As this image shows, the Whirlpool Galaxy’s nucleus is particularly bright in x-ray emission (about 100,000 times brighter than the center of our own Milky Way). There are also peculiar x-ray hot-spots in the spiral arms. 
    Only a highly energetic, compact object could heat gas to this extreme temperature. No star could. Any star so hot and so bright would simply blow itself apart. Many astronomers believe that only a black hole could generate the energy necessary to heat gas to such unfathomable temperatures.