20 The Universe - Grygla Public School...The Fate of Stars 2 The Milky Way and Other Galaxies Galaxies Types of Galaxies How Galaxies Evolve 3 Origin of the Universe ... they fuse

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The UniverseThe Universe

1 The Life and Death of StarsWhat Are Stars?Studying StarsThe Fate of Stars

2 The Milky Way and Other GalaxiesGalaxiesTypes of GalaxiesHow Galaxies Evolve

3 Origin of the UniverseWhat Is the Universe?What Happened at the Beginning?Predicting the Future of the Universe

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C H A P T E R 20

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

The Very Large Array inNew Mexico can makedetailed images ofastronomical objectssuch as the CrabNebula, which scien-tists think is theremains of a super-nova explosion.

Background Optical telescopes take pictures of objects by col-lecting visible light. Visible light is only a small part of the electro-magnetic spectrum. Radio waves have longer wavelengths thanthe light waves that allow us to see with our eyes. In 1932, scien-tists discovered that astronomical objects emit radio waves. Radiotelescopes are sophisticated systems that collect the radio wavesemitted by astronomical objects. One of the largest radio tele-scopes is the Very Large Array in New Mexico. It is composed of27 dish antennas that work together as a single instrument.Unlike your radio at home, a radio telescope does not convertradio waves to sound. The radio waves are processed by a com-puter to form a picture such as the image of the Crab Nebulashown below. When scientists combine the data coming from the27 radio telescopes, the data are as precise as if they had comefrom an antenna 36 km in diameter!

Activity 1 Examine the photograph of the Very Large Array. Lookat the sky behind the antennas—do you see star trails due to therotation of Earth? Knowing that Earth rotates once every 24 h,estimate how many degrees a star travels in the night sky in 1 h.(Hint: One rotation is 360˚.)

Activity 2 Turn to Appendix B, and locate the star maps. Pick aconstellation and brainstorm how it got its name. You may needto look up the name in a dictionary to find out what the namemeans in its original language. Write a few paragraphs thatexplain your hypothesis.

Pre-Reading Questions1. Name as many celestial objects as you can

think of. Which objects are outside oursolar system?

2. Where would you find information aboutthe locations of the objects that younamed in question 1?

ACTIVITYACTIVITYFocusFocus

www.scilinks.orgTopic: Radioastronomy SciLinks code: HK4116

665Copyright © by Holt, Rinehart and Winston. All rights reserved.

The Life and Death of Stars

> Describe the basic structure and properties of stars.> Explain how the surface temperature of a star is measured.> Recognize that all normal stars are powered by fusion reactions

that form elements.> Identify the stages in the evolution of stars.

O B J E C T I V E S

SECTION

1

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K E Y T E R M S

starlight-yearred giantwhite dwarfsupernovablack hole

▲

star a large celestial bodythat is composed of gas andthat emits light; the sun is atypical star

light-year the distance thatlight travels in one year; about9.5 trillion kilometers

▲▲

Figure 1The stars in the constellation

Orion looked like the shape of ahunter to the ancient Greeks.

A

Distance (ly)0 200 400 600 800 1,000 1,200 1,400

Betelgeuse325 ly

Rigel910 ly

On a clear night, you can see about 6000 stars. People haveobserved stars for thousands of years, but only in the last

100 years have we begun to understand the life of stars.

What Are Stars?are huge spheres of very hot gas that emit light and other

radiation. The nearest star to Earth is the sun. Ancient Greek sci-entists thought that the stars were attached to a large, invisiblesphere. The Greeks also grouped stars into shapes and patternscalled constellations. Today, we still use constellations to groupstars, such as those in the constellation Orion, shown in Figure 1.Since ancient times, we have learned that stars are located at dif-ferent distances from Earth. We use the unit (ly) todescribe a star’s distance from Earth. One light-year is the dis-tance that light travels in one year, or 9.5 � 1015 m.

light-year

Stars

The stars in Orion, which appear close together whenviewed from Earth, are located at different distances fromus and from each other.

B


Stars are driven by nuclear fusion reactionsA star is a huge sphere of very hot hydrogen and helium gas thatemits light. A star is held together by the enormous gravitationalforces that result from its own mass. Inside the core, or middle,of a star, these forces create a harsh environment. The pressureis more than a billion times the atmospheric pressure on Earth.The temperature is hotter than 15 million kelvins, and the den-sity is more than 13 times the density of lead.

Nuclear fusion takes place in the core. Fusion combines thenuclei of hydrogen atoms into helium. Positively charged parti-cles, such as the nuclei of hydrogen atoms, normally repel eachother, but inside a star, where the temperature and pressure arevery high, these particles collide at high speeds. When they col-lide, they fuse together to form new nuclei called deuterons,which have one proton and one neutron. Next, two deuteronscollide to form the nucleus of a helium atom. When two particlesfuse, energy is released. The energy from these fusion reactionscreates outward pressure that balances the inward pull of gravity.

Energy moves slowly through the layers of a starFigure 2 shows the layers of the sun. Other stars have similarstructures, although the temperatures and depths of the layersmay differ. Energy moves through the layers of a star by a com-bination of radiation and convection. During convection, risinghot gas moves upward, away from the star’s center, and cooler,denser gas sinks toward the center.During radiation, energy is trans-ferred to individual atoms. The atomsabsorb the energy and then transfer itto other atoms in random directions.Atoms near the star’s surface radiateenergy into space.

The energy from a nuclear fusionreaction may take millions of years towork its way through a star. When theenergy finally reaches the surface, itis released into space as radiation andlight.

Once light leaves the surface of a star, it radiates across space at the speed of light in a vacuum, 3 � 108 m/s. At this speed, it takeslight from the sun about eight min-utes to reach Earth.

4000 to 50 000 K

Corona2 000 000 K

Core15 000 000 K

Radiative zone2 500 000 K

Convective zone1 000 000 K

Photosphere6000 K

Figure 2Energy released by fusion reac-tions in the core slowly works its way through the layers of thesun by the forces of radiation and convection.

T H E U N I V E R S E 667

ACTIVITYACTIVITYQuickQuickQuick

Quick

Using a Star ChartLocate the following stars onthe star chart in Appendix B:Betelgeuse, Rigel, Sirius,Capella, and Aldebaran.Name the constellation towhich each star belongs.Which of these stars appearsclosest in the sky to Polaris,the North Star?


Studying StarsAlthough the ancient Greeks noticed that stars had color anddivided stars by their apparent brightness, astronomers did notreally begin to learn about the nature of stars until after theinvention of the optical telescope.

Why do some stars appear brighter than others?The brightness of a star depends on the star’s temperature, size,and distance from Earth. The brightest star in the night sky isSirius in the constellation Canis Major, which is shown in Figure 3.Sirius appears so bright because it is relatively close to Earth,only about 9 ly away. The surface temperature of Sirius is about10 000 K. The sun’s surface is only 6000 K, but the sun is so closeto Earth that it dominates the sky during the day.

We learn about stars by studying lightWhen we look with our eyes or use binoculars, as in Figure 4, wedetect only light in the visible part of the spectrum. But stars alsoproduce other wavelengths of electromagnetic radiation, fromhigh-energy X rays to low-energy radio waves. Scientists useoptical telescopes to study visible light and radio telescopes tostudy radio waves emitted from astronomical objects. Earth’satmosphere blocks other wavelengths, so telescopes in space areused to study a wider range of the electromagnetic spectrum.

Figure 4You can observe some stars and constellations more easilywith binoculars than with theunaided eye.

Figure 3Sirius is the brightest star in thenight sky, and it is shown on themouth of the larger dog on thisstar chart that dates back to 1725.

668 C H A P T E R 2 0 Copyright © by Holt, Rinehart and Winston. All rights reserved.

A star’s color is related to its temperatureWhen light from a glowing hot object passes through a prism, itgenerates a spectrum of many colors. This spectrum changeswith temperature in a definite way: hotter objects glow with lightthat is more intense and that has shorter wavelengths (closer tothe blue end of the spectrum), while the light from cooler objectshas greater intensity and longer wavelengths (closer to red).

Although the light from a glowing object contains many colors, the color that we see when we look directly at a hot objectis determined mainly by the wavelength at which the object emitsthe most light. Figure 5 is a graph that shows the intensity, orbrightness, of light at different wavelengths for three stars. Thesun appears yellow because the peak wavelength of the sun isnear the color yellow. Yellow also corresponds to a temperaturenear 6000 K. Hot stars emit more energy at every wavelengththan cooler stars do.

