Sunshine StateSTANDARDSSC.E.1.3.3: The studentunderstands that ourSun is one of manystars in our galaxy.SC.E.1.3.4: The studentknows that starsappear to be made ofsimilar chemical ele-ments, although theydiffer in age, size,temperature, and distance.

786 Unit 6: Space Science

VOCABULARYlight-year p. 786parallax p. 787nebula p. 789main sequence p. 790neutron star p. 790black hole p. 790

BEFORE, you learned

• The Sun is our local star• The other stars are outside our

solar system• There are huge distances

between objects in the universe

NOW, you will learn

• How stars are classified• How stars form and change

KEY CONCEPT

Stars change over theirlife cycles.

EXPLORE Characteristics of Stars

How does distance affect brightness?

PROCEDURE

In a darkened room, shine a flashlight onto a dark surface from 30 cm away while your partner shines a flashlight onto the surface from the same distance. Observe the two spots of light.

Move one of the flashlights back 15 cm and then another 15 cm. Compare the two spots of light each time you move the flashlight.

WHAT DO YOU THINK?• How did distance affect the brightness of the light on the dark surface?• How does the distance of a star from Earth affect our view of it?

2

1

MATERIALS• 2 flashlights• meter stick• dark surface

We classify stars by their characteristics.Like our Sun, all stars are huge balls of glowing gas that produce or haveproduced energy by fusion. However, stars differ in size, brightness, andtemperature. Some stars are smaller, fainter, and cooler than the Sun.Others are much bigger, brighter, and hotter.

Stars look like small points of light because they are very far away.At most, only a few thousand can be seen without a telescope.To describe the distances between stars, astronomers often use a unitcalled the light-year. A is the distance light travels in oneyear, which is about 9.5 trillion kilometers (6 trillion mi). Outside thesolar system, the star closest to Earth is about 4 light-years away.

light-year

MAIN IDEA WEBA main idea web would be a good choice for taking notes about the characteristics of stars.

Brightness and DistanceIf you look at stars, you will probably notice that some appear to bebrighter than others. The amount of light a star gives off and its distance from Earth determine how bright it appears to an observer.A star that gives off a huge amount of light can appear faint if it is faraway. On the other hand, a star that gives off much less light canappear bright if it is closer to Earth. Therefore, to determine the truebrightness of a star, astronomers must measure its distance from Earth.

One way astronomers measure distance is by using whichis the apparent shift in the position of an object when viewed fromdifferent locations. Look at an object with your right eye closed. Nowquickly open it and close your left eye. The object will seem to moveslightly because you are viewing it from a different angle. The same kindof shift occurs when astronomers view stars from different locations.

To measure the parallax of a star, astronomers plot the star’s positionin the sky from opposite sides of Earth’s orbit around the Sun. They thenuse the apparent shift in position and the diameter of Earth’s orbit tocalculate the star’s distance.

Check Your Reading What factors affect how bright a star appears from Earth?

parallax,

How does the distance of an objectaffect parallax?PROCEDURE

Stand 1 m away from a classmate. Have the classmate hold up a meter stickat eye level.

With your left eye closed, hold a capped pen up close to your face. Look atthe pen with your right eye, and line it up with the zero mark on the meterstick. Then open your left eye and quickly close your right eye. Observe howmany centimeters the pen seems to move. Record your observation.

Repeat step 2 with the pen held at arm’s length and then with the penheld at half your arm’s length. Record your observation each time.

WHAT DO YOU THINK?• How many centimeters did the pen appear to move each time you observed it?

• How is parallax affected when you change the distance of the pen from you?

CHALLENGE How could you use this method to estimate distances thatyou cannot measure directly?

3

2

1

ParallaxParallaxSKILL FOCUSMeasuring

MATERIALS• meter stick• capped pen

TIME10 minutes

Chapter 22: Stars, Galaxies, and the Universe 787

SizeIt is hard to get a sense of how large stars are from viewing them inthe sky. Even the Sun, which is much closer than any other star, is farlarger than its appearance suggests. The diameter of the Sun is about100 times greater than that of Earth. A jet plane flying 800 kilometers perhour (500 mi/h) would travel around Earth’s equator in about two days.If you could travel around the Sun’s equator at the same speed, thetrip would take more than seven months.

