Why Earth is a Sphere and the Milky Way a Disc?

Why do celestial bodies take the shapes they do? The Earth is unmistakably a sphere, yet the Milky Way stretches out as a disc. Delving into this reveals some fundamental truths about our universe. The cosmos seems to favor two shapes in particular: spheres and discs. Stars, planets, and moons are spherical, while spiral galaxies, solar systems, and phenomena like quasars adopt disc-like forms. These shapes are dictated by the same laws of physics, with gravity being the binding force. So, what determines their specific shapes?

The shape of space and celestial bodies has long fascinated scientists and laypeople alike. The shapes of celestial bodies are determined by the forces acting upon them and the properties of the matter they are composed of. The Earth and the Milky Way galaxy provide excellent examples of how these principles play out on different scales. Our Earth is a sphere, while the Milky Way takes on a disc-like form. Understanding why these shapes occur requires delving into the principles of physics and the nature of gravity.

Earth's spherical shape is a result of gravity. When a planet forms, it accumulates mass, which exerts a gravitational pull towards its center. This force pulls matter equally from all directions, causing the planet to mold into a shape where all surface points are equidistant from the center—resulting in a sphere. While Earth's rotation causes a slight bulging at the equator, making it an oblate spheroid, the dominant shape remains spherical due to gravity's uniform pull.

In contrast, the Milky Way galaxy’s disc shape is influenced by more complex dynamics, including rotation and angular momentum. When galaxies form, they start as massive clouds of gas and dust. As these clouds collapse under gravity, they begin to spin. Conservation of angular momentum, a fundamental principle of physics, dictates that as the cloud contracts, its rotation speed increases. This spinning motion causes the material to flatten out into a disc. Within this disc, stars, gas, and dust orbit the galactic center, creating the familiar flat structure of the Milky Way.

The central bulge of the Milky Way, a dense collection of stars, contrasts with the thin, extended disc. This bulge forms due to the more random and varied motion of stars closer to the galactic center, where gravitational forces are stronger and more chaotic.

Ultimately, gravity determines the shapes of both Earth and the Milky Way, but their unique conditions and forces create different forms. Earth's spherical shape reflects gravity's uniform influence, while the Milky Way's disc shape showcases the effects of rotation and angular momentum on cosmic structures. These ideas highlight the varied and complex nature of our universe's shapes.