Spectral lines reveal the composition of starsHow do we know what stars are made of? The spectra of moststars have dark lines. These dark lines are caused by gases in theouter layers of the stars that absorb the light at these wave-lengths. The temperature of these outer layers determines whichgases produce spectral lines. For example, cool hydrogen has nospectral lines. Because each element produces a unique patternof spectral lines, astronomers can match the dark lines instarlight to the known lines of elements found on Earth. Figure 6shows how the spectral lines of both hydrogen and helium can befound in a star’s spectrum.

Astronomers have analyzed more than 20 000 lines in thesun’s spectrum to find the composition of its atmosphere. Likethe composition of most stars of its age, the sun’s mass is71% hydrogen, 27% helium, and 2% other elements.


Figure 5This graph shows the intensity oflight at different wavelengths forthe sun and two other stars.

Figure 6When light is passed throughhydrogen gas , or helium gas , then through a slit andprism, dark lines appear in thespectrum. If both hydrogen andhelium are present, both sets of lines appear .B

C

A

Wavelength

Inte

nsit

y

Visi

ble

Ultr

avio

let

Infr

ared

Radiation from a blue star

(7500 K)

Radiation from a red star (4500 K)

V I B G Y O R

Radiation from the sun(6000 K)

A

B

C


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Figure 7New stars are constantly beingformed in clouds of gas and dustsuch as these columns in theEagle Nebula.

The Fate of StarsFigure 7 shows stars being formed in a cloud of gas and dustcalled a nebula. Stars are born, go through different stages ofdevelopment, and eventually die. Stars appear different from one another in part because they are at different stages in theirlife cycles. Nearly 90% of all stars in our galaxy, including thesun, are in midlife, still converting hydrogen into helium in theirinteriors.

Some stars, such as Rigel, are younger than the sun, whileothers, such as Betelgeuse, are farther along in their life cycles.Some objects in the universe are remnants of very old stars thatdied long ago. But how do stars form? And how do they keep onshining for billions of years?

The sun formed from a cloud of gas and dustAbout 5 billion years ago, in an arm of the Milky Way galaxy, athin, invisible cloud of gas and dust collapsed inward, pulled bythe force of the cloud’s own gravity. As the cloud fell together, itbegan to spin. The smaller the cloud became, the faster it spun.About 30 million years after the cloud started to collapse, thecenter of the cloud reached a temperature of 15 million kelvins.

Electrons were then stripped from hydrogen atoms to leavehydrogen nuclei, which are positively charged protons. Recallthat positively charged particles repel each other. But at very hightemperatures, protons may get as close to each other as 10–15 m.At such a small distance, the strong nuclear force overpowers theelectrical repulsion. Through this process of nuclear fusion, theprotons combine to form helium. Scientists think that once thisprocess of nuclear fusion started in the core of the cloud, the starwe call the sun turned on.

The sun now has a balance of inward and outward forcesThe fusion reactions in the core of the sun produce an outwardforce that balances the inward force due to gravity. With thesetwo forces evenly balanced, the sun has maintained an equilib-rium for 5 billion years.

The sun is now in the prime of its life; its core is actively con-verting hydrogen into helium. Over time, the percentage of thecore that is helium becomes larger. Eventually, the core will runout of hydrogen, and the fusion reactions that turn hydrogen intohelium will slow down. When these reactions slow down, the sunwill begin to die. Scientists estimate that the sun can continuenuclear fusion for another 5 billion years.

www.scilinks.orgTopic: How Stars EvolveSciLinks code: HK4069


The sun will become a red giant before it diesAs fusion slows, the pressure in the core of the sun will drop andthe core will contract, which will cause the core temperature torise. The sun’s outer layers will expand, and the sun will becomea like the one shown in Figure 8. The star is redbecause its surface is cooler, but the core is hot enough to con-vert helium into carbon and oxygen.

After about 100 million years, the core of the red giant sunwill run out of helium and will contract further, which will causethe outer layers to expand again. At this point, the temperatureat the core is not high enough to fuse these heavier elements. Theouter layers will continue to expand out from the core andwill eventually leave the star. The remnant will become a

a small and very dense star about the size ofEarth. White dwarfs no longer fuse elements, so they slowly cool.Stars with a mass of 1.4 solar masses or smaller will have a sim-ilar life cycle. Most stars in our galaxy will end as white dwarfs.

Supergiant stars explode in supernovasMassive stars evolve faster than smaller stars do. They alsodevelop hotter cores that create heavier elements through fusion.Forming an iron core signals the beginning of a supergiant star’sviolent death because fusing iron atoms to make heavier el-ements requires adding rather than releasing energy. When acore becomes mostly iron, fusion stops. When fusion stops, thereis no longer any outward pressure to balance the gravitationalforce. The core collapses because of its own gravity and thenrebounds with a shock wave that violently blows the star’s outerlayers away from the core. The resulting huge, bright explosionis called a Type II shown in Figure 9. Elements heav-ier than iron (such as gold and lead) form during a supernova. AType I supernova occurs when a white dwarf in a binary system(a system composed of two stars) collects enough mass from itscompanion to exceed 1.4 solar masses.

supernova,

white dwarf,

red giant


red giant a large, reddishstar late in its life cycle

white dwarf a small, hot,dim star that is the leftovercenter of an old star

supernova a giganticexplosion in which a mas-sive star collapses andthrows its outer layers intospace; plural supernovae

▲▲

▲

Figure 9Supernova 1987A, a Type II super-nova, was the first supernova vis-ible to the unaided eye in 400years. The first image shows whatthe original star looked like beforethe explosion.

Red giant sun

Present sun

Earth’sorbit

Figure 8When the sun becomes a redgiant, it will expand out pastEarth’s orbit


After a Type II supernova, either a neutronstar or a black hole formsFigure 10 shows a nebular remnant of a super-nova. If the core that remains after a super-nova has a mass of 1.4 to 3 solar masses, theremnant can become a neutron star. Neutronstars are only a few kilometers in diameter,but they are very massive. A neutron star is asdense as matter in the nucleus of an atom,about 1017 kg/m3. A thimbleful of a neutronstar would weigh more than 100 million tonson Earth. Neutron stars can be detected aspulsars, or sources of pulsating radio waves.

If the leftover core has a mass that isgreater than three solar masses, it will col-lapse to form an even stranger object—a

which consists of matter so mas-sive and compressed that nothing, not even light, can escape itsgravity. Because no light can escape, a black hole cannot be seendirectly. Black holes have a powerful gravitational pull, so theycan be detected indirectly by observing the radiation of light andX rays from objects that revolve rapidly around them.

The H-R diagram shows how stars evolveIn 1911, Ejnar Hertzsprung compared the temperature andbrightness of stars and carefully plotted his data on a graph. In1913, Henry Norris Russell made similar plots. Together, the twographs form the Hertzsprung-Russell diagram, or H-R diagram,which is shown in Figure 11. The vertical axis indicates bright-ness. Absolute magnitude indicates how bright stars would be ifthey were all the same distance from Earth. The horizontal axisindicates surface temperature of the stars, with hotter tempera-tures on the left.

When stars are born, they appear as protostars on a diagonalline called the main sequence. Most stars are main sequencestars. None of them are old enough to have evolved off the mainsequence. The position of a star on the main sequence dependson the initial mass of the star. As stars age and pass through dif-ferent stages in their life cycles, their positions on the H-R dia-gram change. Because most stars spend most of their lives inmidlife, more stars appear on the main sequence than on otherparts of the H-R diagram. Red giant stars are both cool andbright, so they appear in the upper right. White dwarf stars areboth faint and hot, so they appear in the lower left.

black hole,

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black hole an object somassive and dense that noteven light can escape its gravity

▲

Figure 10The Crab Nebula is the remains ofa supernova seen by Chineseobservers in the year 1054.

Connection toSOCIAL STUDIESSOCIAL STUDIES

On July 4, 1054, a brightsupernova appeared in

the constellation Taurus. Itwas visible for three weeks.Imperial Chinese astronomersnamed it a “guest” starbecause it was new to thesky. These astronomers told

the emperor that the star’s brightness meant thatthe emperor was a person of great worth. Thissupernova may have also been observed by NativeAmericans in New Mexico and Arizona where rockpaintings have been found. Later, the remnants ofthis supernova gained the name “Crab Nebula.”



