I am sixth grader and I am doing a science project on galaxies. I wanted to know how galaxies are formed.
In general, we talk about "hierarchical" structure formation - we think that the galaxies we see today are "built up" in the coalescence of smaller objects over time. In the early universe, we think that the gas (which eventually ends up in galaxies) traces the dark matter content of the universe. Structure forms as gravity brings massive objects together, and dark matter "halos" that include some amount of cooling gas likely become the first building blocks of galaxies. There are a number of different ingredients that we think are important for galaxy formation and for the diversity of galaxy structures we see today: the mass of galaxies and their dark matter halos, their central supermassive black holes, their environments, etc.
As far as some of the features of galaxies that are common today: We think we see thin disks in spiral galaxies form because of conservation of angular momentum (the same effect as the spinning up of the dancer when she pulls her arms inside). Elliptical galaxies are thought to be formed as a result of a merger of disk galaxies. When spiral galaxies merge, then the orbits of all the stars are randomized. As a result, all the stars in an elliptical galaxy have random orbits and there is not much collective motion of stars.
Galaxy formation is a hard problem and is an area of active research! Beyond the wealth of observational studies into understanding galaxies, there are many interesting and beautiful efforts to computationally model galaxy formation - you can see videos of some here. Watch these videos to see how accretion of gas onto the galaxies and "bursts" of star formation appear to affect the shape and evolution of the galaxies.
This page was last updated on January 28, 2019