Audio Guide：

Between the Milky Way and the Earth is a lot of low-temperature matter that absorbs or scatter visible light. Therefore, while we cannot directly observe what’s going on at the Galactic Center in visible light, we are able to conduct observations with infrared radiation or radio waves. A celestial object known as Sagittarius A* exists at the Galactic Center. Using the speed of the celestial objects orbiting around Sagittarius A*, scientists are able to deduce that a super-massive black hole whose mass is 4 million times of the solar mass lies at the center of the Milky Way. As the black hole devours matter around it, intense x-rays are emitted.