Some stars are much larger than the Sun. Giant and supergiantstars range from ten to hundreds of times larger. A supergiant calledBetelgeuse (BEET-uhl-JOOZ) is more than 600 times greater in diameterthan the Sun. If Betelgeuse replaced the Sun, it would fill space in oursolar system well beyond Earth’s orbit. Because giant and supergiantstars have such huge surface areas to give off light, they are very bright.Betelgeuse is one of the brightest stars in the sky, even though it is 522 light-years away.

There are also stars much smaller than the Sun. Stars called whitedwarfs are about 100 times smaller in diameter than the Sun, or roughlythe size of Earth. White dwarfs cannot be seen without a telescope.

Color and TemperatureIf you observe stars closely, you may notice that they vary slightly incolor. Most stars look white. However, a few appear slightly blue orred. The differences in color are due to differences in temperature.

You can see how temperature affects color by heating up metal.For example, if you turn on a toaster, the metal coils inside will start toglow a dull red. As they get hotter, the coils will turn a brighter orange.The illustration on page 789 shows changes in the color of a metal baras it heats up.

Like the color of heated metal, the color of a star indicates its temperature. Astronomers group stars into classes by color and surfacetemperature. The chart on page 789 lists the color and temperaturerange of each class of star. The coolest stars are red. The hottest stars areblue-white. Our Sun—a yellow, G-class star—has a surface temperatureof about 6000°C.

Stars of every class give off light that is made up of a range of colors.Astronomers can spread a star’s light into a spectrum to learn about thestar’s composition. The colors and lines in a spectrum reveal whichgases are present in the star’s outer layers.

Check Your Reading How does a star’s temperature affect its appearance?

788 Unit 6: Space Science

A star the sizeof the SunDiameter = 1.4 million kilometers(900,000 mi)

Supergiant star100–1000 timesthe Sun’s diameter

White dwarf1/100 the Sun’sdiameter

Giant star10–100 times theSun’s diameter

Colors have been addedto this photograph of theOmega Nebula in order tobring out details.

Stars have life cycles.Although stars last for very long periods, they are not permanent.Like living organisms, stars go through cycles of birth, maturity, anddeath. The life cycle of a star varies, depending on the mass of the star. Higher-mass stars develop more quickly than lower-mass stars.Toward the end of their life cycles, higher-mass stars also behave differently from lower-mass stars.

Stars form inside a cloud of gas anddust called a (NEHB-yuh-luh).Gravity pulls gas and dust closertogether in some regions of a nebula.As the matter contracts, it forms a hot,dense sphere. The sphere becomes astar if its center grows hot and denseenough for fusion to occur.

When a star dies, its matter doesnot disappear. Some of it may form anebula or move into an existing one.There, the matter may eventuallybecome part of new stars.

Check Your Reading How is gravity involved in theformation of stars?

nebula

Chapter 22: Stars, Galaxies, and the Universe 789

Objects that radiate light changecolor as they heat up.

At about 1200°C the metalgives off yellow light.

When heated to about 1500°C,a steel bar gives off white light.

A steel bar glows red whenheated to about 600°C.

Color and Temperature

Class Color Surface Temperature (°C)

O blue-white above 25,000

B blue-white 10,000–25,000

A white 7500–10,000

F yellow-white 6000–7500

G yellow 5000–6000

K orange 3500–5000

M red below 3500

Classification of Stars

Stars are classified according to their colorsand temperatures. The Sun is a G-class star.

790 Unit 6: Space Science

Stages in the Life Cycles of StarsThe diagram on page 791 shows the stages that stars go through intheir life cycles. Notice that the length of a cycle and the way a starchanges depend on the mass of the star at its formation.

Lower-Mass Stars The stage in which stars produce energy throughthe fusion of hydrogen into helium is called the Because they use their fuel slowly, lower-mass stars can remain in themain-sequence stage for billions of years. The Sun has been a main-sequence star for 4.6 billion years and will remain one for about another5 billion years. When a lower-mass star runs out of hydrogen, it expandsinto a giant star, in which helium fuses into carbon. Over time a giantstar sheds its outer layers and becomes a white dwarf. A white dwarf issimply the dead core of a giant star. Although no fusion occurs in whitedwarfs, they remain hot for billions of years.

