hses 1ete c25.qxd 5/17/04 3:41 pm page 700 section 25.1 25.1...

700 Chapter 25

25.1 Properties of Stars

Reading StrategyPreviewing Copy the table below. Beforeyou read, write two questions about theHertzsprung-Russell diagram on page 704. Asyou read, write answers to your questions.

Key ConceptsWhat can we learn bystudying star properties?

How does distance affectparallax?

What factors determine astar’s apparentmagnitude?

What relationship isshown on a Hertzsprung-Russell diagram?

Vocabulary◆ constellation◆ binary star◆ light-year◆ apparent magnitude◆ absolute magnitude◆ main-sequence star◆ red giant◆ supergiant◆ Cepheid variable◆ nova◆ nebulae

The star Proxima Centauri is about 100 million times farther awayfrom Earth than the moon. Yet, besides the sun, it is the closest star toEarth. The universe is incomprehensibly large. What is the nature ofthis vast universe? Do stars move, or do they remain in one place? Doesthe universe extend infinitely in all directions, or does it have bound-aries? This chapter will answer these questions by examining theuniverse and the most numerous objects in the night sky—the stars.

As early as 5000 years ago, people became fascinatedwith the star-studded skies and began to name the patternsthey saw. These patterns of stars, called constellations,were named in honor of mythological characters or greatheroes, such as Orion, shown in Figure 1.

Although the stars that make up a constellation allappear to be the same distance from Earth, some are manytimes farther away than others. So, the stars in a particularconstellation are not associated with one another in anyphysical way.

Today 88 constellations are recognized. They are usedto divide the sky into units, just as state boundaries dividethe United States. Every star in the sky is in, but is not nec-essarily part of, one of these constellations. Therefore,constellations can be used as a “map” of the night sky.

Betelgeuse

Bellatrix

Mintaka

RigelSaiph

Alnitak

Alnilam

Figure 1 Orion The constellationOrion was named for a hunter.

Questions about the Hertzsprung-Russell Diagram

Question Answer

a. b.

c. d. ??

??

700 Chapter 25

FOCUS

Section Objectives25.1 Describe what astronomers

can learn by studying starproperties.

25.2 Explain how distance affectsparallax.

25.3 List the factors that determinea star’s apparent magnitude.

25.4 Describe the relationshipshown on a Hertzprung-Russelldiagram.

Build VocabularyWord Parts and Roots Have studentslook up the prefix bi- to help themunderstand what binary means. (“two,”“having two distinct parts”) Have themlook up the origin of nova and relate itto its meaning. (from the Latin novus,meaning “new”; refers to a bright starthat suddenly appears in the sky) Alsohave them look up the meaning ofnebulous and relate it to the meaning ofnebula. (“lacking definite form or limits”;describes the appearance of a nebula)

Reading StrategyPossible answers:a. What information does the H-Rdiagram show?b. absolute magnitude and temperaturec. What is the largest group of stars onthe H-R diagram?d. the main sequence

INSTRUCTIntegrateLanguage ArtsGreek Mythology Tell students thatmany of the constellations were namedby the ancient Greeks after characters instories. Other peoples, such as NativeAmericans, had different names andstories associated with various starpatterns. Ask students to each researcha different constellation and write aparagraph describing the story associatedwith it. Students can also make up theirown patterns and stories if they wish.Verbal

L2

2

L2

L2

Reading Focus

1

Section 25.1

HSES_1eTE_C25.qxd 5/17/04 3:41 PM Page 700

Beyond Our Solar System 701

Characteristics of StarsA great deal is known about the universe beyondour solar system. This knowledge hinges on the factthat stars, and even gases in the “empty” spacebetween stars, radiate energy in all directions intospace. The key to understanding the universe is tocollect this radiation and unravel the secrets itholds. Astronomers have devised many ways to dojust that. We will begin by examining some prop-erties of stars, such as color, temperature, and mass.

