Audio Guide：

The rotation of a spiral galaxy isn’t as simple as it seems. Astronomer Vera Rubin observed the rotational speed of galaxies in the 1970s. She realized that the rotational speed of a galaxy does not decline with distance in the outer regions of the spiral. Rather, the speed remains constant in the outward portions. This led astronomers to conclude that there might be invisible matter in the outer regions of galaxies. The invisible matter, called “dark matter” by astronomers, influences the rotational speed of galaxies due to its mass and gravitational force. To explain how spiral galaxies rotate, astronomers Chia-Chiao Lin and Frank Shu proposed the density wave theory based on fluid mechanics. The theory points out that the stars in the spiral arms do not remain fixed in their position. Instead, they are affected by density wave and will move in the disc of the galaxy. These stars sometimes come together and sometimes move apart so that the structure of the galaxy’s spiral arms can be maintained.

You can learn more about the theories of galaxy rotation and the formation of spiral arms using the mechanical devices and videos on display.