Near, Mid and Far-Infrared

Overview

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. 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.

Moving away from visible light towards longer wavelengths of light we enter the infrared region. In the near-infrared region, the hot blue stars seen clearly in visible light fade out and cooler stars such as red dwarfs and red giants come into view. 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. Warm interstellar dust also starts to shine as we enter the mid-infrared region. This dust surrounds newly forming stars as well as stars going through their final phases.

In the far-infrared, the stars have all vanished. Instead we now see very cold matter. 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. The central regions of galaxies, including our own Milky Way, shine brightly in the far-infrared due to 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 far-infrared.

The table below highlights what we see in the different infrared spectral regions.

Spectral Region	Wavelength Range (microns)	Temperature Range (degrees Kelvin)	What We See
Near-Infrared	0.7 - 5	740 - 5,200)	Cooler red stars Red giants Dust is transparent
Mid-Infrared	5 - ~30	~120 - 740	Planets, comets and asteroids Dust warmed by starlight Protoplanetary disks
Far-Infrared	~30 - ~200	~10 - ~120	Emission from cold dust Central regions of galaxies Very cold molecular clouds

Additional Information & Images

Visible (courtesy of Howard McCallon), near-infrared (2MASS), and mid-infrared (ISO)
view of the Horsehead Nebula. Image assembled by Robert Hurt.

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.

Near-Infrared

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. 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 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.

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, many of which are too cool to be detected by optical telescopes.

Mid-Infrared

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. 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 strongly 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.

An infrared view of the Earth

IRAS mid-infrared view of Comet IRAS-Araki-Alcock

The mid-infrared is where dust warmed by starlight is most prominent. 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. Another example is the dust from comets which also has strong emission in the mid-infrared.

Warm interstellar dust starts to shine as we enter the mid-infrared region. This includes dust around stars which have ejected material as they enter their final stages. 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."

IRAS view of infrared cirrus - dust heated by starlight

Michael Hauser (Space Telescope Science Institute), the COBE/DIRBE Science Team, and NASA

The center of our galaxy, as well as most other galaxies, 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. One special class of galaxies, 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.