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                           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.   
      
                                 2023 December 14   
      
                          Supernova Remnant Cassiopeia A   
           Image Credit: NASA, ESA, CSA, STScI; D. Milisavljevic (Purdue   
       University), T. Temim (Princeton University), I. De Looze (University   
                                     of Gent)   
      
      Explanation: Massive stars in our Milky Way Galaxy live spectacular   
      lives. Collapsing from vast cosmic clouds, their nuclear furnaces   
      ignite and create heavy elements in their cores. After only a few   
      million years for the most massive stars, the enriched material is   
      blasted back into interstellar space where star formation can begin   
      anew. The expanding debris cloud known as Cassiopeia A is an example of   
      this final phase of the stellar life cycle. Light from the supernova   
      explosion that created this remnant would have been first seen in   
      planet Earth's sky about 350 years ago, although it took that light   
      11,000 years to reach us. This sharp NIRCam image from the James Webb   
      Space Telescope shows the still hot filaments and knots in the   
      supernova remnant. The whitish, smoke-like outer shell of the expanding   
      blast wave is about 20 light-years across, while the bright speck near   
      center is a neutron star, the incredibly dense, collapsed remains of   
      the massive stellar core. Light echoes from the massive star's   
      cataclysmic explosion are also identified in Webb's detailed image of   
      supernova remnant Cassiopeia A.   
      
                            Tonight watch: The Geminids   
                        Tomorrow's picture: stellar eclipse   
        __________________________________________________________________   
      
          Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)   
               NASA Official: Phillip Newman Specific rights apply.   
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                         A service of: ASD at NASA / GSFC,   
                              NASA Science Activation   
                                & Michigan Tech. U.   
      
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