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Looking at the Night SkyHave you ever looked up at the sky on a clear, dark night and

seen more stars than you could possibly count? If you have, you are lucky. Few people see a night sky dense with stars. Lights from towns and cities make the night sky too bright for people to see the faint stars.

If you look at a clear night sky for a long time, the stars seem to move. But what you are really seeing is Earth’s movement. Earth spins, or rotates, once every 24 hours. Day turns to night and then back to day as Earth rotates. Because Earth rotates from west to east, objects in the sky rise in the east and set in the west.

Earth spins on its axis. Earth’s axis is an imaginary line from the North Pole to the South Pole. The star Polaris is almost directly above the North Pole. As Earth spins, stars near Polaris appear to travel in a circle around Polaris. These stars never set when viewed from the northern hemisphere. They are always present in the night sky.

Naked-Eye Astronomy

You don’t need expensive equipment to view the sky. Naked-eye astronomy means “gazing at the sky with just your eyes, without binoculars or a telescope.” Long before the telescope was invented, people viewed the sky in this way.

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

1. The night sky is divided into constellations.

2. A light-year is a measurement of time.

Key Concepts

• How do astronomers

divide the night sky?

• What can astronomers

learn about stars from

their light?

• How do scientists

measure the distance and

the brightness of objects

in the sky?

Create a Quiz Write five

questions as you read this

lesson. Exchange quizzes

with a partner. After taking

the quizzes, discuss your

answers.

Make a horizontal two-tab

book to organize your

notes on astronomy.

Stars and Galaxies

The View from Earth

618 Statrs and Galaxies The View from Earth

LESSON 1

CHAPTER 20

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Visual Check

1. Analyze Why does

east appear on the left and

west appear on the right on

the sky map? (Hint: Hold

the map over your head, as

you would view the sky.

Position the map so that

you are looking north.)

People have observed stars to tell time and find directions since ancient times. They learned about planets, seasons, and astronomical events merely by watching the sky. As you practice naked-eye astronomy, remember never to look directly at the Sun. Ultraviolet radiation from the Sun could damage your eyes.

Constellations

As people in ancient cultures gazed at the night sky, they saw patterns. The patterns resembled people, animals, or objects, such as the hunter and the dragon shown in the figure above. The Greek astronomer Ptolemy (TAH luh mee) identified dozens of star patterns nearly 2,000 years ago. Today, these patterns and others like them are known as ancient constellations.

Present-day astronomers use many ancient constellations to divide the sky into 88 regions. The sky map in the figure above shows some of these regions, which are also called constellations. Dividing the sky helps scientists communicate to others what area of sky they are studying.

Key Concept

Check

2. Describe How do

astronomers divide the

night sky?

Constellations

The View from Earth Statrs and Galaxies 619

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Telescopes

Telescopes were invented in the early 1600s. They can collect much more light than the human eye can detect. Visible light is just one part of the electromagnetic spectrum.

Look at the figure below. The electromagnetic spectrum is a continuous range of wavelengths. Longer wavelengths have low energy. Shorter wavelengths have high energy. Different objects in space emit different ranges of wavelengths. The range of wavelengths that a star emits is the star’s spectrum (plural, spectra).

Spectroscopes

Scientists study the spectra of stars using an instrument called a spectroscope. A spectroscope spreads light into different wavelengths. Using spectroscopes, astronomers can study stars’ characteristics, including temperatures, compositions, and energies. For example, newly formed stars emit mostly radio and infrared waves, which have low energy. Exploding stars emit mostly high-energy ultraviolet waves and X-rays.

Measuring DistancesExtend your arm, and hold up your thumb. Close one eye, and

look at your thumb. Now open that eye, and close the other eye. Did your thumb seem to jump? This is an example of parallax. Parallax is the apparent change in an object’s position caused by looking at it from two different points.

Astronomers use angles created by parallax to measure how far objects are from Earth. Astronomers do not use the eyes as the two points of view. Instead, they use two points in Earth’s orbit around the Sun.

Visual Check

3. Interpret Which

wavelength has the highest

energy?

Key Concept

Check

4. Assess What can

astronomers learn from a

star’s spectrum?

Reading Check

5. Explain What is

parallax?

Electromagnetic Spectrum



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Distances Within the Solar System

The universe is too large to measure easily in meters or kilometers. Therefore, astronomers use other units of measurement. For distances within the solar system, they use astronomical units (AU). An astronomical unit is the average distance between Earth and the Sun, about 150 million km.

Astronomical units make it easy to compare distances between objects in the solar system and the distance between Earth and the Sun. The figure below shows that Jupiter is 5.2 AU from the Sun. This means that Jupiter is 5.2 times farther from the Sun than Earth is from the Sun. The most distant planet, Neptune, is 30 AU from the Sun.

Distances Beyond the Solar System

Astronomers measure distances to objects beyond the solar system using a larger distance unit—the light-year. Despite its name, a light-year measures distance, not time. A light-year is the distance light travels in 1 year. Light travels at a rate of about 300,000 km/s. That means 1 light-year is about 10 trillion km! Proxima Centauri, the nearest star to the Sun, is 4.2 light-years away.

Looking Back in Time

Because it takes time for light to travel, you see a star not as it is today but as it was when light left it. At 4.2 light-years away, Proxima Centauri appears as it was 4.2 years ago. The farther away an object is, the longer it takes for its light to reach Earth.

Visual Check

6. Apply About how

many million kilometers is

Jupiter from the Sun?