S E C T I O N 1 R E V I E W

1. Determine the distance between Polaris and Earth inmeters. Polaris is 431 ly from Earth. The speed of light is 3.0 x 108 m/s.

2. Arrange the following from smallest to largest: sun, super-nova, red giant, and white dwarf.

3. Describe the stages in the life of a star of 1 solar mass andin the life of a star of 20 solar masses.

4. Critical Thinking Which of the following elements is notlikely to be formed in the sun at some time during its life?a. helium c. oxygenb. carbon d. iron

5. Critical Thinking You and a friend are looking at the stars,and your friend says, “Stars must be shrinking becausegravity is constantly pulling their particles together.”Explain what is wrong with this reasoning.

S U M M A R Y

> Stars are spheres of gas thatproduce energy by fusion.

> The composition of stars ismeasured using spectra.

> In most stars, outward pres-sure balances the inwardpull of the star’s gravity.

> Stars smaller than 1.4 solarmasses become red giantsand then white dwarfs.

> Massive stars becomesupergiants and explode insupernovae to become neu-tron stars or black holes.

Sirius B

Sun

Polaris

Betelgeuse

Aldebaran

SiriusVega

Spica

Naos

Spectral type Spectral typeO B A F F G K M

30,000˚K 10,000˚K 7,500˚K 3,500˚K6,000˚K 5,000˚K

Abs

olut

e m

agni

tude

-10

-5

0

+5

+10

+15 Rel

ativ

e br

ight

ness

(co

mpa

red

wit

h su

n)

10,000

100

1

1/100

Canopus

Proxima Centauri

Alpha Centauri

Red giants and Supergiants

Main sequence stars

Red dwarfs

White dwarfs

Sirius B

Our sun went from a protostar to a main sequence star in tensof millions of years. It will stay on the main sequence for about10 billion years. As it becomes a red giant, it will become brighter,cooler, and redder; it will move up and to the right on the H-Rdiagram for about 100 million years. The sun will become awhite dwarf, in the lower left, about 50 million years later.

Figure 11The H-R diagram is a tool thatastronomers use to help themunderstand how stars change overtime.


The Milky Way and Other Galaxies

> Define galaxy, and identify Earth’s home galaxy.> Describe two characteristics of a spiral galaxy. > Distinguish between the three types of galaxies.> Describe two aspects of a quasar, and identify the tools

scientists use to study quasars.

Imagine that you are in a special space ship that allows you toleave Earth, travel through the solar system to nearby stars,

and explore all of space. What do you imagine you will seebeyond our solar system?

GalaxiesWhile the nearest stars are a few light-years away, the nearestgalaxy to our own is millions of light-years from Earth. A

is a collection of millions or billions of stars. The deeperscientists look into space, the more galaxies they find. There maybe more than 100 billion galaxies. If you counted 1000 galaxiesper night, it would take 275 000 years to count all of them.

Galaxies contain millions or billions of starsGalaxies, such as the Andromeda Galaxy shown inFigure 12, contain millions to billions of starsbound together by gravity. Because stars age atdifferent rates, a galaxy may contain many typesof stars. Young stars are often found near the nebu-lar gas and dust where they were born. Older starsmay be throughout the galaxy or in regions thatcontain no gas and dust. Although galaxies con-tain many stars, scientists do not expect to be ableto observe stellar systems within other galaxies.The distances to other galaxies are so large thatsearches for other planets focus on nearby stars,usually within our own galaxy.

galaxy

O B J E C T I V E S

SECTION

2

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K E Y T E R M S

galaxyclusterinterstellar matterquasar

O B J E C T I V E S

▲

galaxy a collection of stars,dust, and gas bound togetherby gravity

▲

Figure 12The Andromeda Galaxy is 2.2 mil-lion ly from Earth. From a darklocation, this galaxy is visible tothe unaided eye as a faint blur.


Gravity holds galaxies together in clustersWithout gravity, everything in space might be a veil ofgas spread out through space. With gravity, clouds ofgas come together and collapse to form stars. After thefirst stars in a galaxy age, throwing off gas and dust orbecoming supernovae, new stars form. The gas, dust,and stars collapse into galaxies because of gravity.

Galaxies are not spread out evenly through space.They are grouped together in like the oneshown in Figure 13. The members of a cluster ofgalaxies are bound together by gravity. The MilkyWay galaxy and the Andromeda galaxy are two of thelargest members of the Local Group, a cluster ofmore than 30 galaxies. New members of the LocalGroup are being discovered as new telescopes, such as theHubble Space Telescope shown in Figure 14, become available toastronomers.

Clusters of galaxies can form even larger groups called superclusters. A typical supercluster contains thousands of gal-axies containing trillions of stars in individual clusters. Super-clusters can be as large as 100 million ly across. They are thelargest structures in the universe.

clusters


cluster a group of stars orgalaxies bound by gravity

▲

Edwin Hubble used the telescopes atMount Wilson Observatory in California toexplore galaxies beyond the Milky Way galaxy.

A The Hubble Space Telescope, shown here beinglaunched from the space shuttle, now probes thedepths of the universe from its orbit above Earth.

B

Figure 14

Figure 13The Hercules Cluster of galaxies is650 million ly from Earth.


Types of GalaxiesEdwin Hubble divided all galaxies into three major types: spiral,elliptical, and irregular. All three types have many stars, but theyhave different structures. Spiral galaxies have spiral arms madeof gas, dust, and stars. Elliptical galaxies have little gas or dust.Irregular galaxies do not have a particular shape.

We live in the Milky Way galaxyIf you live away from bright outdoor lights, you may be able tosee the Milky Way, a faint, narrow band of light and dark patchesacross the sky. This band, shown in Figure 15, consists of stars,gas, and dust in our galaxy, the Milky Way galaxy.

Most of the objects you can see in the night sky are part of theMilky Way galaxy. Because our solar system is inside the MilkyWay galaxy, we cannot see all of it at once. But scientists can useastronomical data to piece together a picture of the Milky Waygalaxy, such as the one shown in Figure 16. Our solar system islocated within a spiral arm, about 26 000 ly from the center, orabout half of the distance to the edge.

The Milky Way is a spiral galaxyOur galaxy is a huge spiraling disk of stars, gas, and dust. Likemost spiral galaxies, the Milky Way galaxy has a huge bulge inthe center. The nucleus of the galaxy is very dense and has manyold stars. The gas and dust have been used up to form stars.Many astronomers think that a large black hole is at the very cen-ter of our galaxy. Spiral galaxies, such as Messier 74 (M74),which is shown in Figure 17A, have gas and dust between thestars. This gas and dust is called Clouds ofinterstellar matter provide materials that allow new stars toform. Because hot young stars are blue, the spiral arms oftenappear bluish. Because old stars are often red, the bulge in themiddle appears reddish. The arms have both old and new stars aswell as gas and dust.

interstellar matter.

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Figure 15When we see the band of lightcalled the Milky Way, we are look-ing along the plane of our galaxy,the Milky Way galaxy.

interstellar matter the gasand dust located between thestars in a galaxy

▲

Figure 16An idea of what the Milky Waygalaxy might look like from theoutside can be pieced togetherfrom astronomical data.


Elliptical galaxies have no spiral armsElliptical galaxies have no spiral arms and are spherical or eggshaped. They contain mostly older stars and have little interstel-lar matter. Older stars are red, so elliptical galaxies, such as M87in Figure 17B often have a reddish color. Elliptical galaxies arefound in a wide range of sizes. Giant elliptical galaxies containtrillions of stars and can be up to 200 000 ly in diameter. Dwarfelliptical galaxies contain a few million stars and are muchsmaller.

A spiral galaxy can be recognized even when it is tilted at anangle, but because an elliptical galaxy has no regular features,scientists have trouble knowing whether an elliptical galaxy ishead-on or sideways in relation to Earth.

All other galaxies are irregular galaxiesEdwin Hubble named the third category irregular galaxiesbecause they lack regular shapes and do not have a well-definedstructure. Some irregular galaxies contain little interstellar mat-ter, while others have large amounts and contain mostly youngblue stars. Figure 17C shows the large irregular galaxy that isnearest to the Milky Way, the Large Magellanic Cloud. Thisgalaxy is a part of the Local Group of galaxies.

