Why infrared light?
The light we can see with our eyes is composed of many light rays, each with its own colour—like those of a rainbow. For instance, the sky appears to be blue on a bright, sunny day because the light rays emitted by the sky are blue; grass appears as green because its light rays are green. But there are many other kinds of light in the universe that our senses cannot detect, for instance, infrared light, which can offer astronomers different kinds of information that is not readily discernable to the naked eye.
At the beginning of the Universe, the light emitted by the first galaxies was mainly ultraviolet. Because of the expansion of the Universe, this light is now "redshifted" (i.e. it has moved along the spectrum of light to appear more red). Many celestial objects, like brown dwarfs and enormous red giant stars, are too cold to emit much visible light; most of their rays are in infrared light. Because infrared light wavelengths are longer than those of visible light, they can penetrate dust clouds more easily, which allows astronomers to peer inside nebulae to see newly forming stars and planets that would otherwise remain hidden.
Compare Visible and Infrared Light for Yourself!
The Orion Nebula in Visible Light
This composite image compares infrared and visible light views of the famous Orion Nebula, a nearby large region of star formation, and its surrounding clouds. In addition to Orion, which is in the lower half of the image, two other nebulae are present. The small nebula above and to the left of Orion, is called M43 and the nebula at the very top of the image is NGC 1977. Each nebula is surrounded by a ring of dust that is clearly visible in infrared light. These rings are created by the radiation and winds of the massive stars that formed in the region. In visible light, the gas heated by the ultraviolet radiation of these stars can be clearly seen.
Above the Orion Nebula, the image in visible light is rather dark. However, the infrared image makes it possible to see through the dust to reveal stars being formed (in green in the infrared image).
The Orion Nebula – in infrared light
The infrared image from Spitzer shows the light from the dust heated by the star's light in red (8 microns) and orange (5.8 microns). The green (4.5 microns) shows the very hot gas and dust. The blue (3.6 microns) shows the light that comes from the stars.
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