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In extreme environments, where pressure, temperature, and density are high, atoms change the way they emit and absorb light. Electrons are stripped from nuclei, forming a plasma, and the ions emit X-rays when struck by free electrons. To assist astronomers in identifying atoms under these harsh conditions, the Center for Astrophysics | Harvard & Smithsonian maintains the AtomDB database of X-ray spectra. This catalog provides essential information for studying conditions in many astrophysical environments, including those near black holes, stars, and neutron stars.