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 get above our 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.
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.
Red giants are large reddish or orange stars which are running out
of their nuclear fuel.
They can swell up to 100 times their original size and have temperatures
which range from 2000 to 3500 K. Red giants radiate most intensely in
the near-infrared region.
Red dwarfs are the most common of all stars. They are much smaller than
our Sun and are the coolest of the stars having a temperature of about
3000 K which means that these stars radiate most strongly in the near-infrared.
Many of these stars are too faint in visible light to even be detected by
optical telescopes, and have been discovered for the first time in the
near-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.
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."
As we move from the near-infrared into mid and far-infrared
regions of the spectrum, some celestial objects will appear while others
will disappear from view. For example, in the above image you can see
how more stars (generally cooler stars) appear as we go from the visible
light image to the near-infrared image. In the near-infrared, the dust also becomes
transparent, allowing us to see regions hidden by dust in the visible image.
As we go to the mid-infrared image, the cooler dust itself glows.
The table below highlights what we see in the different infrared spectral
regions.
(microns)
(degrees Kelvin)
Red giants
Dust is transparent
Dust warmed by starlight
Protoplanetary disks
Central regions of galaxies
Very cold molecular clouds
NEAR INFRARED:
Visible (left) and Near-Infrared View of the Galactic Center
Visible image courtesy of Howard McCallon. The infrared image is
from the
2 Micron All Sky Survey (2MASS)
Notice in the above images how center of our galaxy, which is hidden by thick
dust in visible light (left), becomes transparent in the near-infrared (right).
Many of the hotter stars in the visible image have faded in the near-infrared
image. The near-infrared image shows cooler, reddish stars which do not
appear in the visible light view. These stars are primarily red dwarfs and
red giants.
MID INFRARED:
An infrared view of the Earth
IRAS mid-infrared view of Comet IRAS-Araki-Alcock
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 strong emission in the mid-infrared.
FAR INFRARED:
Michael Hauser (Space Telescope Science Institute),
the COBE/DIRBE Science Team, and NASA
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.
