Audio Guide：

Active galaxies are brighter than regular galaxies. Their nuclei are also far more luminous than the other parts of the galaxy and thus named “active galactic nuclei”; intense activities can be observed here. This phenomenon makes active galaxies some of the brightest celestial objects in the universe. Scientists speculate that a super-massive black hole exists at the center of these galaxies. When a black hole devours matter, a vast amount of energy is released. The matter that does not fall directly into the center of the accretion disc will be spewed from both the top and bottom of the accretion disc and form incredible jet streams.