Monday, March 16, 2009


Between about 0.7 to 1.1 microns we can use the same observing methods as are use for visible light observations, except for observation by eye. The infrared light that we observe in this region is not thermal (not due to heat radiation). Many do not even consider this range as part of infrared astronomy. Beyond about 1.1 microns, infrared emission is primarily heat or thermal radiation.

As we move away from visible light towards longer wavelengths of light, we enter the infrared region. As we enter the near-infrared region, the hot blue stars seen clearly in visible light fade out and cooler stars come into view. Large red giant stars and low mass red dwarfs dominate in the near-infrared. The near-infrared is also the region where interstellar dust is the most transparent to infrared light.

Visible (left) and Near-Infrared View of the Galactic Center
Visible image courtesy of Howard McCallon. The infrared image is from the 2 Micron All Sky Survey (2MASS)

Notice in the above images how center of our galaxy, which is hidden by thick dust in visible light (left), becomes transparent in the near-infrared (right). Many of the hotter stars in the visible image have faded in the near-infrared image. The near-infrared image shows cooler, reddish stars which do not appear in the visible light view. These stars are primarily red dwarfs and red giants.

Red giants are large reddish or orange stars which are running out of their nuclear fuel. They can swell up to 100 times their original size and have temperatures which range from 2000 to 3500 K. Red giants radiate most intensely in the near-infrared region.

Red dwarfs are the most common of all stars. They are much smaller than our Sun and are the coolest of the stars having a temperature of about 3000 K which means that these stars radiate most strongly in the near-infrared. Many of these stars are too faint in visible light to even be detected by optical telescopes, and have been discovered for the first time in the near-infrared.

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