe that was different from Ptolemaic theory.Tycho believed that the other planets revolved around the sunbut that the sun and the moon revolved around the Earth.While Tycho’s theory was not correct, he did record very exactdata on the positions of the sun, the moon, and the planets.

Johannes Kepler After Tycho died, his assistant, Johannes Kepler, continuedTycho’s work. Kepler did not agree with Tycho’s theory, buthe recognized how valuable Tycho’s data were. In 1609, after analyzing the data, Kepler announced some new laws ofplanetary motion. Kepler stated that all the planets revolvearound the sun in an ellipse, and that the sun is not in theexact center of their orbital paths.

Galileo Galilei In 1609, Galileo Galilei became the first person to use a tele-scope to observe objects in space. His telescope is shown inFigure 5. You will learn more about telescopes in the next sec-tion. Galileo discovered four moons orbiting Jupiter, cratersand mountains on the moon, and sunspots. These discoveriesshowed that the planets are physical bodies like the Earth.Galileo favored Copernicus’s theory over Ptolemy’s.

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Figure 4 Tycho used the muralquadrant, a large quarter-circle ona wall, to measure the positions ofstars and planets.

Figure 5 Galileo’s telescope wasmuch simpler than those used byastronomers today.

Self-CheckName two astronomers who favored an Earth-centered universeand two astronomers who favored a sun-centered universe. (See page 698 to check your answer.)

Isaac Newton Finally, in 1687 a scientist named Sir Isaac Newton explainedwhy planets revolve around the sun and why moons revolvearound planets. Newton explained that gravity keeps all ofthese objects in orbit. Newton’s laws of motion and gravitybrought together the work of Copernicus, Tycho, Kepler, andGalileo.


Modern AstronomyWith Galileo’s successful use of the telescope and Newton’sdiscoveries about planetary motion, astronomy became themodern science that it is today.

Edwin HubbleIn the 1920s, astronomer Edwin Hubble began researchinggalaxies. In 1924 he focused his research on a fuzzy patch inthe sky. Hubble measured the distance of the fuzzy patch fromEarth and determined that the fuzzy patch was beyond ourgalaxy, the Milky Way. Hubble identified the patch as anothergalaxy. This meant that the universe is much larger than justour galaxy. You will learn more about galaxies in Section 3.

Going FurtherToday astronomers still gaze at the skyand try to assign order to the universe.Larger and better telescopes and super-computers, as shown in Figure 6, as well as spacecraft allow us to studyobjects both near and far. Many ques-tions about the universe have beenanswered, but our studies continue toraise new questions.

Which ancient civilization’s calendar gave rise to ourmodern calendar?

What advantage did Galileo have over the astronomerswho lived before him?

Analyzing Relationships Is Copernicus’s theory com-pletely correct? What are some of its strengths andweaknesses? How does his theory relate to what weknow today about the sun’s position in our solar system and in the universe?

Figure 6 Today computers and telescopesare linked together. Computers not only control telescopes but also process the information gathered by the telescopes so that astronomers may better analyze it.

TOPIC: Johnson Space CentersciLINKS NUMBER: HSTX090

TOPIC: The Stars and Keeping TimesciLINKS NUMBER: HSTE430

TOPIC: Early Theories in AstronomysciLINKS NUMBER: HSTE435

Studying the Universe 597Copyright © by Holt, Rinehart and Winston. All rights reserved.

TelescopesIf you’ve ever been to an eye doctor, you’ve probably hadto read the letters on an eye chart. The letters on the toprow are big, but as you go down the chart, the letters getsmaller. You probably had to use lenses to read the tinyletters.

When astronomers look up at the night sky, they experiencethe same difficulty. Thousands of stars in the sky are easilyseen. But even more stars can’t be seen. Astronomers need atelescope to see only some of the distant stars. A telescopeis an instrument that collects light and concentrates it for better observation.

Optical AstronomyThe simplest optical telescope has two lenses. One lens, calledthe objective lens, collects light and forms an image at the backof the telescope. The bigger the objective lens, the more lightthe telescope can gather. The second lens is located in the eye-piece of the telescope. This lens magnifies the image producedby the objective lens.

Without a telescope, you can see about 6,000 stars. Usingan optical telescope, you can see millions of stars. Figure 7shows how much more you can see with an optical telescope.

Terms to Learn

telescoperefracting telescopereflecting telescopeelectromagnetic spectrum

What You’ll Do

● Compare and contrastrefracting telescopes andreflecting telescopes.

● Explain why the atmosphereis an obstacle to astronomersand how they overcome theobstacle.

● List the types of electromag-netic radiation, other thanvisible light, that astronomersuse to study space.

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Figure 7 The image at left shows a section of the sky as seen withthe unaided eye. The image at right shows what the small clusters ofstars in the left image might look like when seen through a telescope.