Star Color and Temperature Study the stars in Figure 2 andnote their color. Color is a clue to a star’s temperature. Very hotstars with surface temperatures above 30,000 K emit most of theirenergy in the form of short-wavelength light and therefore appear blue.Red stars are much cooler, and most of their energy is emitted aslonger-wavelength red light. Stars with temperatures between 5000 and6000 K appear yellow, like the sun.

Binary Stars and Stellar Mass In the earlynineteenth century, astronomers discovered that manystars orbit each other. These pairs of stars, pulled towardeach other by gravity, are called binary stars. More than50 percent of the stars in the universe may occur in pairsor multiples.

Binary stars are used to determine the starproperty most difficult to calculate—its mass. Themass of a body can be calculated if it is attached by grav-ity to a partner. This is the case for any binary starsystem. As shown in Figure 3, binary stars orbit eachother around a common point called the center of mass.For stars of equal mass, the center of mass lies exactlyhalfway between them. If one star is more massive thanits partner, their common center will be closer to themore massive one. If the sizes of their orbits are known,the stars’ masses can be determined.

What is a binary star system?

Two stars of equal mass

One star twice as massive as its companion

Centerof mass

1 unit2 units

Centerof mass

1 unit1 unit

Figure 2 Stars of Orion Thistime-lapse photograph showsstars as streaks across the nightsky as Earth rotates. The streaksclearly show different star colors.

Figure 3 Common Center of Mass A For stars of equal mass, the center of mass lies inthe middle. B A star twice as massive as its partner istwice as close to the center of mass. It therefore hasa smaller orbit than its less massive partner.

A

B

Characteristicsof Stars

Students often think that all stars arefound alone in space, as the sun is.Explain that most stars are actually inpairs or larger groups. Fewer than halfare single stars. Many familiar starsin the sky are actually multiple starsystems. For example, the very brightstar Sirius A has a dimmer companion,Sirius B. Alpha Centauri is actually adouble star and forms a triple systemalong with Proxima Centauri.Verbal

Binary Star MotionPurpose Students observe how stars ofequal and different masses revolvearound a common center of mass.

Materials string, tape, 2 tennis balls,pencil, table tennis ball

Procedure Use string and tape to hanga tennis ball from one end of a pencil.Hang another tennis ball from the otherend of the pencil. Tell students that theballs represent stars and the pencilrepresents gravity holding them together.Tie another string to the center of thepencil so that the pencil is balancedwhen hung from the string. Twist thestring so that the balls rotate aroundeach other. Ask students what figure inthe text this represents. (Figure 3A) Askthem to predict what will happen if oneof the balls is replaced with a smallerone. (The center of mass will be closerto the large ball, and it will move less.)Replace one tennis ball with a tabletennis ball. Adjust the string on thepencil until it balances and sets thesystem rotating. Ask students whatfigure in the text this represents.(Figure 3B)

Expected Outcome The tennis ballswill revolve around an equidistant centerof mass. The tennis ball and table tennisball will revolve around a center of massclose to the tennis ball.Visual, Logical

L2

L2


Customize for English Language Learners

Write each of the vocabulary words at the topof an index card and post them on a wordwall. As students read the section, discuss thedefinitions of each term and add them to theindex cards. Work with students to help themorganize the words into groups based on their

scientific meanings. Revisit the word wall anduse it as an interactive tool to help studentslearn the terms. Refer to the words as often asyou can and incorporate them into homeworkor other assignments.

Answer to . . .

A binary star system ismade up of two stars

that orbit each other.


702 Chapter 25

Original photo

Photo taken 6 months later

Line of sight

Line of sight six months later

Parallax angle

Nearby starSun

Earth’sorbit

Distant stars

Apparentshift

Measuring Distances to StarsAlthough measuring the distance to a star is very difficult, astronomershave developed some methods of determining stellar distances.

Parallax The most basic way to measure star distance is parallax.Parallax is the slight shifting in the apparent position of a nearby star dueto the orbital motion of Earth. Parallax is determined by photographinga nearby star against the background of distant stars. Then, six monthslater, when Earth has moved halfway around its orbit, a second photo-graph is taken.When these photographs are compared, the position of thenearby star appears to have shifted with respect to the background stars.Figure 4 shows this shift and the resulting parallax angle.

