X-ray astronomy deals with phenomena which occur at the end of the stellar lifetimes: supernova explosions, neutron stars, and stellar black holes. Far outside our own Galaxy, the X-ray sky is dominated by active galaxies (radio galaxies, Seyfert galaxies, and quasars) with accreting supermassive black holes in their centers and by clusters of galaxies, the largest physical formations of our universe. Also, normal stars and galaxies can be studied with modern X-ray telescopes. And even comets and planets in the solar system are seen in X-rays. (Figure 21)
What is X-ray Astronomy
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