Messier 13 is a massive collection of hundreds of thousands of stars known as a globular cluster. Because it orbits the center of the Milky Way galaxy, it is constantly being pulled by the galaxy's massive gravity. These pulling forces are called tidal forces. Just as the Moon's gravity creates tides in Earth's oceans, the Milky Way's gravity pulls on the stars within Messier 13.
Over billions of years, these tidal forces influence the cluster in several ways. First, they can strip stars away from the outer edges of the cluster. These lost stars form long streams of light that trail behind or lead ahead of the cluster in its orbit. Second, the gravitational tugging can compress or stretch the cluster, slightly altering its spherical shape. While the core of Messier 13 remains very stable due to its high density, the outer layers are constantly being reshaped by the galaxy's influence. This ongoing interaction is a natural part of how star clusters evolve within a large galaxy. Understanding these forces helps astronomers track the history and movement of the cluster through space.