The nearest stars have the largest parallax angles, while thoseof distant stars are too small to measure. In fact, all parallax angles arevery small. The parallax angle to the nearest star (besides the sun),Proxima Centauri, is less than 1 second of arc, which equals 1/3600 ofa degree. To put this in perspective, fully extend your arm and raiseyour little finger. Your finger is roughly 1 degree wide. Now imaginetracking a movement that is only 1/3600 as wide as your finger.

In principle, the method used to measure stellar distances mayseem simple. But in practice, measurements are greatly complicatedbecause of the tiny angles involved and because the sun, as well as thestar being measured, also move through space. Even with today’s tech-nology, parallax angles for only a few thousand of the nearest stars areknown with certainty.

Light-Year Distances to stars are so large that units such as kilo-meters or astronomical units are often too hard to use. A better unit toexpress stellar distance is the light-year, which is the distance lighttravels in one year—about 9.5 � 1012 or 9.5 trillion kilometers.Proxima Centauri is about 4.3 light-years away from the sun.

What is a light-year?

Figure 4 Parallax The parallaxangle shown here is exaggeratedto illustrate the principle. Becausethe distances to even the neareststars are huge, astronomers workwith very small angles.Relating Cause and EffectWhat caused the star to appearto shift?

702 Chapter 25

Measuring Distancesto StarsUse VisualsFigure 4 Use this figure to explainhow parallax works. Emphasize that thedrawing is not to scale; the star wouldactually be much farther away and theparallax angle very small. Ask: What doesthe simulated photograph at the topleft show? (It shows the star as seen fromEarth’s original position.) What does thesimulated photograph at the bottomleft show? (It shows the star as seen fromEarth 6 months later.) How is the parallaxangle related to the distance betweenthe star and Earth? (The farther awaythe star is, the smaller the angle.)Visual, Logical

Students are often confused by the termlight-year and think that it is a unit oftime. Explain again that the “year” refersto the time it takes for light to travel thedistance known as a light-year. Havestudents calculate the distance toProxima Centauri in kilometers.(4.3 light-years � 9.5 � 1012 km �� 41 � 1012 km)Logical

L2

L1

Section 25.1 (continued)



Stellar BrightnessThe measure of a star’s brightness is its magnitude. The stars in thenight sky have an assortment of sizes, temperatures, and distances, sotheir brightnesses vary widely.

Apparent Magnitude Some stars may appear dimmer thanothers only because they are farther away. A star’s brightness as itappears from Earth is called its apparent magnitude. Three fac-tors control the apparent brightness of a star as seen from Earth:how big it is, how hot it is, and how far away it is.

Astronomers use numbers to rank apparent magnitude. The largerthe number is, the dimmer the star. Just as we can compare the bright-ness of a 50-watt bulb to that of a 100-watt bulb, we can compare thebrightness of stars having different magnitudes. A first-magnitude staris about 100 times brighter than a sixth-magnitude star. Therefore, twostars that differ by 5 magnitudes have a ratio in brightness of 100 to 1.It follows, then, that the brightness ratio of two stars differing by onlyone magnitude is about 2.5. A star of the first magnitude is about2.5 times brighter than a star of the second magnitude.

Absolute Magnitude Astronomers are also interested in howbright a star actually is, or its absolute magnitude. Two stars of thesame absolute magnitude usually do not have the same apparent mag-nitude because one may be much farther from us than the other. Theone that is farther away will appear dimmer.To compare their absolute brightness,astronomers determine what magnitude thestars would have if they were at a standard dis-tance of about 32.6 light-years. For example,the sun, which has an apparent magnitude of�26.7, would, if located at a distance of 32.6light-years, have an absolute magnitude ofabout 5. Stars with absolute magnitude valueslower than 5 are actually brighter than the sun.Because of their distance, however, theyappear much dimmer. Table 1 lists theabsolute and apparent magnitudes of somestars as well as their distances from Earth.

What is absolute magnitude?

What is apparent magnitude?

