Astronomy Picture of the Day

Explanation: What is the Andromeda galaxy really like? To find out, astronomers looked at our largest galactic neighbor in a different light: infrared. Astronomers trained the orbiting Spitzer Space Telescope at the Messier monster (M31) for over 18 hours, creating a mosaic that incorporated 11,000 separate exposures. The result, pictured above, shows M31 in greater infrared detail than ever before. Infrared light in this 24-micron color band is particularly sensitive to dust heated up by stars. Visible above are previously undiscovered features including intricate structure in the spiral arms, a spiral arc near the center, an off center ring of star formation, and an unusual hole in the galaxy's disk. In contrast, the Andromeda galaxy appears much smoother in visible light and even ultraviolet light. Analyses and comparison of this image to other images will likely yield clues not only to the violent past of M31 but to our own Milky Way Galaxy as well.