Refracting Telescopes Telescopes that use a set of lenses to gather and focus lightare called refracting telescopes. The curved objective lens in arefracting telescope bends light that passes through it andfocuses the light to be magnified by the eyepiece, as shownin Figure 8. A refracting telescope’s size is limited by the sizeof the objective lens. If the curved lens is too large, the glasssags under its own weight and distorts images. For this rea-son, most professional astronomers use reflecting telescopes.

Reflecting Telescopes Telescopes that use curved mirrors to gather and focus lightare called reflecting telescopes. Light enters the telescope andis reflected from a large, curved mirror to a focal point abovethe mirror. A focal point is the point on the axis of a mirroror lens through which parallel light rays are focused. As shownin Figure 8, reflecting telescopes use a second mirror in frontof the focal point to reflect the light through the eyepiece.

One advantage of reflecting telescopes over refracting tele-scopes is that mirrors can be very large, which allows them togather more light than refracting telescope lenses do. Also,mirrors are polished on their curved side, which prevents lightfrom entering the glass. Therefore, any flaws in the glass donot affect the light. A third advantage is that mirrors reflectall colors of light to the same place, while lenses focus dif-ferent colors of light at slightly different distances. Reflectingtelescopes allow all colors of light from an object to be seenin focus at the same time.


Refracting telescope

Refracting telescopes use lenses to gather and focus light.

Reflecting telescope

Reflecting telescopes use mirrors to gather and focus light.

Eyepiece

Starlight Eyepiece Starlight

Focal point

Focal point

Up or Down?

1. Using scissors, cuta circle out of darkpaper to fit theopening of agooseneck lamp.

2. Cut out an arrowshape from the cen-ter of the paper circle.

3. Tape the paper circle tothe lamp opening with the arrow pointing up.

4. Place the lamp 2 m fromthe wall, darken the room,and turn the lamp on.

5. Hold a magnifying lens30 cm from the lamp.

6. Move the magnifying lensaround until a clear imageis projected on the wall.

7. Why is the image on thewall upside down?

Figure 8 Optical Telescopes


Very Large Reflecting Telescopes In some very large reflecting telescopes, several mirrors worktogether to collect light and deliver it to the same focal point.The Keck Telescopes, in Hawaii, shown in Figure 9, are twintelescopes that each have 36 hexagonal mirrors workingtogether. Linking several mirrors allows more light to be col-lected and focused in one spot.

Optical Telescopes and the Atmosphere The light gathered by telescopes on Earth is affected by theatmosphere. Earth’s atmosphere causes starlight to shimmerand blur. Also, light pollution from large cities limits anobserver’s ability to view faint objects. Astronomers often place telescopes in dry areas to avoid moisture in the air.Mountaintops are also good places to use a telescope becausethe air is thinner at higher elevations. The fact that there isgenerally less air pollution and light pollution on mountain-tops also increases the visibility of stars.

Optical Telescopes in Space! To avoid interference by the atmosphere al-together, scientists have put telescopes inspace. Although the mirror in the HubbleSpace Telescope, shown in Figure 10, is only2.4 m across, the optical telescope producesimages that are as good as or better than anyimages produced by optical telescopes onEarth.

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Figure 9 The 36 hexagonal mirrorsin each of the Keck Telescopes combine to form a light-reflectingsurface that is 10 m across.

Each hexagon is 1.8 min diameter.

Figure 10 The Hubble Space Telescope providesclearer images of objects in deep space than anyground-based optical telescope can.

Chapter 22Copyright © by Holt, Rinehart and Winston. All rights reserved.

Nonoptical AstronomyFor thousands of years, humans have observed the universewith their eyes. But scientists eventually discovered that thereare more forms of radiation than the kind we can see—visiblelight.

In 1864, James Clerk Maxwell showed that visible light isa form of electromagnetic radiation. Electromagnetic radiationincludes all forms of energy that travel through space as waves.Each color of visible light represents a different wavelength of electromagnetic radiation. Visible light is just a small partof the electromagnetic spectrum, as shown in Figure 11. Theelectromagnetic spectrum is made of all of the wavelengths ofelectromagnetic radiation. Most of the electromagnetic spec-trum is invisible to our eyes.

Earth’s atmosphere blocks most electromagnetic radiation.Think of the atmosphere as a screen that lets in only certainwavelengths of radiation. The atmosphere lets in some ultra-violet and infrared wavelengths as well as all visible light andradio wavelengths. All other wavelengths are blocked.


InfraredRadio waves Micro-waves

Gamma raysUltra-violet

X rays

Visible

Figure 11 Radio waves have the longest wavelengths, andgamma rays have the shortest wavelengths. Visible light isonly a small band of the electromagnetic spectrum.

Artificial light at night is oftenneeded for safety and security.But it also causes light pollutionthat interferes with stargazing. Dosome research on this problem,and list some possible solutions.What compromises can be madeso that people feel safe andstargazers can see objects in thenight sky?


