The Astronomy Book

191

of these kinds of radiation. The
waves are absorbed by the air’s
molecules, reflected back into
space, or scattered in all directions
into a meaningless hodgepodge.
As a result, it is almost impossible
to garner information about most
kinds of non-visible radiation
from terrestrial observatories.
Spitzer’s 1946 paper, entitled
Astronomical Advantages of
an Extra-terrestrial Observatory,
highlighted the problem of detecting
non-visible radiation. His solution
was to put a telescope into space.
But Spitzer also highlighted the
obstacles to such a proposal: first,
the technological challenge of
inventing space travel, and second,
that of designing an instrument
capable of operating in space by
remote control from the ground.

Twinkle, twinkle little star
The rest of Spitzer’s paper was
focused on solving a problem that
had frustrated astronomers for
centuries—the sky itself. Viewed

from Earth, the stars appear to

twinkle. This effect is caused by the

star’s light shifting back and forth,

and rising and falling in brightness.

This is not a property of starlight,

but is caused by Earth’s thick

atmosphere. The twinkle becomes

more marked as magnification

increases, making the objects

appear shaky and fuzzy in the

eyepiece of a telescope or as diffuse

smears of light in photographs.

The scientific term for twinkling is

scintillation. It is caused when light

passes through layer upon layer

of turbulent air in the atmosphere.

The turbulence itself has no effect

on the light, but the density and

temperature differences that are

making the air churn and swirl

do have an effect. As the starlight

passes through one pocket of air

to another of a different density,

it refracts slightly, with some ❯❯

See also: Beyond the Milky Way 172–77 ■ Radio astronomy 179 ■ Studying distant stars 304–05 ■
Gravitational waves 328–31

ATOMS, STARS, AND GALAXIES

The level of the orange curve in this graph represents how

opaque the atmosphere is at the given wavelength of radiation.

The major windows are around visible wavelengths (marked by

the rainbow) and radio wavelengths from about 1 mm to 10 m.

The solution to both problems

is to put telescopes in space.

Earth’s atmosphere

causes astronomical

objects to twinkle, so they

cannot be captured with

sharp definition.

Many kinds of

electromagnetic radiation

cannot pass through

the atmosphere.

ATMOSPHERIC OPACITY (%)

100

150

50

0

WAVELENGTH

0.1nm 1 nm 10nm 100nm 1 μm 10pm 100pm 1 mm 1 cm 10cm 1 m 1 0m 1 00m 1 km