Distance Apparent AbsoluteName (light-years) Magnitude* Magnitude*

Sun NA �26.7 5.0

Alpha Centauri 4.27 0.0 4.4

Sirius 8.70 �1.4 1.5

Arcturus 36 �0.1 �0.3

Betelgeuse 520 0.8 �5.5

Deneb 1600 1.3 �6.9

Table 1 Distance, Apparent Magnitude,and Absolute Magnitude of Some Stars

*The more negative, the brighter; the more positive, the dimmer.

Stellar BrightnessBuild Reading LiteracyRefer to p. 216D in Chapter 8, whichprovides guidelines for comparing andcontrasting.

Compare and Contrast Have studentscreate and fill in a table to compareand contrast apparent and absolutemagnitude. They should consider whatdetermines the value of apparent andabsolute magnitude and whether eachone is a relative value or an absolutevalue.Visual

Apparent and AbsoluteMagnitudePurpose Students observe the differencebetween apparent and absolutemagnitude.

Materials 2 equally bright flashlights,1 dimmer flashlight

Procedure Place two equally brightflashlights near the front of the class,one as close to the students as possibleand one as far away as possible. Dimthe lights and turn the flashlights on.Ask: Which flashlight has the greaterapparent magnitude?(the closer one)Which flashlight has the lowerapparent magnitude? (the more distantone) Place the two flashlights side byside. Ask: How do the flashlights’apparent magnitudes compare?(They are the same.) How do theflashlights’ absolute magnitudescompare? (They are the same.) Replaceone flashlight with a dimmer one. Ask:Which flashlight has the greaterapparent magnitude? (the brighter one)Which flashlight has the greaterabsolute magnitude? (the brighter one)

Expected Outcome Students willobserve how distance and brightnessaffect apparent and absolutemagnitude.Visual

L2

L1


Answer to . . .

Figure 4 Earth moved in its orbit.

the distance light travelsin one year—about

9.5 trillion km

the brightness of a staras seen from Earth

how bright a staractually is

Stars were first classified according to theirbrightness around the second century B.C.,when the Greek astronomer Hipparchusclassified about 1000 stars into six categories.In 1989, the European Space Agency launchedthe satellite Hipparcos (High Precision Parallax

Collecting Satellite). Hipparcos used parallaxto measure the distances to all visible starswithin about 150 light-years of the sun. Theaccuracy of the measurements was about2 milliarcseconds, or within 10 percent.

Facts and Figures

HSES_1eTE_C25.qxd 9/29/04 3:16 AM Page 703

704 Chapter 25

Hertzsprung-Russell DiagramEarly in the twentieth century, Einar Hertzsprung and Henry Russellindependently developed a graph used to study stars. It is now calleda Hertzsprung-Russell diagram (H-R diagram). A Hertzsprung-Russell diagram shows the relationship between the absolutemagnitude and temperature of stars. By studying H-R diagrams, welearn a great deal about the sizes, colors, and temperatures of stars.

In the H-R diagram shown in Figure 5, notice that the stars are notuniformly distributed. About 90 percent are main-sequence stars thatfall along a band that runs from the upper-left corner to the lower-right corner of the diagram. As you can see, the hottest main-sequencestars are the brightest, and the coolest main-sequence stars are thedimmest.

The brightness of the main-sequence stars is also related to theirmass. The hottest blue stars are about 50 times more massive than thesun, while the coolest red stars are only 1/10 as massive. Therefore, onthe H-R diagram, the main-sequence stars appear in decreasing order,from hotter, more massive blue stars to cooler, less massive red stars.

Above and to the right of the main sequence in the H-R diagram liesa group of very bright stars called red giants. The size of these giants canbe estimated by comparing them with stars of known size that have thesame surface temperature. Objects with equal surface temperaturesradiate the same amount of energy per unit area. Therefore, any differ-

ence in the brightness of twostars having the same surfacetemperature is due to their rela-tive sizes. Some stars are so largethat they are called supergiants.Betelgeuse, a bright red super-giant in the constellation Orion,has a radius about 800 timesthat of the sun.