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Figure 12 The Milky Way at Different Wavelengths

Near infrared

Radio

X ray

Gamma ray

The Night Sky Through Different Eyes Astronomers are interested in all forms of electromagnetic radiation because different objects are detected at differentwavelengths. A different type of telescope is required to detecteach type of radiation. For example, infrared telescopes havepolished mirrors similar to those of reflecting telescopes, butinfrared telescopes are more sensitive to infrared waves thanto visible light waves. As you can see in Figure 12, the uni-verse looks much different when observed at other wavelengths.Each image shows the night sky as it would appear if we couldsee other wavelengths of electromagnetic radiation. The “cloud”that goes across each picture is the Milky Way galaxy.

Radio Telescopes Radio telescopes receive and focus radio waves. Radio telescopeshave to be much larger than optical telescopes because radiowavelengths are about 1 million times longer than optical wavelengths. Also, very little radio radiation reaches Earth from objects in space. Radio telescopes must be very sensitiveto detect these faint radio waves.

The surface of a radio telescope doesnot have to be as flawless as the lensof an optical telescope. In fact, the sur-face of a radio telescope does not evenhave to be completely solid. When itwas first built, the Arecibo radio tele-scope, shown in Figure 13, was coveredwith chicken wire! To a radio wave, asurface made of chicken wire is solidbecause its wavelength is so muchlonger than the diameter of the holes.

Figure 13 The Arecibo radio telescope in Puerto Rico is 305 macross. That distance is about thelength of three football fieldsarranged end to end!


Linking Radio Telescopes Together Astronomers can get clearer images of radio wavesby using two or more radio telescopes at the sametime. When radio telescopes are linked together, theywork like a single giant telescope. For example, theVery Large Array (VLA), shown in Figure 14, consistsof 27 separate telescopes that can be spread out over30 km. When the dishes are spread out to the maxi-mum distance, they work as a single telescope thatis 30 km across! The larger the area that linked tele-scopes cover, the more detailed the collected data are.

X-Ray Vision Most electromagnetic waves are blocked by the Earth’satmosphere. To detect these blocked waves, scientistshave put special telescopes in space. These telescopesinclude ultraviolet telescopes, infrared telescopes,gamma-ray telescopes, and X-ray telescopes. Each typeof telescope is made to receive one type of radiation.For example, Figure 15 is an illustration of a tele-scope that is designed to detect X rays.


Name one way refracting telescopes and reflecting telescopes are similar and one way they are different.

Name two ways the atmosphere limits whatastronomers can detect. What single method doastronomers use to solve both problems?

Summarizing Data Construct a table with twocolumns—one for electromagnetic wavelengths thatcommonly penetrate Earth’s atmosphere and one for other wavelengths. Which wavelengths canastronomers detect from Earth? How do they detecteach wavelength?

Figure 14 The radio telescopes of the VeryLarge Array near Socorro, New Mexico, worktogether as one giant telescope.

Figure 15 Launched in 1999, theChandra X-Ray Observatory is the mostpowerful X-ray telescope ever built.

TOPIC: Telescopes in TexassciLINKS NUMBER: HSTX200

TOPIC: TelescopessciLINKS NUMBER: HSTE445


GalaxiesYour complete address is part of a much larger systemthan just your street, city, state, country, and even theplanet Earth. You also live in the Milky Way galaxy.

Large groups of stars in space are called galaxies. Galaxies comein a variety of sizes and shapes. The largest galaxies containmore than a trillion stars. Astronomers don’t count the stars,of course. They estimate from the size and brightness of thegalaxy how many sun-sized stars the galaxy might have.

Types of GalaxiesLook again at the Hubble Deep Field image at the beginningof this chapter. You’ll notice many different types of galaxies.Edwin Hubble, the astronomer for whom the Hubble SpaceTelescope is named, began to classify galaxies, mostly by theirshapes, in the 1920s. We still use the galaxy classification thatHubble developed.

Spiral Galaxies Spiral galaxies are what most people think of when you saygalaxy. Spiral galaxies, such as the one in Figure 16, have abulge at the center and spiral arms. The spiral shape is causedby the rotation of the galaxy. The arms are formed from thestars at the outer edges of the galaxy.

Terms to Learn

galaxynebulaglobular clusteropen clusterquasar

What You’ll Do

● Identify the various types of galaxies.

● Describe the contents and characteristics of galaxies.

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The Andromeda galaxy, shown here, is a spiral galaxy similarto what our galaxy, the Milky Way, is thought to be.

Spiral galaxy

Figure 16 Types of Galaxies


It is hard to tell what type of galaxy we are in because thegas, dust, and stars keep us from having a good view. Observingother galaxies and making measurements inside our galaxy,the Milky Way, lead astronomers to think that Earth is in aspiral galaxy.

