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   71012
   To investigate the chemical evolution of the universe is one of the
   most important subjects in X-ray astronomy. The results of the ASCA
   observations of clusters of galaxies, galaxies, and supernova remnants
   are inconsistent with the standard model of the chemical evolution of
   the universe. However, the connection on the chemical evolution between
   stars and interstellar space are not yet revealed. Therefore we propose
   X-ray observation of a planetary nebula, NGC7293, where metals diffuse
   into interstellar space from a normal star.
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