Why is the Milky Way flat? - Scott Hershberger

Summary

The Milky Way, like most galaxies, is constantly evolving. Our solar system has moved between its spiral arms over millions of years. Initially, the galaxy formed from chaotic gas and dust, which gradually flattened into a spinning disc due to angular momentum. Spiral arms, regions of dense stars and gas, are created by waves of compression traveling through the galaxy. These arms are not permanent, changing over time. In 5 billion years, the Milky Way will merge with the Andromeda galaxy, transforming its shape and marking a new era in its history.

Takeaways

• 🌌 The Earth resides on the inner edge of a spiral arm within the Milky Way, but this is not its permanent location.
• 🦖 During the time when most dinosaurs went extinct, our solar system was located in a different arm of the galaxy.
• 🔄 The Milky Way is constantly evolving, and even its structure continues to change over time.
• 💫 13 billion years ago, the Milky Way formed from gas and dust propelled by supernovae and early universe chaos.
• 🌀 The galaxy's total angular momentum is the sum of the individual particles’ angular momentum, resulting in the galaxy's rotation.
• 💥 The Milky Way became flat due to its high angular momentum, preventing it from forming a spherical shape like stars or planets.
• 📏 Spiral arms are regions with densely packed stars and gas, but they are not static and are created by density waves.
• 🌊 Density waves travel through the galaxy, causing stars to enter and leave spiral arms as the waves move.
• 🎇 The Milky Way’s spirals may have formed due to internal clumps of stars and gas, creating spontaneous density waves.
• 🌌 In 5 billion years, the Milky Way will merge with the Andromeda galaxy, forming a new shape and ending the current spiral era.

Q & A

Where is Earth located within the Milky Way galaxy?

Earth is located on the inner edge of one of the Milky Way's spiral arms.

Has Earth's position within the Milky Way always been the same?

No, Earth's position has changed over time. When most dinosaurs went extinct, the solar system was in a different spiral arm than it is today.

Why does the Milky Way continue to change its structure over time?

The Milky Way never stops changing because it is influenced by the movement of gas, dust, and stars, as well as density waves and gravitational interactions with nearby objects.

What happened 13 billion years ago that contributed to the formation of the Milky Way?

About 13 billion years ago, gas and dust particles propelled by supernovae and early cosmic chaos were moving in all directions, eventually contributing to the formation of the Milky Way.

Why did the Milky Way form as a flat, spiral galaxy rather than a spherical one?

The Milky Way formed as a flat, spiral galaxy because of its high angular momentum, which caused gas and dust particles to fall parallel to the axis of rotation rather than directly toward the center. This process led to the formation of a spinning disc.

What role does angular momentum play in the shape of galaxies?

Angular momentum prevents galaxies from collapsing into a sphere. In galaxies like the Milky Way, the higher angular momentum causes a spinning disc to form. Lower angular momentum could result in an egg-shaped galaxy.

What are spiral arms, and how do they form in galaxies like the Milky Way?

Spiral arms are regions where stars and gas are packed more tightly. They form due to density waves, which are zones of compression traveling through the galaxy. The rotation of the galaxy bends these waves into spirals.

Are the spiral arms of the Milky Way permanent structures?

No, spiral arms are not permanent. They are dynamic structures caused by density waves, and stars and gas are constantly entering and leaving them as the waves travel through the galaxy.

What causes density waves that create spiral arms in galaxies?

Density waves can be triggered by gravitational interactions with nearby companion galaxies or by internal clumps of stars and gas within the galaxy, which can spontaneously generate waves.

How will the future collision between the Milky Way and Andromeda galaxies affect their structures?

In about 5 billion years, when the Milky Way merges with the Andromeda galaxy, the collision will disrupt the angular momentum, causing the formation of an egg-shaped structure, signaling a new phase in the history of the galaxies.