What monster lies at our galaxy’s center?
The answer is in this chapter.
This is an image of the stars near our Milky Way galaxy’s center, which is marked with a small cross. Note that nothing shows up exactly at the center, but slightly off center (at the 8:00 position) there is a small circle. That circle is the image of a star known as S2. The other circles are also images of stars (the halos around them are artificially produced by the method of processing the images). Most stars in our galaxy move so slowly that we cannot actually see them move relative to one another, not even over a lifetime of observations. However, S2 is very different — we can see it move. In fact, it is moving so rapidly that it makes a complete trip around the Galaxy’s center in only 15.2 years. There must be something huge at the center, yet we see nothing there.
One of the long-standing goals of physics is to understand the gravitational force—the force that holds you to Earth, holds the Moon in orbit around Earth, and holds Earth in orbit around the Sun. It also reaches out through the whole of our Milky Way galaxy, holding together the billions and billions of stars in the Galaxy and the countless molecules and dust particles between stars. We are located somewhat near the edge ...