lesson 2 the sun and other stars€¦ · a star is a large ball of gas held together by gravity...

The Sun and Other Stars Stars and Galaxies 267

Lesson 2

Predict three facts that will be discussed in Lesson 2 after reading the headings. Record your predictions in your Science Journal.

The Sun and Other StarsC

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How Stars ShineI found this on page .

I found this on page .

Sequence the process of nuclear fusion.

1. Atoms of hot gas move quickly.

2. Atoms , and nuclei .

3. Great amounts of are released, making the star shine.

Characterize a star.

Star

held together very hotball of

Explain why the Sun is the most easily observed star.

Describe the inner and outer layers of the Sun.

Layer Description

Inte

rior

Lay

ers

Core

Radiative zone

Convection zone

Out

er L

ayer

s Photosphere

Chromosphere

Corona

Composition and Structure of StarsI found this on page .


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268 Stars and Galaxies The Sun and Other Stars

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The Sun and Other Stars

Stars and Galaxies

How Stars ShineThe hotter something is, the more quickly its atoms

move. As atoms move, they collide. If a gas is hot enough and its atoms move quickly enough, the nuclei of some of the atoms stick together. Nuclear fusion is a process that occurs when the nuclei of several atoms combine into one larger nucleus.

Nuclear fusion releases a great amount of energy. This energy powers stars. A star is a large ball of gas held together by gravity with a core so hot that nuclear fusion occurs. A star’s core can reach hundreds of millions of degrees Celsius. When energy leaves a star’s core, it travels throughout the star and radiates into space. As a result, the star shines.

Composition and Structure of StarsThe Sun is the closest star to Earth. Because it is so close,

scientists easily can observe it. They can send probes to the Sun. They can study its spectrum using spectroscopes on Earth-based telescopes. Spectra of the Sun and other stars provide information about the composition of stars. The Sun and most stars are made almost entirely of hydrogen and helium gas. A star’s composition changes slowly over time as hydrogen in its core fuses into more complex nuclei.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

3. Stars shine because there are nuclear reactions in their cores.

4. Sunspots appear dark because they are cooler than nearby areas.

Key Concepts • How do stars shine?• How are stars layered?• How does the Sun change

over short periods of time?• How do scientists classify

stars?

Key Concept Check1. Explain How do stars shine?

Building Vocabulary Work with another student to write a question about each vocabulary term in this lesson. Answer the questions and compare your answers. Reread the text to clarify the meaning of the terms.

LESSON 2

CHAPTER 9

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Reading Check3. Identify What is the hottest layer of a star’s atmosphere?

Interior of StarsWhen first formed, all stars fuse hydrogen into helium in

their cores. Helium is denser than hydrogen, so it sinks to the inner part of the core after it forms. A typical star has three interior layers. The layers are shown in the figure at the bottom of this page. The core is the center layer. The radiative zone is a shell of cooler hydrogen above a star’s core. Hydrogen in this layer is dense. Light energy bounces from atom to atom as it gradually makes its way upward, out of the radiative zone. Above the radiative zone is the convection zone, where hot gas moves up toward the surface and cooler gas moves deeper into the interior. Light energy moves quickly upward in the convection zone.

Atmosphere of StarsBeyond the convection zone are the three outer layers

of a star. These layers make up a star’s atmosphere. The photosphere is the apparent surface of a star. In the Sun, it is the dense, bright part you can see, where light energy radiates into space. From Earth, the Sun’s photosphere looks smooth. But like the rest of the Sun, it is made of gas.

Above the photosphere are the two outer layers of a star’s atmosphere. The chromosphere is the orange-red layer above the photosphere, as shown in the figure below. The corona is the wide, outermost layer of a star’s atmosphere. The temperature of the corona is higher than the photosphere or the chromosphere. It is irregular in shape and can extend outward for several million kilometers.

Key Concept Check2. Name What are the interior layers of a star?

Visual Check 4. Locate Where is the photosphere located in relation to the Sun’s other layers?

Radiativezone

Core

Convectionzone Photosphere

Corona Chromosphere

Layers of the Sun

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The Sun’s Changing FeaturesThe interior features of the Sun are stable over millions

of years. But the Sun’s atmosphere can change over years, months, or even minutes. Some of these features are described below.

Sunspots Regions of strong magnetic activity are called sunspots. Sunspots are cooler than the rest of the photosphere and appear as dark splotches on the Sun. They seem to move across the Sun as the Sun rotates. The number of sunspots changes over time. They follow a cycle, peaking in number every 11 years. An average sunspot is about the size of Earth.

Prominences and Flares Prominences are clouds of gas that make loops and jets that extend into the corona. They sometimes last for weeks. Flares are sudden increases in brightness that often occur near sunspots or prominences. They are violent eruptions that last from minutes to hours. Both prominences and flares begin at or just above the photosphere.

Coronal Mass Ejections (CMEs) Huge bubbles of gas ejected from the corona are coronal mass ejections (CMEs). They are much larger than flares and occur over the course of several hours. Material from a CME can reach Earth, sometimes interfering with radio and satellite communications.

The Solar Wind Charged particles that stream continually away from the Sun create the solar wind. The solar wind passes Earth and extends to the edge of the solar system. Auroras are curtains of light created when they interact with Earth’s magnetic field. Auroras occur in both the northern and southern hemispheres.

Groups of StarsThere are no other stars near the Sun. The star closest to

the Sun is 4.2 light-years away. Many stars are single stars, such as the Sun. Most stars exist in multiple star systems bound by gravity.

Binary Star Systems The most common star system is a binary system. In a binary system, two stars orbit each other. By studying the orbits of binary stars, astronomers can determine the stars’ masses.

Make a vertical four-tab book to organize your notes about the changing features of the Sun.

