Childrens Illustrated Encyclopedia

549

Universe

The expanding universe

You can get an idea of how the

universe is expanding by imagining

several small pieces of paper

glued on to a balloon. each piece

represents a cluster of galaxies. as

you blow up the balloon, all the

paper pieces move farther away from

each other. in the same way, galaxy

clusters are moving farther away from

each other. The farther a cluster is,

the faster it travels away from us.

Balloon expands in

the same way that the

universe is expanding.

galaxY clusTers

Most galaxies belong to groups

called clusters, which may

contain thousands of galaxies

of all types. These clusters

form “walls” with great voids

in between, so that the

universe is like a foam.

Looking back in time

if you look through a telescope

you can see galaxies millions of

light-years away. You are not

seeing them as they are now, but as they were long

ago, when their light first set out on its journey— so

in a sense, you are looking into the past.

Pieces of paper

represent clusters

of galaxies.

Find out more

Big bang

Black holes

comets and meteors

earth

light

Moon

planets

stars

sun

Telescopes

Dinosaurs

lived on Earth

66–215 million

years ago.

In this image the

galaxies are yellow

and red, and the

blue haloes around

them represent

dark matter.

The invisiBle universe

When scientists estimate the mass of a galaxy cluster, the figure 

usually turns out to be much more than the mass of the visible

galaxies alone. The extra, invisible matter is called dark matter,

and no one knows what it is. dark matter and ordinary matter

together account for only 30 percent of the universe. scientists

call the remaining 70 percent dark energy. dark energy is like a

force that acts against gravity and pushes the galaxies apart. it is

causing the expansion of the universe to speed up.

The vasT expanse of space that we call the universe contains

everything there is. it includes the sun, the planets, the Milky Way

galaxy, and all other galaxies, too. The universe is continually

growing, and each part is gradually moving farther away from

every other part. We know about the universe by using

powerful telescopes to study light, radio waves, x-rays,

and other radiations that reach earth from space.

light travels nearly 6 billion miles (9.5 billion km)

in a year. We call this distance a light-year. The

light from a distant star that you can see

through a telescope may have traveled

thousands of light-years to reach

us. Most scientists believe that

the universe was created by a

massive explosive event that

happened billions of years

ago. This idea is called

the Big Bang theory.

Many scientists now

believe that visible

matter makes up

only 7 percent

of the universe

and that the rest

is dark matter

and dark energy.

galaxies

galaxies, which contain

gas, dust, and billions of stars,

belong to one of three main

groups—elliptical, irregular, or spiral.

Most galaxies are elliptical, ranging from

sphere shapes to egg shapes. a few galaxies are

irregular. others, such as the Milky Way, are spirals.

The universe consists of billions of galaxies of all types.

Milky

Way has

a halo of

stars and gas.

MilkY WaY

The sun is just

one of 100 billion stars

in the large spiral galaxy

we call the Milky Way. like

most other spiral galaxies, the

Milky Way has curved arms of stars

radiating from a globe-shaped center.

The Milky Way is 100,000 light-years across,

and the sun is 30,000 light-years from its center.

Galaxy is 100 million

light-years away. Light

left this galaxy when the

dinosaurs lived on Earth.

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