What is Infrared?

Astronomers must rely most of the time on acquiring light, or electromagnetic radiation, from distant celestial objects to determine their nature. To acquire a complete knowledge of the sky and to place individual astronomical objects in the context of the whole Universe, astronomers have surveyed the entire sky at various wavelengths of light.

Light is a form of energy in the Universe, so, the light energy can be thought of as being carried in the form of waves. The wavelength of light is the distance from one light wave crest to the next wave crest, much like the distance between crests of waves in the ocean or on a lake. All light waves travel through space at the speed of light, which is about 300,000 kilometers per second, and so, the number of wavecrests that pass an observer per second is known as the frequency of the light wave. Clearly, if the wavelength of light is short, then many wavecrests will go by per second; alternately, if the wavelength is long, then fewer wavecrests will pass by on observer per second. Therefore, frequency and wavelength are inversely proportional to each other. Short wavelength, high frequency light waves carry high energy, whereas long wavelength, low frequency light waves carry lower energy.

The electromagnetic spectrum, or range of possible wavelengths, frequencies, and energies of light, is shown graphically below.

The light that we see with our eyes is only a very small fraction of the total electromagnetic radiation throughout the Universe. Most of the radiation emitted by celestial objects are unseen visually, and we must use new and amazing technologies to allow us to detect and discover new objects and new aspects of objects we already know that we cannot see. Often this requires going into space. But, much can still be achieved at some wavelengths other than visible wavelengths by observing from the ground.

One such form of light that is invisible to our unaided eye is infrared light. Infrared is just longward in wavelength of visible red light. The longest wavelengths of red that we can still see are the deep, dark reds near the edge of the rainbow; beyond that, our eyes are just not sensitive enough.

The primary source of infrared radiation in the Universe is heat energy or thermal radiation. In fact, any object in the Universe which has a temperature, that is, any object above the temperature "absolute zero" (-459.67 degrees Fahrenheit or -273.15 degrees Celsius or zero degrees Kelvin), radiates in the infrared. Since living things, such as humans, have a temperature, they are sources of infrared radiation. At left is an infrared "image" (made with an infrared detector, such as described above) of a man. In the image, things that are warmer, such as the man's face, look red, while things that are cooler, such as the man's hair or shirt, look bluish or green.

Objects in the sky, such as stars, galaxies, and planets, also have temperatures above absolute zero, and therefore, it is very important for astronomers to observe the Universe in the infrared. Many observations can be made from the ground. First, astronomers must use telescopes to collect the feeble light from distant celestial objects, and they must build the telescopes on the tops of mountains in relatively dry spots around the Earth. This is because water vapor and other molecules in the atmosphere affect how much infrared light from space actually reaches the surface, so it is important that the atmosphere above the telescopes be relatively free of water.

Next, since our eyes cannot see infrared light, astronomers must use "state-of-the-art" digital detectors on telescopes to extend our view of the Universe.
At left is shown an infrared detector, which is actually infrared-sensitive material, made from alloys of exotic metallic substances, such as mercury, cadmium, and tellurium, arranged in a lattice-like array of individual cells called pixels (for "picture elements"). Infrared detectors here on Earth are used for "night vision", such as military night scopes, or other technological applications where detecting thermal emission is necessary. Your remote controls for televisions, VCRs, stereos, and other electronics, for instance, use infrared technology to function. For astronomical purposes, such an infrared array is housed in a vacuum-sealed thermos-like dewar, which is filled with a refrigerant, such as liquid nitrogen, to keep the array cool. The reason the detector must be cooled is to minimize the amount of stray infrared radiation it will detect from its surroundings, since the telescope, the camera, and objects in the telescope dome also emit infrared light (the goal is to detect infrared radiation only from celestial objects.) The dewar is attached to a light-proof camera, which is then attached to a telescope. Observations of the sky are then made on a clear night with the telescope-camera-detector combination. The array of pixels converts the energy from the light waves into electrical current, which is then converted into "bits" of digital data. These data are then stored on a computer, which is connected to the camera, for analysis by the observer or other astronomers at some later time.

The field of infrared astronomy is still fairly new. By today's standards, the ground-based observations in the past were relatively crude. Great advances have been made over just the last decade in detector technology, resulting in larger, more sensitive arrays, making efficient, sophisticated astronomical observations possible now. The Earth's atmosphere still limits a great deal of the celestial infrared radiation that reaches the surface, so detectors have been taken off the ground over the last two decades, including the airborne Kuiper Astronomical Observatory (KAO), a high-flying decommissioned military cargo plane with an on-board telescope, and the space-based Infrared Astronomical Satellite (IRAS), an international mission that orbited the Earth in 1983. Upcoming NASA missions, such as the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), and the Space Infrared Telescope Facility (SIRTF) and Next Generation Space Telescope (NGST), will take the field to new heights. As ground-based infrared astronomy becomes more sophisticated, it paves the way for the success of these future ambitious missions.

For further information about infrared light and its importance to astronomy, see IPAC's excellent, award-winning tutorial on Infrared Astronomy.

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