Childrens Illustrated Encyclopedia

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twinkling

starlight

nuclear reactions inside a

star heat the star up from

the center, causing it to

emit light and heat from its

surface. a star appears to

flicker or twinkle because

its light passes through

Earth’s atmosphere, which

is a constantly shifting

blanket of gases. seen from

a traveling spacecraft, stars

shine steadily because

there is no surrounding

atmosphere to disturb

the path of the light.

Constellations

Modern astronomers group stars into

88 constellations. Each has a latin

name, such as Ursa Major (the

great Bear), or Corona australis

(the southern Crown). the “sun

signs” of astrology have the same

names as the 12 constellations of

the zodiac—the band of sky along

which the sun and planets appear to

pass during the course of a year.

VariaBlE stars

Many stars, called variable stars, appear to vary in

brightness. some stars constantly swell and shrink,

becoming alternately fainter and brighter. Other variables

are really two stars that circle each other and block off

each other’s light from time to time.

Double stars circle around each other.

When one star is in front of the other, the

brightness dims. When both stars can be

seen, the brightness increases.

Some variable stars are produced

by exploding stars. The

explosion makes the star

appear much brighter than

usual for a period that can last

from a few days, to a few years.

star qUalitiEs

the color of a star’s light corresponds to the surface

temperature of the star: red stars are the coolest, blue stars

are the hottest. a star’s brightness (the amount of energy

it gives out) is linked to its mass (the amount of material it

contains): heavier stars are brighter than lighter stars.

astronomers can use the color and brightness of the

light emitted from a star to help calculate its size and

distance from Earth.

Parallax

astronomers use a

technique called parallax

to measure the distance of

a star from Earth. as Earth

moves around the sun, the

closest stars seem to move

very slightly compared

with stars farther away.

astronomers measure the

position of a star once, and

then again six months later.

From their measurements

they can then calculate the

distance of a star.

star BrightnEss

a star’s brightness is called its

magnitude. the brightest

magnitudes are the smallest

numbers, so magnitude 1 stars

are brighter than magnitude 2

stars. how bright a star looks

depends on its distance and

how much light it emits.

Neutron stars

(pulsars) are the

smallest stars. They

have about the same

mass as the sun, but

are only about

10 miles (16 km)

in diameter.

White dwarfs are

small stars at the

end of their life;

some are smaller

than Earth.

Giants have

diameters between

100 and 1,000

times larger than

that of the sun.

Supergiants are the largest

stars, with diameters up to

1,000 times that of the sun.

Yellow dwarfs, or

medium-sized stars,

are about the same

size as the sun.

astronomy

Black holes

gravity

navigation

Planets

sun

telescopes

Universe

When the constellation of Orion (above) is in the

night sky, it can be seen from anywhere on Earth.

Apparent position of nearby star when viewed from B Distant star

Apparent position of

nearby star when

viewed from A

Angle of parallax

gives distance

of star.

Earth in position

for second

measurement, six

months later (B)

Sun

Earth in position for

first measurement (A)

stars

Find out more

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