There are many more dwarf irregular galaxies than largeones. Dwarf galaxies are often found near larger galaxies. Someirregular galaxies may be oddly shaped because the gravitationalinfluence of nearby galaxies distorts their spiral arms.


Seen from above, the Milky Waygalaxy might look like this spiralgalaxy, named Messier 74.

A Unlike the Milky Way galaxy,elliptical galaxies such as Messier 87do not have spiral arms.

B The Magellanic Cloud is a largeirregular galaxy that is easily seenin the Southern Hemisphere.

C

Figure 17

Many new technologies havecome out of the space pro-gram. A company has recentlystarting selling a jacket madeout of the same material thatNASA uses to insulate space-craft. The material, calledaerogel, can withstand tem-peratures from –45°C to1650°C (–50°F to 3000°F), and keeps the person insidethe jacket very warm even inthe coldest weather.


How Galaxies EvolveWhen scientists observe distant galaxies, theyare looking back in time. When astronomersobserve a galaxy that is 1 billion ly away, theyare observing light that left the galaxy one bil-lion years ago. Scientists do not know whatsuch a galaxy is doing now, but by studyingcloser galaxies that might be similar to ancientones, they can slowly piece together the puzzleof how galaxies change over time.

Quasars may be infant galaxiesWhen astronomers first detected radio waves in space, they haddifficulty finding stars that accompanied some of the radiosources. In 1960, a faint object was finally matched with a strongradio signal. This object was the first or quasi-stellarobject, named for its starlike appearance, as shown in Figure 18.On further study, scientists discovered that quasars are the mostdistant and most radiant objects in space. One explanation for thestrong radiation is that each quasar has a huge central black hole(about a billion solar masses) and a large disk of gas and dustaround it. Friction in the disk releases energy into space at manywavelengths, especially radio waves. Optical telescopes showquasars embedded in faint galaxies. Quasars may be the centralparts of distant galaxies, seen as they were when very young.

quasar,

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quasar quasi-stellar radiosources; very luminous objectsthat produce energy at a highrate and that are thought tobe the most distant objects in the universe

▲

Figure 18This quasar, which is named3C273, has a powerful jet shoot-ing from it and produces moreenergy than the sun.

REAL WORLDAPPLICATIONS

REAL WORLD

Radio Telescopes and Cell Phones Electromagnetic radiation comes in many wavelengths. Radio tele-scopes detect radio waves fromspace. Radio waves are especiallyimportant in studying distant objectssuch as quasars and the interstellarmatter within galaxies. But just asoptical telescopes are affected bylight pollution, radio telescopes areaffected by unwanted radio signals.Cell phones can easily drown out

the distant signals of many galaxies.For this reason, cell phones arebanned near many radio telescopes,such as the Arecibo in Puerto Rico.

Applying Information 1. Why do cell phones affect tele-

scopes more than phones thatuse wires do?

2. What other sorts of pollution mayaffect radio telescopes? Wheremight you build a radio telescopeto avoid radio pollution?




1. List the types of galaxies and describe one important featureof each type.

2. Explain why the Milky Way appears as a narrow band oflight in the night sky.

3. Draw a sketch of the Milky Way galaxy. Label the nucleusand the central bulge, and indicate the position of our solar system.

4. Compare the colors of spiral galaxies with the colors ofelliptical galaxies.

5. Describe how quasars got their name.

6. Arrange these structures from largest to smallest: solar system, sun, spiral galaxy, dwarf elliptical galaxy, and cluster of galaxies.

7. Explain how scientists know that elliptical galaxies do not contain many young stars?

8. Critical Thinking Why do stars rarely collide during galactic collisions?

S U M M A R Y

> A galaxy is a collection ofmillions or billions of starsbound together by gravity.

> Our solar system lies in aspiral arm in the Milky Waygalaxy.

> Spiral galaxies have a bulgenear the center and spiralarms made of gas, dust,and stars.

> Elliptical galaxies have littlegas and dust and a spheri-cal or oval shape.

> Irregular galaxies have noregular shape.

> Quasars are stellar inappearance but emit largeamounts of radiation, espe-cially radiowaves.


Figure 19The Hubble Deep Field projectdiscovered many faint and verydistant galaxies.

Galaxies change over timeAll stars change over time. Massive stars explode in supernovae,and lower-mass stars become red giants and, eventually, whitedwarfs. Because gas, dust, and stars make up galaxies, entiregalaxies also change over time. As galaxies consume their gas anddust, they become unable to make new stars. Many galaxies inthe Hubble Deep Field, shown in Figure 19, are blue, indicatingthat we are viewing them when they were young, before theyused their stores of gas and dust.

Galaxies also change as a result of collisions. Stars within agalaxy are far apart and can easily pass each other if two galax-ies collide. But as galaxies approach each other, the mutual grav-itational attraction changes their shapes. While the stars rarelyhit each other, the collision of gas and dust sets off rapid burstsof new star formation. Scientists are trying to discover why ellip-tical galaxies do not contain young stars and how they used uptheir gas and dust. One possibility is that gas and dust werestripped away in collisions that also stripped away many of theyounger stars.


Origin of the Universe> Describe the basic structure of the universe.> Describe red shift, and explain what it tells scientists about

our universe. > State the main features of the big bang theory, and explain

the evidence supporting the expansion of the universe.> Explain how scientists are using tools and models to hypoth-

esize what may happen to the universe in the future.

Just imagine the following: colliding galaxies that rip stars fromeach other, a dead star so dense that one thimbleful of its mat-

ter would weigh more than 100 million tons on Earth, a volcanoon Mars that is nearly three times taller than Mount Everest andthat has a base larger than Louisiana. All of these things are partof the universe.

What Is the Universe?By the term scientists mean everything physical thatexists in space and time. The universe consists of all space, mat-ter, and energy that exists—now, in the past, and in the future.There is only one observable universe. Figure 20 shows objects inthe universe and their relative sizes.

universe,

O B J E C T I V E S

SECTION

3

680 C H A P T E R 2 0

K E Y T E R M S

universered shiftblue shiftbig bang theory

O B J E C T I V E S

▲

universe the sum of allspace, matter, and energy thatexist, that have existed in thepast, and that will exist in thefuture

▲

680 C H A P T E R 3

Person (2 m)

Soccer field (100 m)

Earth (1.3 × 107 m)

Florida (500 km)

1 m 10 m 102 m 103 m 104 m 105 m 106 m 107 m 108 m 109 m 1010 m

Figure 20The sizes of astronomical objectsare so great that measuring unitssuch as the light-year are neededto describe these objects.


Pluto

Neptune

Uranus

SaturnJupiter

MarsSun

Mercury

VenusEarth

Mars

You are part of the universe, as is Earth and everything on it.With the unaided eye, we can see about 6000 stars, 5 of the plan-ets, our moon, and several nebulae, star clusters, and galaxies. Asshown in Figure 20, huge distances are involved in studying theuniverse. Perhaps you can imagine the size of a soccer field, yourcity, your country, or even Earth, but the comparisons becomedifficult as we imagine objects on scales beyond Earth, such asthe solar system. The solar system is only a small part of ourgalaxy, which is but one of many galaxies in one of many clustersof galaxies.

We see the universe now as it was in the pastAstronomers need large units of measure to express distances.As you recall, a light-year is the distance that light travels in oneyear, or 9.5 � 1015 m. This distance is so long that driving it in acar moving at highway speed would take more than 10 millionyears. Remember that while a year is a unit of time, a light-yearis a unit of distance.

It takes time for light to travel in space. The farther away anobject is, the older the light that we get from that object is. Whenwe say the sun is 8 light-minutes away, we are not only express-ing its distance, but also the fact that we see it as it was eightminutes ago. We never see it as it really is, right now. The sameis true for stars, planets (Pluto is more than 5 light-hours away),galaxies, or clusters of galaxies. When we see very distant objects,we see them as they were when they were younger. Astronomerscan compare how galaxies age by looking at many galaxies at dif-ferent distances, and therefore at different ages.


