Galaxies, vast collections of stars, gas, dust, and dark matter, display a remarkable diversity of forms. While some, like our own Milky Way, are distinctly flat and disk-shaped, others appear more spherical or possess irregular structures. This variation prompts a fundamental question: why do some galaxies flatten into disks while others do not? The answer lies in the interplay of physical processes governing their formation and evolution.
The Cosmic Starting Point
Galaxies begin as immense, diffuse clouds of gas and dust scattered throughout the early universe. These primordial clouds were not perfectly uniform, containing slight variations in density and distribution. Such minor irregularities, imprinted shortly after the Big Bang, set the stage for gravitational collapse. These vast clouds represented the material from which all galaxies would eventually emerge.
Gravity’s Pull and the Spin
The primary force shaping these initial cosmic clouds is gravity, pulling matter inward towards regions of higher density. These nascent clouds possessed a slight amount of rotation, arising from tidal torques exerted by surrounding matter. This initial spin introduces angular momentum, a measure of an object’s tendency to continue rotating.
From Cloud to Flat Disk
As gravity pulls the cloud inward, its rotation significantly shapes its ultimate form. Conservation of angular momentum dictates that as the cloud shrinks, its rotation rate increases, much like a spinning ice skater pulling their arms inward. This increasing spin creates a centrifugal effect, resisting gravitational collapse perpendicular to the axis of rotation. Along the axis, there is no such outward resistance, allowing matter to collapse more easily.
Furthermore, gas within these clouds cools by radiating energy, losing kinetic energy and falling deeper into the gravitational well. This cooling enables further contraction, but crucially, it does not remove angular momentum.
As a result, the material flattens into a disk as it collapses and spins faster. Matter continues to fall into this disk, fueling star formation and reinforcing its flattened structure.
Why Some Galaxies Aren’t Flat
Not all galaxies exhibit a flattened, disk-like structure; some appear more spherical, known as elliptical galaxies, while others have chaotic, irregular shapes. Elliptical galaxies are generally thought to form through different evolutionary pathways.
Elliptical Galaxies
One common theory suggests they primarily result from the mergers of two or more spiral galaxies. When galaxies collide and merge, their organized disk structures are disrupted, and the stars within them are thrown into more randomized, three-dimensional orbits, leading to a more spherical or ellipsoidal shape. These galaxies often contain older stellar populations and have little remaining gas and dust, indicating a past burst of star formation followed by a cessation of new star birth. Alternatively, some elliptical galaxies may have formed from initial gas clouds that possessed very little intrinsic angular momentum, precluding the formation of a flat disk from the outset.
Irregular Galaxies
Irregular galaxies, on the other hand, lack a defined shape and appear disorganized. These galaxies are frequently the outcome of strong gravitational interactions or collisions with other galaxies. Such encounters can severely distort a galaxy’s original structure, preventing it from settling into a stable disk or elliptical form. Irregular galaxies often exhibit regions of active star formation and can be rich in gas and dust, reflecting the turbulent processes that shaped them.