Infrared is usually divided into 3
spectral regions: near, mid and far-infrared.
The boundaries between the near, mid
and far-infrared regions are not agreed
upon and can vary. The main factor
that determines which wavelengths
are included in each of these three
infrared regions is the type of detector
technology used for gathering infrared
light.
Near-infrared observations have been made from ground
based observatories since the 1960's. They are done in
much the same way as visible light observations for wavelengths
less than 1 micron, but require special infrared detectors
beyond 1 micron. Mid and far-infrared observations can
only be made by observatories which can observe above
the atmosphere. These observations require the use of
special cooled detectors containing crystals like germanium
whose electrical resistance is very sensitive to heat.
Infrared radiation is emitted by any object that has a
temperature (ie radiates heat). So, basically all celestial
objects emit some infrared. The wavelength at which an
object radiates most intensely depends on its temperature.
In general, as the temperature of an object cools, it
shows up more prominently at farther infrared wavelengths.
This means that some infrared wavelengths are better suited
for studying certain objects than others.
NEAR INFRARED
Between about 0.7 to 1.1 microns
we can use the same observing methods as are use for visible
light observations, except for observation by eye. The infrared
light that we observe in this region is not thermal (not
due to heat radiation). Many do not even consider this range
as part of infrared astronomy. Beyond about 1.1 microns,
infrared emission is primarily heat or thermal radiation.
As we move away from visible light towards longer wavelengths
of light, we enter the infrared region. As we enter the
near-infrared region, the hot blue stars seen clearly
in visible light fade out and cooler stars come into view.
Large red giant stars and low mass red dwarfs dominate
in the near-infrared. The near-infrared is also the region
where interstellar dust is the most transparent to infrared
light.
MID INFRARED
As we enter the mid-infrared region
of the spectrum, the cool stars begin to fade out and cooler
objects such as planets, comets and asteroids come into
view. Planets absorb light from the sun and heat up. They
then re-radiate this heat as infrared light. This is different
from the visible light that we see from the planets which
is reflected sunlight. The planets in our solar system have
temperatures ranging from about 53 to 573 degrees Kelvin.
Objects in this temperature range emit most of their light
in the mid-infrared. For example, the Earth itself radiates
most bly at about 10 microns. Asteroids also emit most
of their light in the mid-infrared making this wavelength
band the most efficient for locating dark asteroids. Infrared
data can help to determine the surface composition, and
diameter of asteroids.
Dust warmed by starlight is also very prominent in the mid-infrared.
An example is the zodiacal dust which lies in the plane
of our solar system. This dust is made up of silicates (like
the rocks on Earth) and range in size from a tenth of a
micron up to the size of large rocks. Silicates emit most
of their radiation at about 10 microns. Mapping the distribution
of this dust can provide clues about the formation of our
own solar system. The dust from comets also has b emission
in the mid-infrared.
Warm interstellar dust also starts to shine as we enter
the mid-infrared region. The dust around stars which have
ejected material shines most brightly in the mid-infrared.
Sometimes this dust is so thick that the star hardly shines
through at all and can only be detected in the infrared.
Protoplanetary disks, the disks of material which surround
newly forming stars, also shines brightly in the mid-infrared.
These disks are where new planets are possibly being formed.
FAR INFRARED
In the far-infrared, the stars have
all vanished. Instead we now see very cold matter (140 Kelvin
or less). Huge, cold clouds of gas and dust in our own galaxy,
as well as in nearby galaxies, glow in far-infrared light.
In some of these clouds, new stars are just beginning to
form. Far-infrared observations can detect these protostars
long before they "turn on" visibly by sensing
the heat they radiate as they contract."
The center of our galaxy also shines brightly in the far-infrared
because of the thick concentration of stars embedded in
dense clouds of dust. These stars heat up the dust and cause
it to glow brightly in the infrared. The image (at left)
of our galaxy taken by the COBE satellite, is a composite
of far-infrared wavelengths of 60, 100, and 240 microns.
Except for the plane of our own Galaxy, the brightest far-infrared
object in the sky is central region of a galaxy called M82.
The nucleus of M82 radiates as much energy in the far-infrared
as all of the stars in our Galaxy combined. This far-infrared
energy comes from dust heated by a source that is hidden
from view. The central regions of most galaxies shine very
brightly in the far-infrared. Several galaxies have active
nuclei hidden in dense regions of dust. Others, called starburst
galaxies, have an extremely high number of newly forming
stars heating interstellar dust clouds. These galaxies,
far outshine all others galaxies in the far-infrared.