Elliptical Galaxies About one-third of all galaxies are simply massive blobs ofstars. Many galaxies look like spheres, while others are moreelongated. Because we don’t know how these galaxies are ori-ented, some of them could be cucumber shaped, with theround end facing us. These galaxies are called elliptical galax-ies. Elliptical galaxies have very bright centers and very littledust and gas. Elliptical galaxies contain mostly old stars. Becausethere is so little gas, few new stars are forming. Some ellipti-cal galaxies, such as M87, shown in Figure 16, are huge andare therefore called giant elliptical galaxies. Other elliptical galax-ies are much smaller and are called dwarf elliptical galaxies.

Irregular Galaxies When Hubble first classified galaxies, he had a group of left-overs. He named them “irregulars.” Irregular galaxies are galaxiesthat don’t fit into any other class. As their name suggests,irregular galaxies have an irregular shape. Many of these galax-ies are close companions of large spiral galaxies. The gravity ofthe spiral galaxies may be distorting the shape of their smallerneighbors. Figure 16 shows an irregular galaxy called the Large Magellanic Cloud.


The Large Magellanic Cloud, an irregular galaxy, islocated within our own galactic neighborhood.

Now that you know the namesthat Edwin Hubble gave to galax-ies of different shapes, look atthe names that you gave thegalaxies in the Start-Up Activity at the beginning of this chapter.Rename your types usingHubble’s names. Look for exam-ples of spiral, elliptical, and irregular galaxies. Describe thecharacteristics of these galaxies in your ScienceLog, and comparethem to the galaxies you havealready drawn.

Unlike the Milky Way, the galaxy known as M87has no spiral arms.

Elliptical galaxy Irregular galaxy


Contents of GalaxiesGalaxies are composed of billions and billions of stars andprobably some planetary systems too. Some of these stars form larger features. Among these features are gas clouds andstar clusters.

Gas Clouds The Latin word for “cloud” is nebula. In space, nebulas (ornebulae) are giant clouds of gas and dust. Some types of nebu-las glow by themselves, while others absorb light and hidestars. Still other nebulas reflect starlight and produce someamazing images. Some nebulas are regions where new starsform. Figure 17 shows part of the Eagle nebula. Spiral galax-ies generally contain nebulas, but elliptical galaxies don’t.

Star Clusters Globular clusters are groups of older stars. A globular clusterlooks like a ball of stars, as shown in Figure 18. There may be20,000 to 100,000 stars in an average globular cluster. Globularclusters are located in a spherical halo, or ring, that surroundsspiral galaxies such as the Milky Way. Globular clusters arealso common around giant elliptical galaxies.

Open clusters are groups of stars that are usually locatedalong the spiral disk of a galaxy. Newly formed open clustershave many bright, blue stars, as shown in Figure 19. There maybe a few hundred to a few thousand stars in an open cluster.

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Figure 19 The open clus-ter Pleiades is barely visiblewithout a telescope.

Figure 17 Part of a nebula inwhich stars are born is shownbelow. The fingerlike shape to the left of the bright star is slightly wider than our solar system.

Figure 18 With 5 million to 10 million stars, Omega Centauriis the largest globular cluster inthe Milky Way galaxy.


Origin of GalaxiesHow did galaxies form in the first place? To answer this ques-tion, astronomers must travel back in time and explore theearly universe through telescopes. Scientists investigate theearly universe by observing objects that are extremely far awayin space. Because it takes time for light to travel through space,looking through a telescope is like looking back in time. Thefarther out one looks, the farther back in time one sees.

Looking at distant galaxies reveals what early galaxieslooked like. Seeing early galaxies helps give scientists an ideaof how galaxies develop through time and perhaps what causesgalaxies to form in the first place. Scientists have already foundsome very strange-looking objects in the early universe.

Quasars Among the most distant objects are quasars. Quasars are star-like sources of light that are extremely far away. They are amongthe most powerful energy sources in the universe. Some sci-entists think that quasars may be the core of young galaxiesthat are in the process of forming. In Figure 20, you can see aquasar that is 6 billion light-years away. A light-year is equal tothe distance that light travels through space in 1 year. That’salmost 9.5 trillion kilometers in one year. You are seeing thisquasar as it was 6 billion years ago!


What type of galaxy is the Milky Way galaxy?

Describe the difference between an elliptical galaxy anda globular cluster.

Analyzing Relationships Suppose the quasar in Figure 20changed suddenly. How long would we have to wait tosee this change? Explain your answer.

TOPIC: GalaxiessciLINKS NUMBER: HSTE520

Figure 20 The quasar known asPKS 0637-752 gives off as muchlight as 10 trillion suns.


Formation of the UniverseExplosions. Bright light. Intense energy. Sound like anaction movie? This scene could also describe a theoryabout the formation of the universe.

Cosmology is the study of the origin, structure, and future ofthe universe. Like other scientific theories, theories about thebeginning and end of the universe must be tested by obser-vations or experiments.

Universal ExpansionTo understand how the universe formed, scientists study themovement of galaxies. Careful measurements have shown thatmost galaxies are moving apart.

