A spiral galaxy is one of the universe’s most recognizable shapes, yet its thin, disc-like form is counterintuitive considering the forces that create it. These vast cosmic structures, which can span over a hundred thousand light-years, look remarkably like flat pancakes. The explanation for this flattened morphology lies in a sequence of events involving gravity, the initial matter, and a conserved physical property known as angular momentum. Understanding why galaxies are not shapeless spheres requires a look back at their earliest stages of formation.
The Starting Point: Diffuse Gas Clouds
The genesis of a galaxy begins as an immense, sparsely distributed cloud of matter within the early universe. These protogalactic clouds are primarily composed of primordial gas, mostly hydrogen and helium, the lightest elements created shortly after the Big Bang. This gaseous material is interspersed with a small percentage of cosmic dust, existing in a vast, turbulent, and irregular volume.
The initial distribution of this material is not uniform, containing slight overdensities that are the seeds of future structures. These clouds are enormous, perhaps hundreds of thousands of light-years across, and are much more spherical than the eventual flattened disc. The gas within this cloud is relatively hot and diffuse, with its particles moving randomly.
The Initial Force: Gravitational Collapse
The first act in galaxy formation is driven by the force of gravity, acting on the slight overdensities within the massive gas cloud. Because the cloud contains an enormous amount of mass, its gravity begins to pull all the material inward toward the center. This process, known as gravitational collapse, causes the cloud to shrink dramatically in size and increase in density.
As the cloud contracts, gravitational potential energy is converted into kinetic energy and heat, increasing the gas temperature. This collapse brings the widely dispersed matter closer together, reducing the volume of the original spherical cloud. However, gravity alone acts symmetrically in all directions and would only create a smaller, denser sphere; it does not inherently produce a flat shape.
The Mechanism of Flattening: Angular Momentum
The factor that transforms a collapsing sphere into a thin, rotating disc is the conservation of angular momentum. Even the turbulent protogalactic cloud possesses a tiny, net overall rotation, which is its initial angular momentum. This rotation is likely inherited from gravitational interactions, or tidal torques, with other nearby massive structures in the early universe.
As gravity forces the cloud to collapse inward, angular momentum conservation dictates that the cloud must spin faster to compensate for the reduction in its radius. This is analogous to a figure skater pulling their arms inward to increase rotational speed. This increased rotation creates a centrifugal force that acts perpendicularly to the axis of spin.
The gas particles within the cloud are collisional, meaning they interact frictionally, allowing the cloud to dissipate energy. This dissipation permits the material to collapse along the axis of rotation. However, the centrifugal force acts as a barrier, resisting collapse along the plane perpendicular to that axis. This constraint causes the material to settle into a flat, rotating plane, forming the thin disc of a spiral galaxy. The collapse stops when the outward centrifugal force balances the inward pull of gravity within the plane of rotation, establishing a stable disc.
Why the Halo and Bulge Remain Spherical
Not every component of a spiral galaxy conforms to the flattened disc shape; the central galactic bulge and the surrounding galactic halo maintain a more spherical distribution. The material forming these non-disc components either settled before the gas fully flattened or lacked the ability to dissipate energy. The central bulge is primarily composed of older stars that formed early in the galaxy’s history, before angular momentum had fully flattened the gas cloud.
The galactic halo is a vast, roughly spherical region extending far beyond the visible disc, dominated by dark matter. Unlike the gas that formed the disc, dark matter is non-collisional; its particles do not interact with each other or with ordinary matter through electromagnetic forces. Because dark matter cannot dissipate its kinetic energy through friction, it cannot flatten into a disc. It remains distributed in a spherical or ellipsoidal shape, retaining the initial large-scale structure of the protogalaxy.