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Infrared Astronomy is the detection and study of theinfrared radiation(heat energy) emitted from objects in the Universe. Every object that has atemperature radiates in the infrared. So, Infrared Astronomy involves the study of just about everything in the Universe. In the field of astronomy, the infrared region lies within the range ofsensitivity of infrared detectors, which is betweenwavelengths of about 1 and 300 microns(a micron is one millionth of a meter).The human eye detects only 1% of light at 0.69 microns, and 0.01% at 0.75microns, and so effectively cannot see wavelengths longer than about 0.75microns unless the light source is extremely bright.

{see an example of theinvisible becoming "visible" in the infrared}

The Universe sends us a tremendous amount of information in the form ofelectromagnetic radiation(or light). Much of this information is in the infrared, which we cannotsee with our eyes or with visible light telescopes. Only a small amount ofthis infrared information reaches the Earth's surface, yet by studying thissmall range of infrared wavelengths, astronomers have uncovered a wealth ofnew information. Only since the early 1980's have we been able to sendinfrared telescopes into orbit around the Earth, above the atmosphere whichhides most of the Universe's light from us. The new discoveries made by theseinfrared satellite missions has been astounding. The first of these satellites -IRAS (Infrared Astronomical Satellite) - detected about 350,000infrared sources, increasing the number of cataloged astronomical sources byabout 70%.

In space, there are many regions which are hidden from optical telescopesbecause they are embedded in dense regions of gas and dust.However, infrared radiation, having wavelengths which are much longer thanvisible light, can pass through dusty regions of space without beingscattered.This means that we can study objects hidden by gas and dust inthe infrared, which we cannot see in visible light, such as the center of our galaxy and regions of newly forming stars.

The images to the left, of the central region of our own Milky Way Galaxy andof the Cygnus star-forming region, show how areas which cannot be seen invisible light can show up very brightly in the infrared.The top row shows these regions in visible red light. At thiswavelength we are seeing the light from billions of stars, particularlythe largest, brightest ones. Note the dark bands where vast clouds ofdust block our view of more distant objects. The middle row shows thesame regions in thenear-infrared(infrared wavelengths closest to visible light). Here the light we see isalso generated by stars, but now it better traces the smaller, cooler ones.Notice how the the lanes of dust havebecome partially transparent, allowing us to see things that are hiddenin visible light. Our view of the central bulge of stars in our own Milky Way galaxy isparticularly striking since it is almost completely obscured at shorterwavelengths!The bottom images show these regions in thefar-infrared (infrared wavelengths farther from visible light). At thesewavelengths, stars hardly emit any light at all. Instead almost everything wesee is generated by the dust clouds themselves. The dust,which is colder than the coldest arctic night on earth, is still warmenough to emit the thermal infrared radiation seen here.

Many objects in the universe which aremuch too cool and faint to be detected in visible light, can be detected inthe infrared. These include cool stars, infrared galaxies, clouds ofparticles around stars, nebulae, interstellar molecules, brown dwarfs and planets.For example, the visible light from a planet is hidden by the brightness of the star thatit orbits. In the infrared, where planets have their peak brightness, the brightness of the star isreduced, making it possible to detect a planet in the infrared.Some of the most exciting discoveries in infrared astronomy have been the detectionof disks of material and possible planets around other stars. Recently, an infrared survey of the Trapezium star cluster in the Orion Nebularevealed over 100 low mass objects which are brown dwarf candidates.Click on the image for details.

In the infrared, astronomers can gather information about the universe asit was a very long time ago and study the early evolution of galaxies.As a result of the Big Bang (the tremendous explosion which marked thebeginning of our Universe), the universe is expanding and most of thegalaxies within it are moving away from each other. Astronomers havediscovered that all distant galaxies are moving away from us and that thefarther away they are, the faster they are moving. This recession of galaxiesaway from us has an interesting effect on the light emitted from thesegalaxies. When an object is moving away from us, the light that it emits is"redshifted". This means that the wavelengths get longer and thereby shiftedtowards the red part of the spectrum.This effect, called the Doppler effect, is similar to what happens to soundwaves emitted from a moving object. For example, if you are standingnext to a railroad track and a train passes you while blowing its horn,you will hear the sound change from a higher to a lower frequency asthe train passes you by.As a result of this Doppler effect, at large redshifts, all of the ultravioletand much of the visible light from distant sources is shifted into the infrared part of thespectrum by the time it reaches our telescopes. This means that the only way to study this light is in the infrared.Infrared astronomy will provide a great deal of information on how and whenthe universe was formed and on what the early universe was like. The image to the left is an infrared view of some of the farthest galaxiesever seen. (Image credit:R.I. Thompson (U. Arizona), NICMOS, HST, NASA)
Objects which can be seen in visible light can also be studied in the infrared. Infrared astronomy can not only allow us to discover new objects and viewpreviously unseen areas of the universe, but it can add to what we alreadyknow about visible objects.To get a complete picture of any object in the Universe we needto study all of the radiation that it emits. Infrared Astronomyhas, and will continue to, add a greatdeal to our knowledge about the Universe and the origins of our Solar System.