Sky & Telescope - June 2023 - 76

BEGINNER'S SPACE by Diana Hannikainen
How Can We Observe Objects
Invisible to Our Eyes?
WHO DOESN'T LOVE a rainbow? When
seeing that delicate sight arcing in the
sky, you might intone the colors in their
order: violet, indigo, blue, green, yellow,
orange, red. Or maybe you start at the
other end.
Regardless, it's easy to forget that
those vibrant colors we're seeing represent
only a tiny slice of the electromagnetic
spectrum. Most of the universe's
light comes to us in a form invisible
to our eyes. While we can't see that
radiation, we've built detectors and
telescopes that can.
Visible vs. Invisible
Amateur astronomers observe the universe
at the familiar visible wavelengths
(for the most part). That's what our eyes
have adapted to. But the electromagnetic
spectrum stretches far beyond the
red and the violet, into both the long
and the short wavelength ends. Capturing
this radiation is challenging.
Earth's atmosphere blocks the transmission
of much of the electromagnetic
spectrum outside the visible range.
(Thankfully for us, as some radiation is
dangerous to life.) Early experiments in
which scientists sent sounding rockets
and balloons to the upper atmosphere
first opened the ultraviolet, X-ray, and
gamma-ray universe to us. But to effectively
capture this radiation, we must
loft satellites well above the atmosphere.
Most radio wavelengths (apart from
the very longest) do reach Earth's surface,
but we need large collecting areas
in order to capture those long waves.
That's why, for example, we build radio
dishes with diameters of tens of meters.
And to exploit their capabilities even
further, we often operate them in sync
with one another as a radio interferometer
(as in the movie Contact, in which
76 JUNE 2023 * SKY & TELESCOPE
viewers got to see glimpses of the iconic
Karl G. Jansky Very Large Array in New
Mexico). Building and maintaining
such large structures - let alone making
them work together - is no mean feat.
Even the visible-light regimes suffer
from our protective atmosphere. If
you've ever tried to observe on a hot,
humid night, you'll have noticed how
moisture and turbulence in the atmosphere
affect views of the stars and
planets. This is even more pronounced
for the longer-wavelength infrared
bands; water vapor can block their
passage entirely. In order to somewhat
mitigate the disturbing effects of the
atmosphere, we place telescopes as far
above sea level as possible. Several locations
around the world are renowned
for their clutches of telescopes at high
elevation: Hawai'i, the Canary Islands, a
handful of mountaintops in Chile, and
Siding Spring in Australia, to name a
few of the better-known.
From Radio Waves to Gamma Rays
Radio
Infrared
Visible
Electromagnetic Spectrum
Radio waves
Microwaves
Infrared
Ultraviolet
X-rays
Gamma
rays
X-ray
Visible light
Longer wavelength
(lower energy)
Shorter wavelength
(higher energy)
We only see a tiny slice of the electromagnetic spectrum, the visible. Everything beyond
the red at one end and violet at the other is invisible to our eyes. Electromagnetic radiation
propagates like waves that vibrate perpendicularly to the direction of motion. The
wavelength is the distance between successive crests (tops of the waves) or alternatively
troughs (bottoms). The number of crests (or troughs) that pass through a point per second
is the frequency. Low frequency corresponds to long wavelengths, high frequency to
short wavelengths.
EM SPECTRUM: LEAH TISCIONE / S&T; CENTAURUS A INSETS: NASA / CXC /
SAO / NASA / JPL-CALTECH / NRAO / AUI / NSF / UOH / M. J. HARDCASTLE /
ASTROPHOTOGRAPHY BY RON OLSEN (4)

Sky & Telescope - June 2023

Table of Contents for the Digital Edition of Sky & Telescope - June 2023