The Solar System

PERSPECTIVE: ORIGINS 177

Astronomers estimate that our galaxy is about 80,000 light-
years in diameter. Th at is, light takes 80,000 years to travel from
one edge to the other. If you had a photograph of the Milky Way
Galaxy as big as North America, the entire solar system would be
about the size of a small cookie, and the sun and planets would
all be too small to see without a powerful microscope.
Large astronomical telescopes reveal other galaxies scattered
across the sky. You have already met the Andromeda Galaxy in
Figure P-6. It is so close you can see its nucleus with the unaided eye
as a hazy patch in the constellation Andromeda. Other galaxies are
all around us, and some are dramatically beautiful (■ Figure P-7).

The Universe

of Galaxies

Our galaxy is our home, but ours is only one of billions of
galaxies. Some, like our Milky Way Galaxy, are disk shaped with
graceful spiral arms marked by clouds of gas and bright newborn
stars. But many galaxies are great swarms of stars with relatively
little gas and dust.
Study Galaxy Classifi cation on pages 178–179 and
notice three important points and four new terms:

P-4

Many galaxies have no disk, no spiral arms, and almost no

gas and dust. Th ese elliptical galaxies range from huge giants

to small dwarfs.

Notice that disk-shaped galaxies usually have spiral arms and

contain gas and dust. Many of these spiral galaxies have a

central region shaped like an elongated bar and are called

barred spiral galaxies. Th e Milky Way Galaxy is a barred spi-

ral. A few disk galaxies contain little gas and dust.

Finally, notice the irregular galaxies, which are generally

shapeless and tend to be rich in gas and dust.

You might expect such titanic objects as galaxies to be rare,

but large telescopes reveal that the sky is fi lled with galaxies. Like

leaves on the forest fl oor, galaxies carpet the sky. Th ey fi ll the

universe in every direction as far as telescopes can see (■ Figure

P-8). Grouped in clusters and superclusters, galaxies are the

homes of the billions of stars that illuminate the universe and

create the chemical elements.

When two galaxies collide, the stars swirl past each other

without bumping, but the great clouds of gas and the magnetic

fi elds inside the galaxies do collide and compress each other. Th e

compression of the gas clouds can stimulate two colliding galax-

ies to form vast numbers of new stars and massive star clusters.

1

2

3

Canis

Major

Orion

Taurus

Gemini

a

Visual-wavelength image

b

■ Figure P-6

(a) Nearby stars look bright, but the vast majority of the stars in

our galaxy merge into a faintly luminous path that circles the sky,

the Milky Way. This artwork shows the location of a portion of the

Milky Way near a few bright winter constellations. (See the star charts

at the end of this book to further locate the Milky Way in your sky.)

(b) This photograph of the Andromeda Galaxy, a spiral galaxy about

2.5 million ly from Earth, shows approximately what our own galaxy

would look like if you could view it from a distance. (AURA/NOAO/NSF)