? 681

Our solar system (6 × 1012 m)

1 light-year (ly)(9.5 × 1015 m)

The sun (1.4 × 109 m)

A typical galaxy (1021 m, 100 thousand ly)

A typical cluster (1023 m, 10 million ly)

1013 m 1014 m 1015 m 1016 m 1017 m 1018 m 1019 m 1020 m 1021 m 1022 m 1023 m

Because light travels at a con-stant speed of 3 � 108 m/s,we can measure how long ittakes starlight to reach Earth.Light from Sirius travels8 years and 7 months. If youlook at Altair (in the constella-tion Aquila), you see light thatleft the star 16 years and9 months ago. You couldeven find a star whose lightleft the year you were born.

www.scilinks.orgTopic: Origin of the

UniverseSciLinks code: HK4098


Most of the universe is empty spaceDespite the variety of objects in the universe, suchas interstellar matter, stars, and galaxies, thereis almost nothing between objects. Figure 21shows an astronaut in a suit designed to help aperson survive in space. It is extremely hot fac-ing the sun in space, and it is very cold facingaway from it. Space is a vacuum with no air andno air pressure. The suit provides the insulation,breathable air, and air pressure that the humanbody needs to survive. In this case, the astronautis bathed in particles streaming from the sun.Farther out, there is so little between stars thatthe space can truly be called “empty.”

What Happened at the Beginning?How the universe came to be is an age-old question. Ancient cul-tures had myths to explain the origin of the universe. Today, sci-entists study stars and galaxies for clues by using new tools andtechniques. Scientists interested in the early history of our uni-verse use large telescopes to study the most distant objects,whose light was emitted billions of years ago.

The universe is expandingIn 1929, Edwin Hubble announced that the universe is expand-ing. Hubble based his conclusion on observations of the spectrallines in light from other galaxies. He found that these spectrallines were almost always shifted toward the red end of the spec-trum. This effect, called can be explained by theDoppler effect. The Doppler effect states that when an object ismoving away from us, waves emitted from the object stretch out.The faster a light source moves away, the more that lightstretches to longer wavelengths and shifts toward the red end ofthe spectrum, as shown in Figure 22.

When an object is approaching us, the shift is toward shorterwavelengths at the spectrum’s blue end and is called Table 1 shows the distance, velocity, and frequency shift of severalgalaxies. Hubble found that most galaxies have red shifts andthat galaxies that are farther away have greater red shifts. Hubbleexplained this by proposing that almost every galaxy is movingaway from Earth. Therefore, galaxies are also moving away fromeach other, and the universe is expanding.

blue shift.

red shift,

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red shift an apparent shifttoward longer wavelengths oflight caused when a luminousobject moves away from theobserver

blue shift an apparent shifttoward shorter wavelengths oflight caused when a luminiousobject moves toward theobserver

▲▲

Figure 21Astronaut Bruce McCandlessmaneuvers through space in asuit specially designed to allowhim to propel himself. His space-suit also protects him from theconditions of space.


Expansion implies that the universe was once smallerAlthough galaxies that are close to each other are gravitationallyattracted to each other, galaxies are moving away from eachother in general. Imagine time running backward, like a moviebeing rewound. If every galaxy normally moves away from everyother galaxy, then as time goes backward, the galaxies movecloser together. Long ago, the entire universe might have beencontained in an extremely small space, effectively a point.

If time moves forward again from that point, all of the mat-ter in the universe appears to expand rapidly outward like agigantic explosion. Scientists call this hypothetical explosion thebig bang. If the expansion has been constant since the big bang,we can estimate the age of the universe. Velocity is equal to dis-tance divided by time. The velocities of galaxies can be measuredby using red shift and the Doppler effect, but the distances tothese faint objects are difficult to measure. Using estimates fordistance and velocity, scientists have estimated that the age of theuniverse is between 10 and 20 billion years.


Figure 22The spectral lines of hydrogen

gas can be seen and measured ina laboratory.

When this pattern appears instarlight, we know that the starcontains hydrogen. In this case,the lines show a red shift, suggest-ing that the star is moving awayfrom us.

B

A

CHEMISTRYOne of the major dis-coveries of the 1800swas that spectral lines

found in chemistry labs werealso found in celestial objects.This discovery showed thatobjects act predictably every-where in the universe andallowed scientists to identifyelements found in space asidentical to elements found on Earth. Astronomers beganto study how atoms react inconditions that differed fromthose on Earth.

FPOspec #35

Normal hydrogen spectrum

Hydrogen spectrum with red shift

Andromeda galaxy (M31) spiral –10 blue 2.4 � 106

Barnard’s galaxy (NGC 6822) irregular 15 red 2.2 � 106

NGC 55 (in Sculptor) spiral 115 red 1.0 � 107

Sunflower galaxy (M 63) spiral 550 red 3.6 � 107

Virgo A (M87) elliptical 822 red 7.2 � 107

Fornax A (NGC 1316) spiral 1713 red 9.8 � 107

Velocity Red shift or DistanceName of galaxy Type (km/s) blue shift (ly)

Table 1 Velocity, Frequency Shift, and Distance from Earth of Several Galaxies

A

B


Did the universe start with a big bang?Although scientists have proposed several different theories toexplain the expansion of the universe, the most complete andwidely accepted is the big bang theory. Thestates that the universe began with a gigantic explosion 10 billionto 20 billion years ago. In this book, we will assume that the uni-verse is 15 billion years old.

According to this theory, nothing existed before the big bang.There was no time and no space. But out of this nothingnesscame the vast system of space, time, matter, and energy that nowmakes up the universe. The explosion released all of the matterand energy that still exist in the universe today.

Cosmic background radiation supports the big bang theoryIn 1965, Arno Penzias and Robert Wilson were making adjust-ments to a new radio antenna that they had built. They could notexplain a steady but very dim signal from all over the sky in the form of radiation at microwave wavelengths. They realizedthat the signal they were receiving was the cosmic backgroundradiation predicted by the big bang theory.

Imagine the changes in color that occur as the burner on anelectric stove cools off. First, the hot burner glows yellow orwhite. As the burner cools, it becomes dimmer and glows red. Itmay still be rather hot when it finally looks black. The color yousee corresponds to the wavelength at which the burner radiatesthe most light. In outer space, the burner would cool until itreached the temperature of space.

Many scientists believe that the microwaves detected byPenzias and Wilson are dim remains of the radiation producedduring the big bang. Using maps of cosmic background radia-tion, such as the one shown in Figure 23, scientists have foundthat the universe has an overall temperature of about 2.7 K.

big bang theory

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big bang theory the theorythat all matter and energy inthe universe was compressedinto an extremely small vol-ume that 10 to 20 billionyears ago exploded and beganexpanding in all directions

▲

Figure 23The colors in this computerizedmap of cosmic background radia-tion across the entire sky representslight differences in temperatureabove and below 2.7 K.

Quick

ACTIVITYACTIVITYQuickQuickQuick

Modeling the ExpandingUniverse

1. Inflate a round balloonto about half full, andthen pinch it closed tokeep the air inside.

2. Use a marker to drawseveral dots closetogether on the balloon.Mark one of the dotswith an M to indicate theMilky Way galaxy.

3. Now continue inflatingthe balloon. How do thedots move relative toeach other?

4. How is this inflating bal-loon a good model forthe expanding universe?

5. In what ways might thisballoon not be a goodmodel for the expandinguniverse?


Time from big bang (years)

BIG BANG

First atoms form (1 million years)

Hydrogen and helium nuclei form (100 seconds after big bang)

First galaxies form (100 million years)

Our solar system forms (10 billion years)

Present (15 billion years)

10-6

10-4

10-2

100

10102

104

106

108

1010

Radiation dominated the early universeAccording to the big bang theory, expansion cooled the universeenough for matter such as protons, neutrons, and electrons toform from the radiation within a few seconds after the big bang.Hydrogen and helium nuclei and other particles were present,but the temperature was still too high for entire atoms to formand remain stable. The universe was dominated by radiation,which immediately overcame the attraction between electronsand nuclei. Figure 24 shows key points in the evolution of the uni-verse as predicted by the big bang theory. Note that the timelineuses a logarithmic scale, so the last 5 billion years can be foundin a small area near the end.

Processes in stars lead to bigger atomsIn a million years, the universe had expanded and cooled enoughfor hydrogen and helium atoms to form. Hydrogen comprised75% of the matter, and helium comprised 25%. Hydrogen fuelsstars and acts as a building block for other elements. Oncehydrogen atoms formed, stars and galaxies began to form, too.Our solar system is thought to be 4.6 billion years old, forming10 billion years after the big bang.

All elements other than hydrogen and helium form in stars.Nuclear fusion in stars produces helium and elements up to theatomic number of iron. Heavier elements form during super-novae. Figure 25 shows helium and lead that were produced in astar. The lead is in the form of galena, or lead sulfide.