A Raisin-Bread ModelTo understand how the galaxies are moving, imagine a loaf ofraisin bread before it is baked. Inside the dough, each raisinis a certain distance from every other raisin. As the dough getswarm and rises, it expands and all of the raisins begin to moveapart. No matter which raisin you observe, the other raisinsare moving farther apart from it. The universe, like the risingbread dough, is expanding. Think of the raisins as galaxies. Asthe universe expands, the galaxies move farther apart.

The Big Bang TheoryWith the discovery that the universe is expanding, scientistsbegan to wonder what it would be like to watch the forma-tion of the universe in reverse. The universe would appear tobe contracting, not expanding. All matter would eventuallycome together to a single point. Thinking about what wouldhappen if all of the matter in the universe were squeezed intosuch a small space led scientists to the big bang theory.

Terms to Learn

cosmologybig bang theory

What You’ll Do

● Describe the big bang theory.

● Explain evidence used to sup-port the big bang theory.

● Research and describe histori-cal scientific theories of theorigin of the universe.

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Figure 21 Some astronomersthink the big bang caused the universe to expand in all directions.


The big bang theory states that the universe began with atremendous explosion. According to the theory, 12 billion to 15 billion years ago all the contents of the universe gatheredtogether under extreme pressure, temperature, and density ina very tiny spot. Then the universe rapidly expanded, and mat-ter began to come together and form galaxies. The galaxiescontinued to move outward as they do today. Figure 21 illus-trates what the big bang might have looked like.


There are other scientific theoriesabout the origin of the universe.Use the Internet or library re-sources to research these othertheories. Describe in your ownwords the different theories ofthe origin of the universe. Usecharts or tables to examine andevaluate these differences.

Graphing Expansion

Suppose you decide to make some raisin bread. You wouldform a lump of dough, as shown in the top image. The lowerimage represents dough that has been rising for 2 hours. Lookat raisin B in the top image. Measure how far it is from each ofthe other raisins—A, C, D, E, F, and G—in millimeters. Now mea-sure how far each raisin has moved away from B in the lowerimage. Using a computer or graph paper, make a graph of speed(in units of millimeters per hour) versus original distance (inmillimeters). Remember that speed equals distance divided bytime. For example, if raisin E was originally 15 mm from raisinB and is now 30 mm away, it moved 15 mm in 2 hours. There-fore the speed of raisin E is 7.5 mm/h. Repeat the procedure,starting with raisin D. Plot your results on the same graph, andcompare the two results. What valid conclusion can you drawfrom the information you graphed?

A

BD F

GE

C

A

B

DF

G

E

C


Cosmic Background Radiation In 1964, two scientists, using the antenna shownin Figure 22, accidentally found radiation comingfrom all directions in space. One explanation forthis radiation is that it is cosmic background radia-tion, left over from the big bang.

To understand the connection between the bigbang theory and cosmic background radiation,think about a kitchen oven. When an oven dooris left open after the oven has been used, thermalenergy is transferred throughout the kitchen andthe oven cools. Eventually the room and the ovenare the same temperature. According to the bigbang theory, the thermal energy from the originalexplosion was distributed in every direction as theuniverse expanded. This cosmic background radia-tion fills all of space.

Dating the Universe One way scientists can calculate the age of the universe is tomeasure the distance from Earth to various galaxies. By usingthese distances, scientists can calculate the age of the universeand predict its rate of expansion. Another way to estimate theage of the universe is to calculate the ages of old, nearby stars.Because the universe must be at least as old as the oldest starsit contains, the ages of the stars provide a clue to the age ofthe universe. But according to these calculations, some starsare older than the universe! These calculations are not con-sistent with the big bang theory. Astronomers continue tosearch for evidence that will solve this puzzle.

A Big Crunch? What will eventually happen to the universe? As the galaxiesmove apart, they get older and eventually stop forming stars.The next step depends on how much matter the universe contains. One possibility is that if there is enough matter, gravity could eventually stop the expansion of the universe.The universe may even start collapsing to its original state andcause a “big crunch.”

If there is not enough matter, expansion of the universecould continue. As stars age and die, the universe would eventually become cold and dark. Some scientists suggest thatthere may not be enough matter to stop the universe fromexpanding forever, but they are still uncertain.

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Objects in very distant spacelook younger than they reallyare. In fact, we cannot even besure they still exist. If a distantgalaxy disappeared, for exam-ple, people on Earth wouldn’tknow about it for billions ofyears.

Figure 22 Robert Wilson (left) andArno Penzias (right) discovered the cosmic background radiation,which gave a big boost to the bigbang theory.


Structure of the UniverseThe universe is an amazing place. From our homeon planet Earth, the universe stretches out fartherthan we can see with our most sensitive instru-ments. The universe contains a variety of objects,some of which you have just learned about. Butthese objects are not simply scattered through theuniverse at random. The universe has a structurethat is repeated over and over again.

