Astronomy in the deep freeze
Box 2 | A different window on the universe
Devices using infrared radiation are part of modern living. A beam from the remote control turns the television on or off or changes the channel. Infrared is also used in car locking systems and home security systems.
Infrared imaging is widely used in medicine, fire fighting, police and security work, and by the military. Infrared satellites are used to monitor the weather, to study vegetation patterns and geological formations, and to measure ocean temperatures.
Infrared radiation is simply another form of electromagnetic radiation, similar to visible light and radio waves, but differing in its wavelength (or frequency). Infrared wavelengths begin in the ‘near’ infrared at around 0.0007 millimetres, just beyond the reddest light that the human eye can see, and grow in size to about 1 millimetre in the ‘far’ infrared.
Wavelengths longer than this belong to the sub-millimetre, microwave and radio parts of the electromagnetic spectrum. Different types of telescopes are required to view the universe through these other ‘windows’ of the spectrum.
Infrared radiation is emitted by any object with a temperature above absolute zero (-273ºC). In other words, the object radiates heat. Basically, all objects in space emit some infrared, but the wavelength at which the object radiates most intensely depends on its temperature. A very hot object has its peak emission near the middle of the wavelength range of visible light. As the object cools, the wavelength of peak emission moves to longer wavelengths (infrared). Very hot objects, such as stars, dominate views of the universe at wavelengths of visible light, but they are much less prominent at wavelengths of infrared.
Objects in space emit radiation at all wavelengths across the electromagnetic spectrum. However, most of this radiation does not reach the Earth’s surface. Fortunately for life on Earth, the atmosphere blocks out harmful high energy radiation such as gamma rays and X-rays, and most ultraviolet rays. It also blocks out radio waves with long wavelengths and most infrared radiation. However, the atmosphere lets through visible light, most radio waves, and narrow ranges of infrared wavelengths, allowing astronomers to view the universe through these wavelength windows.
Pros and cons of infrared
Most of the infrared radiation reaching Earth is absorbed by water vapour (H2O), carbon dioxide (CO2) and ozone (O3) in the atmosphere. Only in a few wavelength ranges can the radiation make it through to ground level. Overcoming this limitation is the greatest challenge faced by infrared astronomers.
Besides blocking out most infrared radiation, the Earth’s atmosphere poses another tricky problem. The atmosphere is warm and radiates strongly in the infrared, often swamping the astronomical object being observed. For the infrared astronomer, the sky itself glows brightly day and night.
Thus, the best view of the infrared universe is at wavelengths that pass easily through the Earth’s atmosphere and where the background radiation from the atmosphere itself is at a minimum. For this reason infrared observatories are usually placed near the summit of high mountains to get above as much of the atmosphere as possible. Low temperatures at mountain summits are also an advantage because they cause water vapour to condense out of the air.
Despite these difficulties, infrared observations are important for several reasons. Infrared radiation passes through the vast gas and dust clouds in interstellar space more easily than visible light, revealing objects hidden from optical telescopes. For example, young stars are usually surrounded by a cocoon of gas and dust. This makes them invisible at optical wavelengths, but their heat warms the dust grains and produces infrared radiation to reveal their presence.
Page updated July 2006.