Figure 24This timeline shows major events inthe evolution of the universe. Thefirst 1 million (106) years are basedon the big bang theory.

Figure 25Helium is found in stars, butheavier elements, such as lead,are the result of supernovae.

T H E U N I V E R S E 685Copyright © by Holt, Rinehart and Winston. All rights reserved.

Predicting the Future of the UniverseScientists use their ever-increasing knowledge to hypothesizewhat might happen in the future. They depend on a mixture oftheory and precise observations of very faint objects. Theseobservations depend on technology, such as the telescopes shownin Figure 26. New space telescopes that collect infrared radiationand X rays are being built and launched. Data in these regionsof the electromagnetic spectrum may provide important cluesabout the beginning and future of the universe.

The future of the universe is uncertainThe universe is still expanding, but it may not do so forever. Thecombined gravity of all of the mass in the universe is also pullingthe universe inward, in the direction opposite to the expansion.The competition between these two forces leaves three possibleoutcomes for the universe:

1. The universe will keep expanding forever.2. The expansion of the universe will gradually slow down,

and the universe will approach a limit in size.3. The universe will stop expanding and start to fall back in

on itself.

The fate of the universe depends on massFigure 27 shows three possible fates of the universe. Which oneoccurs depends on the amount of matter in the universe. If thereis not enough mass, the gravitational force will be too weak tostop the expansion, so the universe will keep expanding forever.If there is just the right amount of mass, the expansion will con-tinually slow down but will never stop completely. If there is

more mass than this right amount, gravitywill eventually overcome the expansion andthe universe will start to contract.Eventually, a contracting universe could col-lapse back to a single point in a “bigcrunch.” As things drew closer to each other,galaxies and stars would collide. The uni-verse would become extremely hot and verysmall. At this point, the universe may end, oranother big bang may start the cycle all overagain.

It is hard to predict what will happen inthis very distant future. Much of the mass inthe universe is very difficult to detect, so we donot yet know the total mass of the universe.

686 C H A P T E R 2 0

Figure 26Astronomers observe the universeby using modern telescopes, suchas the telescopes at the CerroTololo Inter-American Observatoryin Chile.

Size

of

the

univ

erse

Time

Bigbang

Universe keepsexpanding forever

Universe stopsexpanding

Bigcrunch

T = 0

Figure 27There are three possible fates forthe universe.


New technology helps scientists test theoriesPredictions of the future of the universe rest ontheories of the past. Scientists test theories bymaking observations to see whether the theoriesmake accurate predictions. If observations donot agree with theory, new theories are needed.To make these important observations, verypowerful telescopes and other sensitive equip-ment are needed.

One example of new, more sensitive technol-ogy is the Chandra X-Ray Observatory, shown in Figure 28A. The presence of X rays indicates matter at temperatures of more than one milliondegrees. The Crab Nebula emits radiation atmany wavelengths, including X rays, as shownin Figure 28B. Compare this image with the visi-ble light picture in Figure 10 and the radio imageon the first page of this chapter. Observations ineach wavelength region tell us something aboutthe Crab Nebula and about supernovae and theirrelease of elements in general.

There is debate about dark matterAstronomers estimate the mass of the universe by measuringstars, galaxies, and matter in the interstellar medium. But obser-vations of gravitational interactions between galaxies, such as theinteraction shown in Figure 29 indicate that there is more matterthan what is visible. Some scientists call this undetectable matterdark matter. Dark matter may be planets, black holes, or browndwarfs. Brown dwarfs are starlike objects that lack enough massto begin fusion. Dark matter could also be exotic atomic particlesthat no one knows how to observe. As much as 90% of the uni-verse may be composed of dark matter. What it is, where it is, andhow to detect it remain a mystery.


Figure 29Spiral galaxies NCG 2207 and IC 2163 are colliding.

Figure 28The Chandra X-Ray Observatory

collects information from matter atvery high temperatures.

The Chandra X-Ray Observatorycreated this image of the CrabNebula.

B

A

A

B

www.scilinks.orgTopic: Dark MatterSciLinks code: HK4030


Scientists use mathematics to build better modelsPeople easily accept Newton’s theory of gravitybecause it corresponds to our experiences of the world.Newton wrote his theory in a mathematical form thatcan be applied to many circumstances on Earth and inspace. In 1916, Albert Einstein expanded on Newton’stheories by developing the general theory of relativity,which he also expressed in a mathematical form.Einstein’s theory is hard to understand, in part becauseits effects are only noticeable at a very large scale.

According to Einstein’s theory, mass curves space,much in the same way that your body curves a mat-tress when you sit on it. In 1919, observations of a totalsolar eclipse showed that Einstein was correct. Stars inthe direction of the sun, which could be seen only dur-ing the eclipse, were in slightly different positions thanexpected. The mass of the sun had curved space, caus-ing light to come from a slightly different location, asshown in Figure 30. Larger masses, such as galaxies,will distort space even more. In this way, a mathemat-ical model was tested and supported by observation.


1. Define the word universe, and list three things that arefound in the universe.

2. Define the terms red shift and blue shift.

3. Describe the evidence that the universe is expanding.

4. Explain why the microwave background radiation is nowless than 3 K even though the universe was originally very hot.

5. Compare the features that you see in the three images of the Crab Nebula in this chapter. Make a list of similaritiesand a list of differences.

6. Critical Thinking Why didn’t the first stars to form havesolar systems with Earth-like planets and satellites?

7. Critical Thinking If an object is moving away from us at a high speed and is observed in the radio region of the spectrum, what does red shift mean? Explain.

8. Critical Thinking Why is it unlikely that dark matter is composed mostly of stars?

S U M M A R Y

> The universe is all of thespace, matter, and energythat exist, have existed, and will exist.

> The big bang theory statesthat the universe began 10billion to 20 billion yearsago as an explosion.

> The discovery of cosmicbackground radiation sup-ports the big bang theory.

> Red shift shows that mostgalaxies are moving awayfrom each other and thatthe universe is expanding.

> Astronomers use mathe-matical models and obser-vations to discern the pastand future of the universe.

Orbit of Mercury

Star(real position)Star

(apparentposition)

Earth

Path of light

Figure 30According to Einstein’s theory of relativity,the mass of the sun curves the space nearthe sun.


G R A P H I N G S K I L L S 689

Stars have different colors depending on their masses and ages. The graph above indicatesthe colors and types of the 100 stars nearest Earth (within 26 ly). Examine the graph andanswer the following questions. (See Appendix A for help interpreting a graph.)

What variables are shown in this graph?

What is the most common type of star nearest Earth? What stars are least common?

Most stars are main sequence stars. The more massive a main sequence star is, thegreater its surface temperature and brightness. Using the graph and H-R diagram,what can you conclude about the brightness, temperature, and mass of the starsnearest Earth? Which form most easily: stars with large or small masses?

If you were to construct a similar graph from a list of the 100 brightest stars as seenfrom Earth, the number of red and orange main sequence stars would decrease and thenumber of giant stars would increase. What can you conclude from this information?

Construct a graph best suited for the information listed below. How many ellipticalgalaxies are among the nearest (within 26 million ly) to Earth?

Beyond 26 million ly, the number of irregular galaxies does not increase as much as thenumbers of the other types of galaxies. What can you conclude from this information?

6

5

4

3

2

1

Graphing SkillsGraphing SkillsGraphing Skills

0 10 20

Number of Stars

Population of Stars Within 26 ly of Earth

30 40 6050

Star

Typ

e

Giants/White dwarfsRed

OrangeYellow

Yellow-whiteBlue-white

Galaxy Type Percentage among 190 nearest galaxies

Elliptical 10.0

Spiral 38.4

Irregular 51.6

Bar Graphs


8. If two stars have the same temperature and are the same distance from Earth, thebrighter star is a. faster. c. slower.b. larger. d. smaller.

