One website titled curious about astronomy talks a little bit about
radio astronomy being more sucessful at mapping the milky way than
visible light. It states on its page about the milky way galaxy [
http://curious.astro.cornell.edu/milkyway.php ] that "There are dark
patches in the band of light, due to dark clouds of interstellar
matter. Radio telescopes can see through these clouds of dust and have
shown that the material in our Galaxy is distributed in a disk with
spiral arms of material trailing through."
This website goes on to link to another good website about the milkey
way galaxy [ http://www.seds.org/messier/more/mw.html ] that states
this "In the infrared light, the structure of the Milky Way can be
better investigated, as the obscurring dust clouds are of better
transparency for long wavelength IR than for the visible light."
I interpret these two things to mean that there is a good amount of
interfering matter present in the milky way galaxy that would prevent
seeing everything it contains simply from the visible spectrum. As
such it is necessary to look at things from other spectra such as
infrared and radio waves.
This website led to a gem that looks like just what you are looking
for. It linked to a site titled "The Multiwavelength Milky Way" [
http://adc.gsfc.nasa.gov/mw/milkyway.html ], which has some good
information on why different wavelengths of light and electromagnetic
spectrum are used to get a better view of the milky way.
On their page about the laws of light energy and temperature [
http://adc.gsfc.nasa.gov/mw/mmw_bbody.html ], they explain the
following "II. An object emits radiation at several wavelengths.
However the peak wavelength emitted depends on the object's
temperature. The cooler the object, the longer the wavelength at which
most of the radiation is emitted. This is known as Wien's law." For
this reason, different types of stars and gasses would emit different
wavelengths of light (some of them well outside the visible spectrum)
based on their temperature. But this is only part of the picture.
Their page titled "accross the spectrum, seeing at different
wavelengths" [ http://adc.gsfc.nasa.gov/mw/mmw_across.html ] explains
this a bit more. It has this to explain different wavelength pictures
of the milky way "An astronomical object can look very different
depending on what wavelength is used in its detection. The object's
appearance often changes across the electromagnetic spectrum because
various physical processes result in emission in different wavelength
regions. The optical image of the Andromeda galaxy (below left) shows
glowing stars. And dark dust lanes trace out a spiral arm structure.
An infrared image of the galaxy (below right) shows several concentric
rings of dust rather than spiral arms. The dust is too cold (-260
degree C) to be detected in optical light."
They go on to say what types of things you will see at various
wavelengths of light:
"Radio Active galaxies (such as radio galaxies and quasars), some
stars (particularly binary stars), supernovae, pulsars, and clouds of
cold molecular gas are typical emitters of radio waves.
Infrared If an object has a temperature above zero, it emits in the
infrared. The longer wavelengths of infrared light can pass
un-scattered through dust that blocks visible light. In the infrared,
we see stars, including smaller and cooler stars which are hard to see
in visible light. Stars do not shine as you move into the far-infrared
(towards the radio region). The far-infrared emission comes from
protostars, and the dust clouds that were transparent in the
near-infrared. The dust is very cold, but still warm enough for
Visible (optical) In the visible region of the electromagnetic
spectrum, we see large and bright stars in our galaxy, nebulae,
supernova remnants, planets, and galaxies.
Ultraviolet (optical) In the ultraviolet, we see hot stars in our
galaxy, and quasars in other galaxies. Gas that has been heated to a
million degrees can be seen in the ultraviolet.
X-rays An object shines in x-rays if it is very hot (like millions of
degrees hot!). High temperatures can be reached in the presence of
magnetic fields or extreme gravity, or events such as supernovae can
heat surrounding material. Hot interstellar gas, neutron stars, and
supernova remnants are some objects that are studied in x-rays.
Gamma rays Gamma rays are produced by changes in atomic nuclei. They
are also products of collisions between cosmic rays and interstellar
matter. The objects and phenomena most often studied in gamma rays
include neutron stars, quasars, black holes, and gamma ray bursts."
Therefore, the reason it is so hard to map out the milky way galaxy
with visible light is that you cannot see everything with visible
light. Some types of astronomical bodies do not emit visible light,
and others absorb it. As such, in order to get as complete a view of
the sky as possible, it is necessary to take pictures with all the
various spectra, including radio waves.
Radio waves are not "better" in general than visible light, but
visible light and radio waves have the abilities to see different
kinds of objects. Therefore, for those applications where radio waves
are often used, it is because it is not easy to get any information
from visible light, but at the same time, radio waves does not offer
all of the information that visible light does.
So it is important to use a combination of these methods when trying
to map anything as large and complex as the milky way galaxy.
I hope that this fully answers your question. If you feel anything is
lacking, please request a clarification before rating this answer so
that I may improve my answer to better meet your needs.