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What's special about the infrared?

When it comes to understanding galaxies and how they have changed with time, or "evolved", the infrared region of the electromagnetic spectrum is an important and unique one for several key reasons. The infrared spectral range is that part of the spectrum that starts beyond the reddest light that our eyes can see, and extends to the radio wavelengths. A general property of astronomical objects like stars is that they shine brightly over a great range of wavelengths, and the hotter they are the shorter the wavelength that their light peaks at; thus very hot stars look bluer than cooler stars. It turns out that the most common stars in the Universe actually peak beyond the optical band, in the infrared, specifically the so-called near-infrared (the infrared wavelength range "nearest" to the optical). This is the first reason that Spitzer excels at charting the sky: galaxy light peaks in it's near-infrared bands. SWIRE is detecting galaxies full of these typical stars using the [IRAC] camera.

SWIRE is also using the [MIPS] camera, which operates at mid- and far-infrared wavelengths, because regions of star formation inside galaxies are best seen at these wavelengths, and so Spitzer can readily home in on such regions which other telescopes find hard to see. The reason this wavelength range is so important is that young stars form inside great clouds of interstellar material -- shrouds of dense gas and dust -- which blocks them from sight at optical wavelengths, much like fog or a dust storm obscures vision on Earth. Moreover these hot young stars heat up the dust surrounding them until that dust itself warms up enough that it radiates energy away in the infrared, like warm ashes after a fire. So young star forming regions "light up" in the infrared. SWIRE is charting galaxies with lots of stars forming in them using this technique.

For more details about this fascinating region of the electromagnetic spectrum, visit IPAC's award-winning infrared tutorial .

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by Dr. Carol Lonsdale and Dr. Russ Laher