The Solar System

PERSPECTIVE: ORIGINS 181

■ Figure P-9

The colliding galaxies NGC

4038 and NGC 4039 are known

as the Antennae because their

long curving tails resemble the

antennae of an insect. Earth-

based photos (left) show little

detail, but a Hubble Space

Telescope image (right) reveals

the collision of the two galaxies

producing thick clouds of dust

and raging star formation, creat-

ing roughly a thousand massive

star clusters such as the one at

the top (arrow). Such collisions

between galaxies are common.

(Brad Whitmore, STScI, and NASA)

Visual-wavelength images

Name of cluster
containing the galaxy

Redshifts shown

as red arrows

Unshifted position

of calcium lines

17 Mpc

215 Mpc

310 Mpc

550 Mpc

860 Mpc

Hydra

Boötes

Corona Borealis

Ursa Major

Virgo

17 Mpc

215 Mpc

310 Mpc

550 Mpc

860 Mpc

1,200 km/s

15,000 km/s

22,000 km/s

39,000 km/s

61,000 km/s

■ Figure P-10

These galaxy spectra extend from the near-ultraviolet at left to the blue

part of the visible spectrum at right. The two dark absorption lines of

once-ionized calcium are prominent in the near-ultraviolet. The redshifts

in galaxy spectra are expressed here as velocities of recession. Note that

the apparent velocity of recession is proportional to distance. (Caltech)

the galaxies moving toward each other, and eventually you

would see the galaxies pushing into each other, compressing

and heating the gas until your video screen was fi lled with the

glare of an intensely hot, fantastically dense gas fi lling the

universe. Th is, the beginning of the universe, is a state called

the big bang.

How can anyone know the big bang really happened? Th e

evidence is conclusive—astronomers can see it (How Do

We Know P-2). To understand how you might see an event

that occurred billions of years ago, you need to look once

again at the galaxies. Nearby galaxies, such as the Andromeda

Galaxy, are only a few million light-years away. Th e hazy light

you see in the sky coming from that direction in the constel-

lation Andromeda has been traveling through space for only

2.5 million years, and you see the Andromeda Galaxy as it was

2.5 million years ago when the light began its journey. If you

used a telescope and looked at more distant galaxies, you

would see them as they were a billion years ago or more; light