A Cosmic RepetitionYou already know that the Earth is a planet. Butplanets are part of planetary systems. Our solar system is the one we are most familiar with, butplanets have recently been detected in orbit aroundother stars. Scientists think that planetary systemsare actually quite common in the universe. Starsare grouped in larger systems that range from star clusters to galaxies. Galaxies are arranged ingroups that are bound together by gravity. Evengalaxy groups form galaxy clusters and super-clusters, as shown in Figure 23. The universe continues farther than the eye can see. In the universe there are great collections of galaxy clus-ters and vast empty regions of space in between.Astronomers estimate that there are between 50 billion and 1 trillion galaxies in the known part of the universe. But what lies in the unknownparts of the universe? Perhaps someday astrono-mers will be able to see the unknown parts of theuniverse. Maybe students in future classrooms willhave much more to study!


Name one observation that supports the big bang theory.

Describe how the big bang theory explains the observedexpansion of the universe.

Understanding Technology What type of telescope wouldyou use to study distant galaxies? Explain your answer.

Figure 23 The Earth is only part of a vast system of matter.

TOPIC: Structure of the UniversesciLINKS NUMBER: HSTE525


612 Chapter 22

Through the Looking GlassHave you ever looked toward the horizon or up into the sky and wishedyou could see farther? Do you think atelescope might help? Astronomers usehuge telescopes to study the universe.You can build your very own telescopeto get a glimpse of what astronomerssee with their incredible equipment.

Ask a Question

1 How can you build a telescope to produce a sharp image on a piece of paper?

Form a Hypothesis

2 In your ScienceLog, formulate a testable hypoth-esis about how to build a telescope.

Conduct an Experiment

3 Use modeling clay to form a base to hold one of the lenses upright on your desktop. When the lights are turned off, your teacher will turn on a lamp at the front of the class-room. Rotate your lens so that the light fromthe lamp passes through it.

• modeling clay

• 2 convex lenses, 3 cm in diameter

• desk lamp

• sheet of constructionpaper

• metric ruler

• cardboard wrapping-paper tube

• cardboard toilet-papertube

• scissors

• masking tape


4 Hold the construction paper so that thelight passing through the lens lands on thepaper. Slowly move the paper closer to orfarther from the lens until you see thesharpest image of the light on the paper.Hold the paper in this position.

5 Using the metric ruler, measure the dis-tance between the lens and the paper.Record this distance in your ScienceLog.

6 How far is the paper from the lens? Thisdistance, called the focal length, is how farthe paper has to be from the lens for theimage to be in focus.

7 Repeat steps 3–6 using the other lens.

8 From one end of the long cardboard tube,measure and mark the focal length of thelens with the longer focal length. Place amark 2 cm past this line toward the otherend of the tube, and label the mark “Cut.”

9 From one end of the short cardboard tube,measure and mark the focal length of thelens with the shorter focal length. Place amark 2 cm past this line toward the otherend of the tube, and label the mark “Cut.”

10 Shorten the tubes by cutting along themarks labeled “Cut.” Wear safety goggleswhen making this cut.

11 Tape the lens with the longer focal lengthto one end of the longer tube. Tape theother lens to one end of the shorter tube.Slip one tube inside the other. Be sure the lenses are at each end of this new,longer tube.


12 Congratulations! You have just constructeda telescope! To use your telescope, lookthrough the short tube (the eyepiece) andpoint the long end at various objects in theroom. You can focus the telescope byadjusting its length. Are the images rightside up or upside down? Observe birds,insects, trees, or other outside objects. Inyour ScienceLog, record the images you see. Caution: NEVER look directly at the sun! Looking directly at the sun couldcause permanent blindness.

Analyze the Results

13 Which type of telescope did you just construct—a refracting telescope or areflecting telescope? What makes it onetype and not the other?

14 What differences did you see when observ-ing the images through the telescope ver-sus looking at them with your naked eye?Record these differences in yourScienceLog.

Draw Conclusions

15 Would upside-down images negativelyaffect astronomers looking at stars through theirtelescopes? Explain your answer.


Chapter 22614

Section 1

Vocabularyuniverse (p. 594)astronomy (p. 594)year (p. 594)month (p. 594)day (p. 594)leap year (p. 594)

Section Notes

•Calendars are based onmovements of objects inthe sky.

•The Ptolemaic theory statesthat Earth is at the center ofthe universe, while Coper-nicus’s theory states that thesun is at the center of theuniverse.

•Isaac Newton was the firstscientist to explain whycelestial objects move asthey do.

•Galileo’s use of the tele-scope brought the technol-ogy of astronomy to a newlevel.

LabBookThe Sun’s Yearly Trip

Through the Zodiac (p. 694)

Section 2

Vocabularytelescope (p. 598)refracting telescope (p. 599)reflecting telescope (p. 599)electromagnetic

spectrum (p. 601)

Section Notes

•Telescopes collect and focuselectromagnetic radiation.

•Humans can see only vis-ible light. To detect other wavelengths of radiation,astronomers use special telescopes.