SunspotsProminencesand

Flares

Solar WindCoronalMassEjections

Key Concept Check5. Name Which parts of the Sun change over short periods of time?

6. Relate Look up the word binary in a dictionary. How does the definition relate to a binary star system?



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7. Apply Star A is 12 billion years old. Star B is in the same cluster. About how old is Star B? Why?

Reading Check8. Relate How does star color relate to mass?

Key Concept Check9. Describe What is the Hertzsprung-Russell diagram?

Star Clusters Many stars exist in large groups called clusters. There are two types of star clusters—open clusters and globular clusters. Open clusters contain fewer than 1,000 stars. Globular clusters can contain hundreds of thousands of stars.

All the stars in a cluster formed at about the same time and are the same distance from Earth. If astronomers know the distance to or the age of one star in a cluster, they know the distance to or the age of every star in the cluster.

Classifying StarsHow do you classify a star? Which properties are

important? Scientists classify stars according to their spectra. Recall that a star’s spectrum is the light the star emits spread out by wavelength. Stars have different spectra and different colors depending on their surface temperatures.

Temperature, Color, and MassHave you ever seen coals in a fire? Red coals are the

coolest. Blue-white coals are the hottest. Stars are similar. Blue-white stars are hotter than red stars. The temperatures of orange, yellow, and white stars are between the hottest blue-white stars and cooler red stars.

There are some exceptions, but color in most stars is related to mass. Blue-white stars tend to have the greatest mass, followed by white stars, yellow stars, orange stars, and red stars. The most massive stars are normally the hottest. The smallest stars tend to be cooler and red.

The Sun is a yellow star. It is tiny compared to blue-white stars. However, scientists suspect that most stars—as many as 90 percent—are smaller than the Sun. These stars are called red dwarfs.

Hertzsprung-Russell DiagramWhen scientists plot the temperatures of stars against

their luminosities, the result is a graph like that shown on the next page. The Hertzsprung-Russell diagram (or H-R diagram)is a graph that plots luminosity v. temperature of stars. The y-axis of the H-R diagram displays increasing luminosity. The x-axis displays decreasing temperature.

The H-R diagram is an important tool for categorizing stars. Astronomers also use it to determine distances of some stars. If a star has the same temperature as a star on the H-R diagram, astronomers often can determine its luminosity. As you read earlier, if astronomers know a star’s luminosity, they can calculate its distance from Earth.



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The Main SequenceMost stars spend the majority of their lives on the main

sequence. On the H-R diagram, main sequence stars form a curved line from the upper left corner to the lower right corner of the graph. The mass of a star determines both its temperature and its luminosity; the higher the mass the hotter and brighter the star. Because high-mass stars have more gravity pulling inward than low-mass stars, their cores have higher temperatures and produce and use more energy through fusion. High-mass stars have a shorter life span than low-mass strars. High-mass stars burn through their hydrogen much faster and move off the main sequence. A downside to a large-mass star is that the life span of the star is much shorter than average- or low-mass stars.

As shown in the figure above, some groups of stars on the H-R diagram lie outside of the main sequence. These stars are no longer fusing hydrogen into helium in their cores. Some of these stars are cooler, but brighter and larger, such as supergaints. Other stars are dimmer and smaller, but much hotter, such as white dwarfs.

Reading Check11. Explain Supergiants are cool yet luminous because _______. (Circle the correct answer.)a. they produce more

energyb. they are farther from

Earthc. they are unusually large

Hertzsprung-Russell Diagram

40,000

White dwarfs

Blue-whitestars

White stars

Yellow stars

Main-sequencestars

Supergiants

Giants

Red dwarfs

20,000 10,000 6,000 4,000 3,000 2,000

10-3

10-2

10-1

1

10

102

103

104

105

107

106

Lum

inos

ity

(Sun

= 1

)

Star Surface Temperature (K)

Visual Check 1 0. Interpret Where is the Sun on this diagram? Draw a circle to indicate the approximate location.



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Mini Glossary

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

ConnectED

chromosphere: the orange-red layer of a star above the photosphere

convection zone: a layer of a star where hot gas moves up toward the surface and cooler gas moves deeper into the interior

corona: the wide, outermost layer of a star’s atmosphere

Hertzsprung-Russell diagram: a graph that plots luminosity v. temperature of stars

nuclear fusion: a process that occurs when the nuclei of several atoms combine into one larger nucleus

photosphere: the apparent surface of a star

radiative zone: a shell of cooler hydrogen above a star’s core

star: a large ball of gas held together by gravity with a core so hot that nuclear fusion occurs

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains why nuclear fusion occurs in a star.

2. Label each star below as yellow, red, white, blue-white, or orange, based on mass. Then label each end of the temperature arrow as higher or lower.

3. How did writing and answering questions about vocabulary terms in this lesson help you learn about the Sun and other stars?

TEMPERATURE



Lesson 2 | The Sun and Other Stars (continued)C

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Groups of StarsI found this on page .

Detail features that change on the Sun over short periodsof time.

Feature Details

Sunspots

Prominences

Flares

Coronal Mass Ejections (CMEs)

Solar wind

Categorize ways stars exist in space.

Arrangementsof Stars




Lesson 2 | The Sun and Other Stars (continued)C

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Order stars’ colors, generally, by temperature. Draw arrows to represent increasing mass.

Color Temperature Mass

Red

Characterize the Hertzsprung-Russell diagram.

Named after Developed in

H ertzsprung- Russell

DiagramGraph that plots Tool for•

•

Generalize trends on the main sequence of the Hertzsprung-Russell diagram.

Classifying StarsI found this on page .


Synthesize It Compare and contrast the Sun with other stars in the universe.



lesson 2 the sun and other stars€¦ · a star is a large ball of gas held together by gravity...

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