9. A star like the sun will end its life as a a. pulsar. c. white dwarf.b. black hole. d. supernova.

10. What kind of galaxy is the Milky Way galaxy?a. elliptical c. clusterb. spiral d. irregular

11. Giant elliptical galaxies have _____ of stars.a. dozens c. hundredsb. thousands d. millions

12. A Type II supernova explodes when it beginsto fuse _____ in its core.a. hydrogen c. helium b. carbon d. iron

13. Most astronomers agree that quasars area. very old. c. very bright.b. very distant. d. All of the above

14. Which of the following are in the universe?a. Mars c. Milky Wayb. stars d. All of the above

15. Which of the following is a possible age of theuniverse, according to the big bang theory?a. 4.6 million years c. 4.6 billion yearsb. 15 million years d. 15 billion years

16. If the big bang theory is correct, what per-centage of the universe is helium?a. 10% to 25% c. more than 25%b. exactly 25% d. 0%

17. Dark matter is detected because ita. is bright. c. is dark.b. has gravity. d. is hot.

18. According to Einstein’s theory of relativity,space is curved by a great _____ nearby.a. mass c. vacuumb. comet d. satellite

Chapter HighlightsBefore you begin, review the summaries of thekey ideas of each section, found at the end ofeach section. The key vocabulary terms arelisted on the first page of each section.

1. What are the three basic types of galaxies? a. spiral, elliptical, and irregularb. closed, elliptical, and openc. spiral, quasar, and pulsard. open, binary, and globular

2. A pattern of stars seen from Earth is aa. galaxy. c. Milky Way.b. nebula. d. constellation.

3. By studying starlight, astronomers may learn a. the elements that are in the star.b. the surface temperature of the star.c. the speed at which the star is moving

toward or away from Earth.d. All of the above

4. The core of a star that remains after a super-nova may be any of the following excepta. a black hole. c. a red giant.b. a neutron star. d. a pulsar.

5. A light-year is a unit ofa. time. c. temperature.b. mass. d. distance.

6. The spectral lines of galaxies that are mov-ing away from us shift toward the _____ end of the spectrum.a. red c. greenb. yellow d. blue

7. If two stars have the same diameter and are at the same distance from Earth, thebrighter star is a. hotter. c. faster.b. colder. d. slower.

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R E V I E WC H A P T E R 20

UNDERSTANDING CONCEPTSUNDERSTANDING CONCEPTS


19. Arrange the following from largest to small-est: dwarf elliptical galaxy, spiral galaxy, pulsar, red giant, and cluster of galaxies.

20. Describe nuclear fusion, and identify inwhich part of a star it takes place.

21. Describe spectral lines, and explain how they help scientists study the composition of stars.

22. Define supernova, and describe the differ-ence between a Type I supernova and a Type II supernova.

23. Write a paragraph that explains the origin of the universe as presented in this chapter.Use the following terms: big bang theory, red shift, galaxy, interstellar matter, and star.

24. Describe the arrangement of the componentsof the Milky Way galaxy. Use the terms inter-stellar matter, stars, bulge, and spiral arms.

25. Describe the various stages in the life of astar like the sun. Use the terms white dwarf,red giant, and star.

26. Explain the current theory of what a quasar is.

27. Write a paragraph that describes howthe mass of a star determines thedeath of the star. Include theterms star, white dwarf, super-nova, neutron star, and black hole.

28. Describe the difference between blue shiftand red shift.

29. Explain what makes up a cluster, andapproximate how large a cluster may be.

30. Describe how cosmic background radiationwas discovered.

31. Using Graphics The figure below shows theintensity of radiation at different wave-lengths for three stars. Draw the curve for astar whose surface temperature is 20 000 K.

32. Graphing Graph the following data. Put dis-tance on the horizontal axis.


WRITINGS K I L L

Wavelength

Inte

nsit

y

Visi

ble

Ultr

avio

let

Infr

ared

Radiation from a blue star

(7500 K)

Radiation from a red star (4500 K)

V I B G Y O R

Radiation from the sun(6000 K)

BUILDING MATH SKILLSBUILDING MATH SKILLSUSING VOC ABULARYUSING VOC ABULARY

Distance VelocityObject (thousand ly) (km/s)

Andromeda galaxy 224 –10

Centaurus A 2116 251

M66 3680 593

M49 6746 822

Fornax A 9200 1713

BUILDING GR APHING SKILLSBUILDING GR APHING SKILLS


43. Interpreting Graphics The spectra shownbelow were taken for hydrogen, helium, andlithium in a laboratory on Earch. The spec-tra labeled as “Star 1” and “Star 2” weretaken from starlight. What elements arefound in Star 1 and Star 2.

44. Interpreting and Communicating Researchhow astronomers find distances to stars andgalaxies. Make a poster or presentation thatdescribes at least three methods.

45. Working Cooperatively Working in teams,research how ancient cultures around theworld explained the Milky Way. Present yourfindings to the class.

46. Applying Technology Use a computerart program to illustrate differenttypes of galaxies.

47. Communicating Effectively Write an articlefor your school newspaper in which youexplain why developing theories aboutthe future of the universe is important.

33. Estimate the distance between Earth and agalaxy whose velocity is 2000 km/s.

34. The Andromeda galaxy has a negative valuefor velocity. What does this mean physically?

35. Critical Thinking Why are most of the starsin the Milky Way galaxy red?

36. Understanding Systems Given that verymassive stars fuse hydrogen into helium at a faster rate than less massive stars, explainwhy the most massive stars have the shortest lifetimes.

37. Critical Thinking Name two ways that twostars that are the same distance from Earthcan have different brightnesses.

38. Understanding Systems What keeps a starfrom collapsing under its own weight?

39. Critical Thinking When looking at very dis-tant galaxies, astronomers see the galaxiesas they were when the galaxies were veryyoung. Will these galaxies have more bluestars or fewer blue stars than nearby galax-ies do? Explain your answer.

40. Applying Knowledge If Edwin Hubble hadobserved that the spectral lines from everygalaxy were blue shifted, what might hehave concluded about the universe? Whatcould we conclude about the fate of the universe?

41. Critical Thinking Where in a galaxy would ablack hole most likely be?

42. Applying Knowledge Could a black hole consume an entire galaxy?Explain your answer in a para-graph. Use concepts you learnedin the chapter.

692 C H A P T E R 2 0

R E V I E WC H A P T E R 20

DEVELOPING LI FE/WORK SKILLSDEVELOPING LI FE/WORK SKILLS

TH INKING CR ITIC ALLYTHINKING CR ITIC ALLY

COMPUTERS K I L L

WRITINGS K I L L

WRITINGS K I L L

Hydrogen

Helium

Lithium

Star 1

Star 2


48. Connection to Literature Robert Frostwrote a poem entitled “Fire and Ice” thatbegins “Some say the world will end in fire,Some say in ice.” Explain how these linesrelate to possible fates of the universe.

49. Connection to Literature In Following theEquator, Mark Twain wrote, “Constellationshave always been troublesome things toname. If you give one of them a fancifulname, it will always refuse to live up to it; it will always persist in not resembling thething it has been named for.” Choose a con-stellation that you have tried to observe orhave seen on a star map. Draw the stars andconnect them in a new way. Give the con-stellation a new name, and explain how youarrived at the name.

50. Connection to Chemistry Based on the desc-riptions within this chapter, how did hydro-gen and helium first form? What are thepossible sources of the elements found on theperiodic table from lithium to carbon? Whatare the possible sources of elements fromcarbon to iron? How could atoms heavierthan iron form?

51. Connection to Science Fiction Manyauthors, such as Poul Anderson, IsaacAsimov, David Brin, Larry Niven, and FredPohl, have incorporated black holes, neutronstars, or supernovae into their stories. Readone of their stories and compare the author’suse of scientific concepts with informationthat you learned in this chapter.

52. Concept Mapping Copy the unfinished concept map below onto a sheet of paper.Complete the map by writing the correctword or phrase in the lettered boxes.


INTEGR ATING CONCEPTSINTEGR ATING CONCEPTS

neutron stars

b.

a.

helium

c. supergiants

e.

all stars

which leavebehind coresthat become

white dwarfs

carbon

all the heavierelements

undergoes nuclearfusion to form

which undergoesfusion to form


whichbecome

which mayslowly cool as

which may explodeviolently in

in

d.

in

in

in

or

or


Art Credits: Fig. 1B, Stephen Durke/Washington Artists; Fig. 2, Tony Randazzo/American Artist’s Rep.,Inc.; Fig. 8, Uhl Studios, Inc.; Fig. 11, Stephen Durke/Washington Artists; Fig. 16, Uhl Studios, Inc.; Fig.20 (sun and solar system), Sidney Jablonski, (meter bar), Uhl Studios, Inc.; Fig. 24, Uhl Studios, Inc.;Fig. 30, Craig Attebery/Jeff Lavaty Artist Agent.

