🌌 The Story of the Milky Way
Explore the dynamic history and evolution of our home galaxy.
Within the Milky Way, the Earth resides on the inner edge of a spiral arm. However, this is not our permanent address. When most dinosaurs went extinct, our solar system was located in a different spiral arm. The Milky Way is constantly changing, and even its structure continues to evolve.
Around 13 billion years ago, the gas and dust that eventually formed the Milky Way were moving rapidly in every direction. Each particle possessed angular momentum, which describes the magnitude and direction of its rotation around the galaxy's center. Together, these individual motions created the overall rotation of the young galaxy.
🌀 How the Galactic Disc Formed
As the particles frequently collided, they lost energy as heat. Moving more slowly, gravity pulled them closer to the galactic center. Unlike stars and planets, however, the Milky Way became a flat disc rather than a sphere. This happened because galaxies have relatively low density but high angular momentum. To conserve angular momentum, particles tended to move parallel to the axis of rotation instead of falling directly toward the center. Over billions of years, they formed a rapidly rotating galactic disc.
🌊 Spiral Arms and Density Waves
The spiral arms are regions where stars and gas are more densely packed. They are not fixed structures but are created by density waves moving through the galaxy, much like waves moving across water. As these waves travel, stars continuously move into and out of the spiral arms.
Density waves can form in different ways. A nearby companion galaxy may disturb a galaxy's structure through its gravitational pull, creating waves that can last for billions of years. Alternatively, a dense clump of stars and gas within the galaxy can trigger waves on its own. Scientists believe this is how the Milky Way's spiral arms formed.
Our solar system orbits the center of the galaxy faster than the spiral arms themselves. As a result, it will continue moving deeper into its current spiral arm before eventually leaving it millions of years from now.
⏳ A Constantly Changing Galaxy
Recent observations suggest that spiral galaxies often contain multiple overlapping density waves moving at different speeds. Because of this, spiral arms are constantly forming, breaking apart, and reforming over tens or hundreds of millions of years. Therefore, the Milky Way likely looked very different when Earth formed 4.5 billion years ago.
🚀 Looking Far Into the Future
About five billion years from now, the Milky Way will begin merging with the Andromeda galaxy. This collision will disrupt the galaxy's balance of angular momentum, creating an egg-shaped galaxy and marking the beginning of a new chapter in our galaxy's evolution.