These four images of the Whirlpool galaxy show how different wavelengths of light can reveal different features of a cosmic object.
Also known as Messier 51 and NGC 5194/5195, and located some 23 million light-years away in the constellation Canes Venatici, the Whirlpool is, in fact, a pair of galaxies that are tugging and distorting each other through their mutual gravitational attraction.
Panel (a) is the galaxy in visible light. Taken with the Kitt Peak National Observatory 2.1-metre telescope, it shows light at 0.4 microns (blue) and 0.7 microns (green).
Panel (b) combines two visible-light wavelengths (blue and green) and infrared light (red). The infrared was captured by NASA’s Spitzer Space Telescope and emphasises how the dark dust veins that block our view in visible light begin to light up at these longer, infrared wavelengths.
The right two panels are composed entirely of Spitzer data. Panel (c) shows three wavelengths of infrared light: 3.6 microns (blue), 4.5 microns (green) and eight microns (red).
The blended light from the billions of stars in the Whirlpool is brightest at the shorter infrared wavelengths and appear as a blue haze. The individual blue dots across the image are mostly nearby stars and a few distant galaxies. Red features (at eight microns) show dust composed mostly of carbon that is illuminated by the stars in the galaxy.
Panel (d) expands the infrared view to include light at a wavelength of 24 microns (red), which is particularly good for highlighting areas where the dust is especially hot. The bright reddish-white spots trace regions where new stars are forming and, in the process, heating their surroundings.
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