Cassiopeia A Light Echoes in Infrared

Why is the image of Cassiopeia A changing? Two images of the nearby supernova remnant
taken a year apart in infrared light appear to show outward motions at tremendous speeds.
This was unexpected since the supernova that created the picturesque nebula was seen 325
years ago. The reason is likely light echoes. Light from the supernova heated up distant
ambient dust that is just beginning to show its glow. As time goes by, more distant dust
lights up, giving the appearance of outward motion. The above image is a composite of X-ray,
optical, and infrared light exposures that have been digitally combined. The infrared light
image was taken by the orbiting Spitzer Space Telescope and was used in the discovery of the
light echo. Cassiopeia A spans about 125 light years and lies 10,000 light years away toward
the constellation of Cassiopeia.