Higher-Mass Stars Stars more than eight times as massive as our Sunspend much less time in the main-sequence stage because they usetheir fuel rapidly. After millions of years, a higher-mass star expands tobecome a supergiant star. In the core of a supergiant, fusion producesheavier and heavier elements. When an iron core forms, fusion stopsand gravity causes the core to collapse. Then part of the core bouncesoutward, and the star erupts in an explosion called a supernova.

For a brief period, a supernova can give off as much light as a galaxy.The outer layers of the exploded star shoot out into space, carryingwith them heavy elements that formed inside the star. Eventually thismatter may become part of new stars and planets.

Neutron Stars and Black HolesThe collapsed core of a supergiant star may form an extremely densebody called a Neutron stars measure only about 20 kilo-meters (12 mi) in diameter, but their masses are one to three timesthat of the Sun.

Neutron stars emit little visible light. However, they strongly emitother forms of radiation, such as x-rays. Some neutron stars emit beamsof radio waves as they spin. These stars are called pulsars because theyseem to pulse as the beams rotate.

Sometimes a supernova leaves behind a core with a mass more thanthree times that of the Sun. In such a case, the core does not end up asa neutron star. Instead, it collapses even further, forming an invisibleobject called a The gravity of a black hole is so strong thatno form of radiation can escape from it.

Check Your Reading How do lower-mass stars differ from higher-mass stars after themain-sequence stage?

black hole.

neutron star.

main sequence.

A pulsar emits beams ofradio waves as it spinsrapidly. The pulsar seemsto pulse as the beamsrotate toward and awayfrom Earth.

RESOURCE CENTERCLASSZONE.COM

Learn more about lifecycles of stars.

reminder

Notice that in the lives ofstars, as useful energy ofthe star system decreases,disorder in the systemincreases.

Content ReviewFLORIDA

A star forms inside a cloud of gas and dust called a nebula. The life cycle of a star depends on its mass.

Life Cycles of Stars

Chapter 22: Stars, Galaxies, and the Universe 791

Lower-Mass Stars Higher-Mass Stars

A higher-mass starremains in themain-sequencestage for millionsof years.

After the main-sequencestage, the starexpands into asupergiant.

When fusioncan no longeroccur in thesupergiant, itundergoes anexplosion calleda supernova.

A high-mass star leavesbehind a denselypacked core called aneutron star.

A star with an extremelyhigh mass leaves behind aninvisible black hole. Astronomers can some-times detect matter andenergy around a black hole.

A lower-mass starcan fuse hydrogeninto helium for bil-lions of years. Thisstage is called themain sequence.

After the main-sequencestage, the starexpands into agiant star.

When a giant starsheds its outerlayers, it leavesbehind a deadcore called awhite dwarf.

How do the stars shown in this illustration differ in the main-sequence stage of their life cycles?

792 Unit 6: Space Science

Star SystemsUnlike our Sun, most stars do not exist alone. Instead, they aregrouped with one or more companion stars. The stars are held togetherby the force of gravity between them. A binary star system consists oftwo stars that orbit each other. A multiple star system consists of morethan two stars.

In many star systems, the stars aretoo close together to be seen individ-ually. However, astronomers havedeveloped ways of detecting such sys-tems. For example, in a binary starsystem, one of the stars may orbit infront of the other when viewed fromEarth. The star that orbits in frontwill briefly block some of the otherstar’s light, providing a clue thatmore than one star is present. Theillustration at right shows a binarystar system that can be detected thisway. Sometimes astronomers can alsofigure out whether a star is really astar system by studying its spectrum.

Star systems are an importantsource of information about starmasses. Astronomers cannot measure the mass of a star directly.However, they can figure out a star’s mass by observing the effect of the star’s gravity on a companion star.

Check Your Reading Why are star systems important to astronomers?

KEY CONCEPTS1. Why must astronomers figure

out a star’s distance to calculateits actual brightness?

2. How are color and tempera-ture related in stars?

3. How does a star’s mass affectits life cycle?

CRITICAL THINKING4. Analyze Some of the bright-

est stars are red supergiants.How can stars with cooler redsurfaces be so bright?

5. Infer Will the Sun eventuallybecome a black hole? Why orwhy not?

CHALLENGE6. Infer At what stage in the life

cycle of the Sun will it beimpossible for life to exist onEarth? Explain.

Binary Star System

Some binary star systems appearto dim briefly when one starorbits in front of the other andblocks some of its light.

When neither star is in front of the other, the star systemappears to give off more light.