A galaxy may look calm and stable from far away, but new research suggests it can actually behave in surprisingly chaotic ways.
Scientists at Leiden University have discovered that changing the position of just a single star in a computer simulation can eventually reshape the structure of an entire galaxy.
The study, carried out by astronomers Tetsuro Asano and Simon Portegies Zwart, explored how galaxies like the Milky Way evolve over time.
Their findings help explain why astronomers often get different results when simulating galaxies, even when they begin with almost exactly the same starting conditions.
To investigate this mystery, the researchers created hundreds of virtual galaxies similar to the Milky Way.
Each galaxy contained a flat disk filled with stars and surrounded by a huge invisible cloud of dark matter, which provides the gravity needed to hold the galaxy together.
The scientists then compared pairs of simulations that were nearly identical. In some cases, the only difference was a tiny adjustment in the position of a single star.
At first, the galaxies looked almost the same. But over time, those tiny differences grew larger and larger. Spiral arms formed in different patterns, and the rotating bar of stars at the center of the galaxy developed differently.
This is similar to the famous “butterfly effect,” where a very small change can eventually produce a major outcome. In weather science, for example, the flap of a butterfly’s wings is sometimes used as a symbol for how small events can influence large systems.
For many years, astronomers believed galaxies should behave more smoothly because they contain hundreds of billions of stars. The traditional idea was that tiny disturbances would average out and become unimportant. However, this new research shows that small disturbances can have major long-term effects.
The team also solved an important puzzle about why previous galaxy simulations sometimes produced conflicting results. In many computer models, gravity between nearby stars is artificially softened to make calculations easier and faster. Instead of treating stars as exact points, the simulations treat them more like fuzzy clouds. This reduces the strong close-range interactions that normally create chaotic behavior.
By carefully changing the amount of this “softening,” the researchers showed when a simulation behaves realistically and when it hides the galaxy’s natural chaos.
The study revealed that some galactic features are stable and predictable. For example, nearly every simulation still formed a central bar of stars. But the exact shape of that bar and the patterns of the spiral arms could vary greatly depending on very tiny differences at the beginning.
According to Portegies Zwart, the Milky Way becomes effectively unpredictable after only about one million years. That sounds long to humans, but compared to the galaxy’s age of roughly ten billion years, it is incredibly short.
Even so, the chaos has limits. The galaxies may end up looking different, but they still remain recognizable spiral galaxies. The researchers say this finally explains how galaxies can appear both orderly and chaotic at the same time.