Proportions can be used to

calculate distances to

astronomical objects. Light

can travel nearly 10 trillion

km in 1 year (y). How many

years would it take light to

reach Earth from a star that

is 100 trillion km away?

a. Set up a proportion.

10 trillion km

_________

1 y =

100 million km

___________ x y

b. Cross multiply.

10 trillion km × (x)y =

100 trillion km × 1 y

c. Solve for x by dividing

both sides by

10 trillion km.

x = 100 trillion km

__________

10 trillion km

= 10 y

7. Use Proportions How many

years would it take light

to reach Earth from a star

60 trillion km away?

Math Skills

Astronomical Units



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Measuring BrightnessSome stars look dim and some look bright. Astronomers

measure the brightness of stars in two ways: by how bright they appear from Earth and by how bright they actually are.

Apparent Magnitude

Scientists measure how bright stars appear using a scale developed by the ancient Greek astronomer Hipparchus (hi PAR kus). Hipparchus assigned a number to every star he saw based on the star’s brightness. Astronomers today call these numbers magnitudes. The apparent magnitude of an object is a measure of how bright it appears from Earth.

As shown in the figure below, the fainter a star appears, the greater its apparent magnitude. Note that some objects have negative apparent magnitudes. That is because Hipparchus assigned a value of 1 to the brightest stars, but he did not assign values to the Sun, the Moon, or Venus. Astronomers later assigned negative numbers to the Sun, the Moon, Venus, and a few bright stars.

Visual Check

8. Interpret What is the

apparent magnitude of

Sirius?

Key Concept

Check

9. Summarize How do

scientists measure the

brightness of stars?

ACADEMIC

VOCABULARYapparent(adjective) appearing to the eye or mind

Absolute Magnitude

A star can appear bright or dim depending on its distance from Earth. But a star also has an actual, or absolute, magnitude. Luminosity (lew muh NAH sih tee) is the true brightness of an object. The luminosity of a star is measured on an absolute magnitude scale. A star’s luminosity depends on the star’s temperature and size, not on its distance from Earth. A star’s luminosity, apparent magnitude, and distance from Earth are related. If scientists know two of these factors, they can determine the third using mathematical formulas.

Apparent Magnitude



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Mini Glossary

apparent magnitude: a measure of how bright an object appears from Earth

astronomical unit (AU): the average distance between Earth and the Sun

light-year: the distance light travels in 1 year

luminosity (lew muh NAH sih tee): the true brightness of an object

spectroscope: an instrument that spreads light into different wavelengths

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains why scientists use light-years rather than astronomical units to measure distances beyond the solar system.

2. In the diagram below, name and describe measurement units used in astronomy.

3. Why do objects in the sky appear to rise in the east and set in the west?

Units of Distance Units of Brightness

name:

description:

name:

description:

name:

description:

name:

description:

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 dis-

agree. 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.



624 Stars and Galaxies

Name Date

Stars and Galaxies What makes up the universe, and how does gravity

affect the universe?

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Before You ReadBefore you read the chapter, think about what you know about stars and galaxies. Record three things that you already know about the universe in the first column. Then write three things that you would like to learn about the universe in the second column. Complete the final column of the chart when you have finished this chapter.

Lesson 1 Lesson 2 Lesson 3 Lesson 4

NEWspectroscope

astronomical unit

light-year

apparent magnitude

luminosity

ACADEMIC

apparent

NEW nuclear fusion

star

radiative zone

convection zone

photosphere

chromosphere

corona

Hertzsprung-Russell

diagram

NEW nebula

white dwarf

supernova

neutron star

black hole

REVIEW

neutron

NEW galaxy

dark matter

Big Bang theory

Doppler shift

Chapter Vocabulary

K

What I Know

W

What I Want to Learn

L

What I Learned

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Stars and Galaxies 625

Lesson 1

Scan Lesson 1. Read the lesson titles and bold words. Look at the pictures. Identify threefacts you discovered about how astronomers observe the night sky. Record your facts in your Science Journal.

The View from EarthC

opyr

ight

© M

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tion.

Looking at the Night Sky

I found this on page .

Explain facts associated with viewing the night sky.

Few people see a night

sky very full of stars.

Objects in the night sky

appear to move.

Characterize astronomy before the invention of the telescope.

Naked-Eye Astronomy

and Ancient People

identified

patterns resembling

•

•

• learn about

•

•

•

observed stars to

Assess the usefulness of constellations to astronomers.




626 Stars and Galaxies

Lesson 1 | The View from Earth (continued)

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Categorize observations that astronomers can study at various wavelengths.

Waves Used to study

Radio

Infrared

Ultraviolet

X-rays and

gamma rays

Draw an arrow beside the table above to show the energy level of wavelengths in the direction of lowest energy to highest energy.

Identify three characteristics of stars that astronomers can study using spectroscopes.

Characteristics of Stars

Contrast terms associated with measuring distance in the universe.

Parallax Astronomical unit

about

km

Light-year

about

km



Measuring Distances



Stars and Galaxies 627

Lesson 1 | The View from Earth (continued)

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Measuring Brightness


Identify two ways astronomers measure the brightnessof stars.

Relate the appearance of celestial objects with the number values of their apparent magnitudes.

The

brighter the

object

Contrast apparent magnitude and absolute magnitude. Circle the magnitude that measures luminosity.

Apparent Magnitude Absolute Magnitude

Complete the statement below.

If a scientist knows a star’s and its

, he or she can calculate

the star’s .

1.

2.


Explain why it is not possible to know what is going on elsewhere in the universe at exactly this moment.




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