The Solar System

114 PART 1^ |^ EXPLORING THE SKY

Because astronomers understand how light interacts with
matter, a spectrum carries a tremendous amount of information,
and that makes a spectrograph the astronomer’s most powerful
instrument. In the next chapter, you will learn more about
the information astronomers can extract from a spectrum.
An astronomer once remarked, “We don’t know anything about
an object till we get a spectrum,” and that is only a slight
exaggeration.

Radio Telescopes

Celestial objects such as clouds of gas and erupting stars emit
radio energy, and astronomers on Earth can study such objects by
observing at wavelengths in the radio window where Earth’s atmo-
sphere is transparent to radio waves (see Figure 6-3). You might
think an erupting star would produce a strong radio signal, but
the signals arriving on Earth are astonishingly weak—a million to
a billion times weaker than the signal from an FM radio station.
Detecting such weak signals calls for highly sensitive equipment.

6-4

The Operation of a Radio Telescope

A radio telescope usually consists of four parts: a dish refl ector,

an antenna, an amplifi er, and a recorder (■ Figure 6-18). Th ese

components, working together, make it possible for astronomers

to detect radio radiation from celestial objects.

Th e dish refl ector of a radio telescope, like the mirror of a

refl ecting telescope, collects and focuses radiation. Because radio

waves are much longer than light waves, the dish need not be as

smooth as a mirror; wire mesh will refl ect all but the shortest

wavelength radio waves.

Th ough a radio telescope’s dish may be many meters in

diameter, the antenna may be as small as your hand. Like the

antenna on a TV set, its only function is to absorb the radio

energy collected by the dish. Because the radio energy from celes-

tial objects is so weak, it must be strongly amplifi ed before it can

be measured and its strength is recorded in computer memory.

Radio telescopes do not produce images. A single observa-

tion with a radio telescope measures the amount of radio energy

coming from a specifi c point on the sky. So the radio telescope

White

light

Prism

Ultraviolet

Short wavelengths

Infrared

Long wavelengths

Visible light spectrum

■ Figure 6-1 7

A prism bends light by an angle that depends on the wavelength of the light. Short wavelengths bend most and long wavelengths
least. Thus, white light passing through a prism is spread into a spectrum.