NASA's Spitzer Space Telescope captured this infrared image of NGC 5194, or M51, the 'Whirlpool Galaxy'. The spiral structure and star formation in M51 are thought to be triggered by an ongoing collision with its companion, NGC 5195 (blue galaxy at top). M51 was originally discovered by Charles Messier in 1773; the companion, NGC 5195, was discovered in 1781 by Pierre Mechain. This image is a four-color composite of invisible light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange) and 8.0 microns (red). A large number of thin, spoke-like filaments of red emission appear in infrared between the arms of the large spiral galaxy and in the gaps all over the face of the galaxy. While the spiral is rich in dust, bright in the longer infrared wavebands, and actively forming new stars, its blue companion shows little infrared emission and hosts an older stellar population.