One of the first datasets publicly released after the launch of NASA’s Spitzer Space Telescope in August 2003 was for Messier 81, a galaxy just 12 million light-years from Earth.
To mark Spitzer’s 16th anniversary this new infrared image has been released, with extended observations and improved processing.
It is, in fact, is a composite mosaic combining data from the Infrared Array Camera (IRAC) at wavelengths of 3.6/4.5 microns (blue/cyan) and eight microns (green) with data from the Multiband Imaging Photometer (MIPS) at 24 microns (red).
The 3.6-micron near-infrared data (blue) traces the distribution of stars, although the Spitzer image is virtually unaffected by obscuring dust and reveals a very smooth stellar mass distribution, with the spiral arms relatively subdued.
At longer wavelengths, the spiral arms become the dominant feature of the Messier 81 galaxy. The eight-micron emission (green) is dominated by infrared light radiated by hot dust that has been heated by nearby luminous stars.
Dust in the galaxy is bathed by ultraviolet and visible light from nearby stars.
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