APOD: 2004 August 26- Cassiopeia A in a Million . ... Credit Copyright: U. Hwang ( GSFC /UMD), J.M. Lamming (NRL), et al., CXC , NASA , Explanation: One million seconds of x-ray image data were used to construct this view of supernova remnant Cassiopeia A, the expanding debris cloud from a stellar explosion. ... Seen in false-color , Cas A's outer green ring, 10 light-years or so in diameter, marks the location of the expanding shock from the original supernova explosion. ...