Near, Mid, & Far Infrared

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.

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.

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.

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.

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