The Milky Way is our home galaxy, where the Sun and its planetary system are located. Why has the Milky Way, like most other galaxies, become flat rather than spherical like stars and planets?
The Milky Way.
In a star or planet, matter is so dense that the outward pressure combines with gravitational force to create a predominantly spherical shape. However, young galaxies like the Milky Way form from a chaotic cloud of dust and gas. Over time, the gas and dust collide with each other. Their momentum cancels out in the direction of their motion, but angular momentum is conserved.
The laws of physics also dictate that in the absence of external forces, the total angular momentum of an object or system must remain constant over time. In the Milky Way, particles tend to fall parallel to the axis of rotation to keep the total angular momentum constant and greater than zero.
Over billions of years, the cloud of particles gradually falls inward, accelerating along its orbit. As it spins faster, collisions increase, and it begins to flatten out and gradually takes on a disk shape. Simply put, due to angular momentum, as the initial sphere shrinks and becomes denser, any object outside the plane of the disk can be influenced or attracted by objects within the disk, ultimately forming a flat disk shape.
However, this shape of the Milky Way will not last forever. In about 5 billion years, the Milky Way will begin to merge with the Andromeda Galaxy, disrupting the balance of angular momentum and creating an egg-like shape—marking the beginning of a new era in the history of our galaxy.