Stars in the lower-centralpart of the H-R diagram aremuch fainter than main-sequence stars of the sametemperature. Some probablyare no bigger than Earth. Thisgroup is called white dwarfs,although not all are white.

-10

-5

0

+5

+10

+15

20,000 10,000 7000 5000 3000

Ab

solu

te m

agni

tud

e

Surface temperature (K)

Sun

Supergiants

Giants

Main sequence

White dwarfs

14,000+20

Figure 5 Hertzsprung-RussellDiagram In this idealized chart,stars are plotted according totemperature and absolutemagnitude.

704 Chapter 25

Hertzsprung-RussellDiagram

Students often think that the H-R diagramis a star chart that shows the locationsof stars in the sky. Emphasize that theH-R diagram is a graph that shows thecharacteristics of stars. Go over the unitsshown on the axes so that studentsunderstand why it is a graph. Explainthat the stars are shown different sizeson the graph for illustration only. Alsoexplain that an H-R diagram can beused to plot any sample of stars.Visual

Use VisualsFigure 5 Use this diagram to explainthe main groups of stars on the H-Rdiagram. Ask: To what group of starsdoes the sun belong? (It belongs tothe main sequence.) How are absolutemagnitude and temperature relatedwithin the main sequence? (As absolutemagnitude increases, so does temperature.)What types of stars have high absolutemagnitude but low temperature?(giants and supergiants) What types ofstars have low absolute magnitudesand medium temperatures? (whitedwarfs)Visual, Logical

L1

L2


Einar Hertzsprung was born in 1873 inDenmark. He worked as a chemist beforeswitching to studying astronomy. In 1912, hediscovered that for most stars temperature isclosely related to absolute magnitude. Beforethis discovery, astronomers had thought thatstars could have any combination oftemperature and absolute magnitude.

Henry Norris Russell was born in 1877 in theUnited States. While working in England in1921, Russell independently found the samerelationship that Hertzsprung had.Hertzsprung continued to work into hisnineties. Russell spent six decades at PrincetonUniversity.

Facts and Figures



Soon after the first H-R diagrams weredeveloped, astronomers realized theirimportance in interpreting stellar evolu-tion. Just as with living things, a star is born,ages, and dies. After considering some vari-able stars and the nature of interstellarmatter, we’ll return to the topic of stellarevolution.

Variable Stars Stars may fluctuate inbrightness. Some stars, called Cepheid vari-ables, get brighter and fainter in a regularpattern. The interval between two succes-sive occurrences of maximum brightness iscalled a light period. In general, the longerthe light period of a Cepheid, the greater itsabsolute magnitude is. Once the absolutemagnitude is known, it can be compared tothe apparent magnitude of the Cepheid.Measuring Cepheid variable periods is animportant means of determining distanceswithin our universe.

A different type of variable is associatedwith a nova, or sudden brightening of astar. During a nova eruption, the outer layerof the star is ejected at high speed. A nova,shown in Figure 6, generally reaches maxi-mum brightness in a few days, remainsbright for only a few weeks, then slowlyreturns in a year or so to its original bright-ness. Only a small amount of its mass is lostduring the flare-up. Some stars have expe-rienced more than one such event. In fact,the process probably occurs repeatedly.

Scientists think that novas occur inbinary systems consisting of an expandingred giant and a nearby hot white dwarf.Hydrogen-rich gas from the oversized giant is transferred by gravity tothe white dwarf. Eventually, the added gas causes the dwarf to igniteexplosively. Such a reaction rapidly heats and expands the outer layer ofthe hot dwarf to produce a nova. In a relatively short time, the whitedwarf returns to its prenova state, where it remains inactive until thenext buildup occurs.

A

B

Figure 6 Nova Thesephotographs, taken two monthsapart, show the decrease inbrightness that follows a novaflare-up.

Build Science SkillsRelating Cause and EffectGo through the cyclical process thatmay cause some novas to flare uprepeatedly. Ask: What is the first stepin the process? (Hydrogen-rich gas istransferred from a red giant to a whitedwarf.) What effect does this have onthe white dwarf? (The gas eventuallycauses the dwarf to ignite explosively.)What effect does this reaction haveon the white dwarf? (The outer layer isheated and expands, producing a nova.)Have students draw cycle diagrams ofthe entire process.Logical

Build Reading LiteracyRefer to p. 92D in Chapter 4, whichprovides guidelines for this usingcontext clues strategy.

