Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2002 August 24 Cas A Supernova Remnant in X-Rays Credit: John Hughes et al. (Rutgers), NASA / CXC / SAO Explanation: The complex shell of a star seen to explode 300 years ago is helping astronomers to understand how that star exploded. This Chandra Observatory image of supernova remnant Cassiopeia A (Cas A) shows unprecedented detail in three x-ray colors. The relationship between brightness, color, and position of...