•Types of telescopes includeoptical, radio, ultraviolet,infrared, X-ray, and gamma-ray.

•Some telescopes are put inspace to avoid the effects of Earth’s atmosphere or tocollect radiation that can’tpenetrate Earth’s atmosphere.

Section 3

Vocabularygalaxy (p. 604)nebula (p. 606)globular cluster (p. 606)open cluster (p. 606)quasar (p. 607)

Section Notes

•Edwin Hubble classifiedgalaxies according to theirshape including spiral, elliptical, and irregular galaxies.

•A nebula is a cloud of gasand dust. New stars are bornin some nebulas.

•Open clusters are groups ofstars located along the spiraldisk of a galaxy. Globularstar clusters are found in thehalos of spiral galaxies andin elliptical galaxies.

•Scientists look at distantgalaxies to learn what earlygalaxies looked like.

Section 4

Vocabularycosmology (p. 608)big bang theory (p. 609)

Section Notes

•Observations show that the universe is expandingoutward.

•The big bang theory statesthat the universe began withan explosion about 12 billionto 15 billion years ago.

•Some scientists think thatcosmic background radiationfills the universe with radia-tion that is left over fromthe big bang. Cosmic back-ground radiation providessupport for the big bangtheory.

•All matter in the universe,from planets to superclustersof galaxies, is a part of largersystems.


615Studying the Universe

USING VOCABULARY

For each pair of terms, explain how the mean-ings of the terms differ.

1. leap year/year

2. reflecting telescope/refracting telescope

3. X rays/microwaves

4. elliptical galaxy/spiral galaxy

5. big bang/cosmology

UNDERSTANDING CONCEPTS

Multiple Choice

6. According to ? , the Earth is at the center of the universe.a. the Ptolemaic theoryb. Copernicus’s theoryc. Galileo’s theoryd. None of the above

7. In which part of a spiral galaxy do youexpect to find nebulas?a. the spiral armsb. the central regionc. the halod. all parts of the galaxy

8. Telescopes that work best from Earth’s sur-face include all of the following EXCEPTa. radio telescopes.b. refracting telescopes.c. X-ray telescopes.d. reflecting telescopes.

9. Which of the following is true about X-ray and radio radiation from objects in space?a. Both types of radiation can be observed

using the same telescope.b. Separate telescopes are needed to

observe each type of radiation, andboth telescopes can be on Earth.

c. Separate telescopes are needed toobserve each type of radiation, andboth telescopes must be in space.

d. Separate telescopes are needed toobserve each type of radiation, but onlyone of the telescopes must be in space.

10. The first scientist to successfully use a telescope to observe the night sky wasa. Tycho. c. Herschel.b. Galileo. d. Kepler.

11. Which statement about the big bang theory is true?a. The universe will never end.b. New matter is continuously being

created in the universe.c. The universe is filled with radiation

coming from all directions in space.d. We can locate the center of the

universe.

Short Answer

12. How does a reflecting telescope work?

13. Name six types of astronomical objects inthe universe. Arrange them by size.

14. What does the big bang theory have to say about how the universe will end?


Chapter 22616

MATH IN SCIENCE

20. The brightest star seen from Earth isSirius. It is 8.6 light-years away. The North Star is 431 light-years away. What is the difference in the distance expressed inlight-years between these two celestialobjects? What is this distance expressed in kilometers? (Hint: One light-year isequal to 9.46 trillion kilometers.)

INTERPRETING GRAPHICS

The data in the following graph was used byEdwin Hubble in 1931 to compare how fargalaxies are from us and how fast they aremoving away from us.

21. Look at galaxy A in the graph. What is itsspeed and distance?

22. If a new galaxy with a speed of 15,000 km/s were found, at what distance would you expect it to be?

Take a minute to reviewyour answers to the Pre-Reading Questionsfound at the bottom

of page 592. Have your answers changed? If necessary, revise your answers based on whatyou have learned since you began this chapter.

ReadingCheck-up

CONCEPT MAPPING

15. Use the following terms to create a concept map: spiral galaxy, globular clusters, nebulas, galaxy,elliptical galaxy, and irregular galaxy.

CRITICAL THINKING AND PROBLEM SOLVING

Write one or two sentences to answer the following questions:

16. Why was it easier for people in ancientcultures to see celestial objects in the skythan it is for most people today?

17. Because many forms of radiation do notpenetrate Earth’s atmosphere, astrono-mers’ activities are limited but humansbenefit in general. Explain how.

18. While looking at a galaxy through a nearby university’s telescope, you notice that there are no blue stars present. What kind of galaxy is it most likely to be?

19. Explain in your own words the big bangtheory of the formation of the universe.Describe a strength and a weakness of thetheory.