Photo Credits: Chapter Opener photo of Very Large Array radio telescopes by Roger Ressmeyer/COR-BIS; radio image of Crab Nebula by R.A. Perely/NRAO; Fig. 1A, Roger Ressmeyer/CORBIS; Fig. 3,Stapelton Collection/CORBIS; Fig. 4, Victoria Smith/HRW; Fig. 7, Jeff Hester and Paul Scowen (ArizonaSt. University)/NASA; Fig. 9, Anglo-Australian Observatory; “Connection to Social Studies,” Tha BritishLibrary Picture Library; Fig. 10, Malin/Pasachoff/Caltech/Anglo-Australian Observatory; FIg. 12, JohnGleason/Celestial Images; Fig. 13, Dr. Victor Anderson (University of Alabama, KPNO), courtesy W.Keel; Fig. 14A, The Observatories of the Carnegie Institution of Washington; Fig. 14B, NASA; Fig. 15,Jerry Schad/Photo Researchers, Inc.; Fig. 17A, Gemini Observatory, GMOS team; Fig. 17B, Anglo-Australian Observatory; FIg. 17C, Dennis Di Cicco/Peter Arnold, Inc.; Fig. 18, NASA/CXC/SAO/H.Marshall et. al.; “Real World Applications,” Arecibo Observatory, Courtesy Dr. Jose Alonso; Fig. 19,R. Williams and the HDF Team (ST Sci)/NASA; Fig. 20(soccer player), Doug Pensinger/Allsport/GettyImages; (field), Bob Long/AP/Wide World Photos; (earth), ESA/PLI/Corbis Stock Market; (galaxies),Dr. Victor Anderson (University of Alabama, KPNO), courtesy W. Keel; Fig. 21, Digital image (c) 1996CORBIS; Original image courtesy of NASA/CORBIS; NASA/SPL/Photo Researchers, Inc.; Fig. 25, SamDudgeon/HRW/Courtesy Dr. Ann Molineux, Texas Memorial Museum, University of Texas at Austin;Fig. 26, Roger Ressmeyer/CORBIS; Fig. 28A, TRW; Fig. 28B, NASA/CXC/SAO; Fig. 29, NASA; “SkillsPractice Lab,” Dr. Victor Anderson (University of Alabama, KPNO), courtesy W. Keel; “Career Link,”(Dr. Cohn portraits), Peg Skorpinski/HRW; (galaxy “chalkboard”), Digital Image copyright (c)2004PhotoDisc; (nebula), D. Walter(South Carolina State University) and P. Scowen (Arizona StateUniversity)/NASA.

www.scilinks.orgTopic: Formation of the ElementsSciLinks code: HK4056


Investigating DifferentTypes of Galaxies

� Procedure

Preparing for Your Experiment1. Examine the photographs of galaxies in this chapter.

Make sketches of what each galaxy might look like ifyou rotated it from a “top-down” view to a “side” view.

2. Examine the large photograph of the Hercules Clusterof galaxies in Figure 31 on the next page. Yourteacher may also provide you with a larger version of the photograph. The photograph contains bothstars that are between us and the cluster and galaxiesthat are within the cluster. Write your criteria for dis-tinguishing between a nearby star and a galaxy.

Classifying Galaxies3. Set up a classification system that divides different

galaxies into categories. Ignore the individual identityof stars. You should have at least three different typesof galaxies. Discuss in your group what types you willuse, and what characteristics define each type.

4. Find at least one example of each type in the photo-graph and write down the coordinates of each example. Compare your examples with others in your group.

5. Classify each galaxy you see in the photograph, andnote the coordinates of each galaxy. If necessary, usea magnifying glass to view the picture more clearly. If you can identify something as a galaxy but areunclear of its type, classify it as “uncertain” galaxy.

Measuring Galaxies6. Locate the largest and smallest galaxy for each of

your classification types.

7. Measure the sizes of these galaxies in millimeters.This process may be easier to do if you use a mag-nifying glass and a clear ruler.

Galaxies are of many sizes and types.How can you tell the differences in typeamong galaxies by simple measure-ments of their images?

Objectives> Recognize the orientation of galaxies.

> Classify galaxies according to type.

> Measure the diameters of galaxies.

> Analyze theresults by calculating the ratio of spi-ral galaxies to elliptical galaxieswithin a cluster.

Materialsmagnifying glassmetric ruler, clear plasticphotograph of a galaxy cluster

USING SCIENTIFIC METHODS

Introduction

Objectives

Materials


� Analysis1. Count how many galaxies you have of each type. Add up the totals for all types

to get the total number of galaxies observed.

2. Calculate the ratio of each type to the other types by dividing the total of onetype by the total number of galaxies observed. For example if you have 33 of typeA and 100 total galaxies, then the ratio would be 33 divided by 100, or aboutone-third.

3. Make a table showing the types of galaxies, their total numbers, and their calcu-lated ratios.

� Conclusions4. What type of galaxy is most common in this cluster?

5. Which of your classification types typically has larger galaxies?

6. Do you think there may be smaller galaxies that you missed seeing? Explain.

7. Which type of galaxy is easiest to confuse with foreground stars?

8. All the galaxies in a cluster are about the same distance from Earth. Therefore,differences in cluster size are due to differences in the sizes of the galaxies. If thelargest spiral you measured was the same size as the Milky Way galaxy, estimatethe total diameter of this cluster of galaxies.

A

B

C

D

E

321 4 5 6 7


Figure 31The Hercules Cluster


696 C A R E E R L I N K

CareerLinkCareerLinkCareerLink

Cosmologists study the origin, evolution, andfuture of the universe. They use observationsand scientific theory to try to answer someof science’s most fundamental questions:How old is the universe? What happenedduring the Big Bang? What are Black Holes?Is the universe expanding? And if so, howfast? Read on to hear from Joanne Cohn, acosmologist at the University of Californiain Berkeley, California.

Joanne Cohn, Cosmologist

What does a cosmologist do?

A cosmologist studies the universe as a physi-cal system. To study a system, you want toknow what it is made of, and that’s one ofthe important questions in the field today—what is the universe filled with? We want toknow what ingredients make up our universeso we can understand various scenarios, likehow matter assembles into large collapsedobjects such as galaxies, how light travelsthrough what is out there to us, and howlight is given off by stars. It is very much like a detective story, where you try to figure outthe players (types of matter in the universe)and the story (how they interacted in the past to get where they are today).

Why do you think your workis important?

My work is important to people who want to know what the universe is made of andhow it is evolving. The universe contains usand all physical phenomena we know about,so it is of interest to people who want toknow what is out there and what is happen-ing and what has happened.

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What question about the universe are you most interested in answering?

I’m interested in knowing how gravity froma galaxy changes the way light rays thattravel near it behave. Light that travels neara galaxy is deflected and ‘lensed’ by thegalaxy, and it's possible to use this lensingto learn about the matter in the galaxy.

What kinds of tools andmodels do you use in yourwork?

I use theoretical descriptions of how gravityand other physical forces work, and then usecomputers to make calculations and simula-tions. I also use simple models of galaxyshapes as a starting point for comparisonswith real observations from galaxies.

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Joanne Cohn is a cosmologist at the University ofCalifornia inBerkeley, California.

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“I’m still amazed thatthe laws of physicsthat we use on Earthcan help us measureand describe the entireknown universe!”


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What’s the most challeng-ing part of your job?

The field moves very fast, so you always feellike you'd like to be faster to make sure youcan finish things that you started beforesomeone else does it first. Also, you want to calculate things very carefully and thor-oughly, and this often means being boggeddown in details like finding a ‘2’ somewherein a calculation that doesn't quite work.

What kinds of skills andqualities are important fora cosmologist?

The most important skills are physics andmath skills, being able to do calculationsand make models. Writing and speakingskills are also important because you need to be able to communicate yourresults, and write papers for the scientificcommunity.

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What advice do you have forstudents who are interestedin cosmology?

Get involved in either astronomy or physicsas a college student, and start learningabout independent science research to findout if you like it.

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“I really feel like an explorer, find-ing out what is happening out therein the universe.”

—Joanne Cohn

www.scilinks.orgTopic: AstrophysicistSciLinks code: HK4010


Documents

20 The Universe - Grygla Public School...The Fate of Stars 2 The Milky Way and Other Galaxies Galaxies Types of Galaxies How Galaxies Evolve 3 Origin of the Universe ... they fuse