Using Context Clues Tell studentsthat the term nova derives from theLatin word novus, meaning new. Askstudents to explain why the term novagot this name. (The sudden brighteningof an existing star may have beeninterpreted as the creation of a new star.)Encourage students to look up wordsand consider their roots as theyencounter new vocabulary.Verbal

L1

L2


HSES_1eTE_C25.qxd 9/29/04 3:17 AM Page 705

706 Chapter 25

Section 25.1 Assessment

Reviewing Concepts1. What can astronomers learn by studying a

star’s color?

2. Binary stars can be used to establish whatproperty of stars?

3. How does distance affect parallax?

4. What factors determine a star’s apparentmagnitude?

5. The H-R diagram shows the relationshipbetween what two factors?

Critical Thinking6. Problem Solving How many times brighter

is a star with a magnitude of 7 than a star witha magnitude of 12?

7. Inferring Scientists think that only a smallamount of a star’s mass is lost during a nova.Based on what you have learned about novas,infer what evidence scientists use to supportthis theory.

Interstellar Matter Betweenexisting stars is “the vacuum ofspace.” However, it is not a purevacuum, for there are clouds of dustand gases known as nebulae. If thisinterstellar matter is close to a veryhot star, it will glow and is called abright nebula. The two main types ofbright nebulae are emission nebulaeand reflection nebulae.

Emission nebulae consist largelyof hydrogen. They absorb ultravioletradiation emitted by a nearby hot

star. Because these gases are under very low pressure, they emit thisenergy as visible light. This conversion of ultraviolet light to visiblelight is known as fluorescence. You can see this effect in fluorescentlights. Reflection nebulae, as the name implies, merely reflect the lightof nearby stars. Reflection nebulae are thought to be composed ofdense clouds of large particles called interstellar dust.

Some nebulae are not close enough to a bright star to be litup. They are called dark nebulae. Dark nebulae, such as the one shownin Figure 7, can easily be seen as starless regions when viewing theMilky Way.

Although nebulae appear very dense, they actually consist of thinlyscattered matter. Because of their enormous size, however, their totalmass may be many times that of the sun. Astronomers study nebulaebecause stars and planets form from this interstellar matter.

Figure 7 Dark Nebula TheHorsehead Nebula is found in theconstellation Orion.

Web Site Make an educational Web siteabout the H-R diagram for younger stu-dents. Use Figure 5 as a guide. Include acolor key and other elements to help clarifyconcepts such as star temperature, theKelvin scale, and absolute magnitude.

706 Chapter 25

ASSESSEvaluateUnderstandingAsk students to write a summaryparagraph explaining how scientistsuse parallax to determine the differenceto nearby stars.

ReteachUse Figure 5 to review how differenttypes of stars are plotted on an H-Rdiagram.

Student Web sites should explain howstars are plotted on the H-R diagramaccording to absolute magnitude andtemperature. Web sites should alsoinclude definitions of key terms and acolor key explaining the relationshipbetween star color and temperature.

L1

L2

3


5. absolute magnitude and the temperatureof stars6. The magnitude 7 star is 100 times brighter.7. Following the nova flare-up, the starreturns to its prenova state. If a great amountof mass had been lost, this would not be pos-sible, nor would it be possible for the star toexperience more than one nova eruption.

Section 25.1 Assessment

1. Astronomers can learn about a star’stemperature by studying its color.2. Binary stars can be used to establish astar’s mass.3. The nearest stars have the largest parallaxangles, while the parallax angles of distantstars are too small to measure.4. Three factors determine the apparentmagnitude of a star: how big it is, howhot it is, and how far away it is.

HSES_1eTE_C25.qxd 9/29/04 3:18 AM 06

hses 1ete c25.qxd 5/17/04 3:41 pm page 700 section 25.1 25.1...

Documents