In the center of the galaxy in which we live, seen in the constellation Sagittarius (Archer), there lies a black hole. We can see its imprint in the radio and the X-ray image. Material that falls down to the black hole heats up. This results in energetic processes that generate radio waves and X-rays that we can observe. By contrast, in visible light dust particles in our galaxy obscure our view. So we cannot directly see the center of our galaxy; the stars seen in the visible light image are all in the foreground.

Like all galaxies, our galaxy contains billions of stars. Most of them live in a flat disk-shaped structure. Our Sun lies about two-thirds out from the center. Looking along the plane of the disk we see many stars, but looking perpendicular to it we see very few. Our galaxy therefore appears to us as the luminous band across the sky that our ancestors named the Milky Way.

The Sun moves around the center of our galaxy in the same way that the Earth moves around the Sun -- but it takes 250 million years to complete one orbit. The Sun never gets close to the black hole in the center of the Milky Way, but some other stars do. Astronomers have used observations in infrared light to measure the speeds of such stars. Infrared light has longer wavelengths than visible light, which allows it to penetrate the dust in our Milky Way. It can be calculated from the measured speeds that the black hole in the center of our Milky Way is three million times heavier than the Sun. Such black holes are called, not surprisingly, supermassive.