Why the galaxies forms 2D plane (or spiral-like) instead of 3D ball (or spherical-like)?

The formation of galaxies in a 2D plane or spiral structure, rather than a 3D spherical shape, is a fascinating aspect of astrophysics that involves the interplay of gravity, rotation, and angular momentum. To understand this phenomenon, let’s break it down step by step.

The Role of Gravity and Angular Momentum

When a cloud of gas and dust in space begins to collapse under its own gravity, it doesn't just fall straight inwards. Instead, as the material moves closer together, it also retains some of its initial rotation. This is where angular momentum comes into play. Angular momentum is a property of rotating bodies that depends on their mass, shape, and speed of rotation.

Why a Disk Shape Forms

As the gas cloud collapses, it starts to spin faster due to the conservation of angular momentum. Imagine a figure skater pulling in their arms to spin faster; the same principle applies here. As the material falls inward, it flattens out into a rotating disk. This flattening occurs because particles in the cloud collide and interact, losing energy and settling into a more stable configuration.

• Collisions: As particles collide, they lose energy and settle into a plane.
• Rotation: The conservation of angular momentum causes the material to spread out into a disk rather than a sphere.

Formation of Spiral Galaxies

Within this disk, gravitational forces and the dynamics of rotation lead to the formation of spiral structures. The density waves in the disk create areas of higher and lower density, which can lead to the formation of stars and other structures. This is why we see spiral galaxies, which are characterized by their beautiful, swirling arms.

Examples of Spiral Structures

One of the most well-known examples of a spiral galaxy is the Milky Way. Its structure is a direct result of the processes described above. The arms of the galaxy are regions where star formation is actively occurring, often triggered by the density waves moving through the disk.

Why Not a Spherical Shape?

A spherical shape would imply that the material is evenly distributed in all directions, which is less stable for rotating systems. In a spherical configuration, any slight perturbation could lead to instability, causing the material to collapse into a disk shape over time. The dynamics of rotating systems naturally favor a disk-like structure due to the reasons outlined above.

Visualizing the Concept

Think of a pizza dough being spun in the air. As you spin it, the dough flattens out into a circle. If you were to just drop the dough without spinning it, it would remain more spherical. This analogy helps illustrate why rotating systems tend to form flat disks rather than spherical shapes.

Conclusion

In summary, galaxies form in a 2D plane or spiral-like structure due to the effects of gravity, rotation, and angular momentum. The initial collapse of gas clouds, combined with the conservation of angular momentum, leads to the formation of a disk. This disk can then develop into the beautiful spiral shapes we observe in many galaxies today. Understanding these processes not only helps us appreciate the structure of our universe but also the fundamental laws of physics that govern it.