A

0 30 60 90 120

Distance (millions of light-years)

Spee

d (k

m/s

)Galaxy Speed Vs. Distance

20,000

15,000

5,000

10,000


617Studying the Universe

1 What type of galaxy is the Andromedagalaxy? A EllipticalB CircularC SpiralD Irregular

2 You visited an observatory and lookedthrough the telescope. You saw a ball ofstars through the telescope. What typeof object did you see? F Spiral galaxyG Open clusterH Globular clusterJ Irregular galaxy

3 All of the following are contained in agalaxy EXCEPT A globular cluster.B an open cluster.C a gas cloud.D the universe.

4 Which of the following is NOT a charac-teristic of elliptical galaxies? F They contain very little dust and gas.G They contain mostly old stars.H They contain lots of gas clouds.J Many look like spheres.

5 The graph below is based on collecteddata. According to the graph, how longdoes a star with 1.2 times the mass ofthe sun live?

Chapter 22

Relationship Between Age and Mass of a Star

Life

time

of s

tar

(bill

ions

of y

ears

)

Mass of star (compared to sun)

15

0

5

10

2 3 41

Sun

A 10 billion yearsB 8 billion yearsC 5 billion yearsD 6 billion years

6 The most widely accepted theory of theformation of the universe is F Copernicus’s theory.G the big bang theory.H cosmology.J Kepler’s theory.


Chapter 22618

1 According to the chart at left, what wasthe mean number of stargazers at the starparty during these 4 months?A 45B 48C 44D 52

2 Star A is 5 million kilometers from Star B.What is this distance expressed in meters?F 0.5 mG 5 � 109 mH 5 � 106 mJ 5000 m

Quasars are some of the most puzzlingobjects in the sky. Viewed through anoptical telescope, a quasar appears as asmall, faint star. Quasars are the mostdistant objects that have been observedfrom Earth. Yet many quasars are hundredsof times brighter than the brightest galaxy.Because quasars are so far away and theyare so bright, they must emit a largeamount of energy. Scientists do not yetunderstand exactly how quasars can emitso much energy.

1 Which of these statements is a FACT inthis passage?A Quasars, unlike galaxies, include bil-

lions of bright objects.B Galaxies are brighter than quasars.C Quasars are hundreds of times

brighter than the brightest galaxy.D Galaxies are the most distant objects

observed from Earth.

2 Based on the information in the passage,the reader can conclude thatF quasars are the same as galaxies.G quasars appear as small, faint stars,

but they emit a large amount of energy.

H quasars can be viewed only with anoptical telescope.

J quasars will never be understood.

Read the passage. Then read each question that follows the passage. Decide which is the best answer to each question.

Math

Reading

Chapter 22

Attendance at Star Party

Month Number of stargazers

January 36

February 52

March 44

April 48


At Cambridge University in 1967, Jocelyn Bell-Burnell and heradviser, Antony Hewish, completed work on a huge radio tele-

scope designed to pick up signals from quasars. Bell-Burnell’s jobwas to operate the telescope and analyze the “chart paper” record-ings of the telescope on a graph. Each day, the telescope recorded29.2 m of chart paper! After a month, Bell-Burnell noticed a few“bits of scruff” that she could not explain—they were very short,pulsating radio signals. The signals were only 6.3 mm long, andthey happened only once every 4 days. What Bell-Burnell had acci-dentally found was a needle in a cosmic haystack!

LGM 1Bell-Burnell and Hewish struggled to find the source of this mys-terious new signal. They double-checked the equipment andbegan eliminating all of the possible sources of the signal, such as satellites, television, and radar. Because they could not rule out that the signal was coming from aliens, Bell-Burnell andHewish called it LGM 1. Can you guess why? LGM stood for Little Green Men!

The Answer: Neutron StarsShortly after finding the first signal, Bell-Burnell discovered yetanother strange, pulsing signal within the vast quantity of chartpaper. This signal was similar to the first, except that it came fromthe other side of the sky. To Bell-Burnell, this second signal was

Jocelyn Bell-Burnell becameinterested in astronomy at anearly age. As a research stu-dent at Cambridge University,Bell-Burnell discovered pul-sars, celestial objects that emitradio waves at short and regu-lar intervals. Today Bell-Burnellis a leading expert in the fieldof astrophysics and the studyof stars. She is currently headof the physics department atthe Open University, in MiltonKeynes, England.

A S T R O P H Y S I C I S T

exciting because it meant that her first signal was not of local origin and that she had stumbledon a new and unknown signal from space. By January 1968, Bell-Burnell had discovered two morepulsating signals. In March of that year, her findings were published and amazed the scientificcommunity. The scientific press named thenewly discovered stars pulsars, from pulsatingradio stars. Bell-Burnell and other scientistsdecided that her “bits of scruff” were causedby rapidly spinning neutron stars.

Star Tracking� Pick out a bright star in the sky, and keep arecord of its position in relation to a referencepoint, such as a tree or building. Each night,record what time the star appears at thispoint in the sky. Do you notice a pattern?Where will it be next week?

� An artist’s depiction of a pulsar

619Copyright © by Holt, Rinehart and Winston. All rights reserved.

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