Britannica
Sources used in this answer
How did the top-down and bottom-up models explain galaxy formation?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
Galaxy
The Whirlpool Galaxy (left), also known as M51, an Sc galaxy accompanied by a small irregular companion galaxy, NGC 5195 (right).
During the latter half of the 20th century, astronomers debated two primary models regarding how galaxies originated from early density fluctuations in the universe:1
- Top-down model: This theory proposed that galaxies formed through the gravitational collapse of much larger gas clouds.1
- Bottom-up model: This theory suggested that galaxies were built from the merger of smaller entities, roughly the size of globular clusters.1
In both models, the angular momentum of the initial clouds was considered a key factor in determining the final shape of the galaxy. A protogalaxy with high angular momentum typically evolved into a flat, rotating spiral galaxy, while one with little angular momentum formed a more spherical elliptical galaxy.1
Modern understanding has shifted toward the view that galaxies grow through ongoing collisions and the accretion of smaller objects, a process supported by deep-space surveys and computer simulations.1