Credit: NASA/JPL-Caltech

This image provides a close-up look at two of the extremely bright infrared galaxies revealed by NASA's Spitzer Space Telescope. While they are very faint (bottom) or even completely invisible (top) in the deepest-ever optical images obtained by NASA's Hubble Space Telescope, Spitzer easily picked them up because of their strong infrared emissions. Astronomers believe these galaxies are particularly "red" because they are very old and appear to go back in time to a period when the universe was only two billion years old.