Stars do not form one at a time. Rather, hundreds of stars are formed from the collapse of large molecular clouds known as stellar nurseries. One such stellar nursery is the Orion Nebula, where stars and whole planetary systems can be observed at various stages of formation.
While the period of active star formation is relatively short on a cosmological scale (on the order of hundreds of thousands of years), it is far too slow to observe directly. This means we study stellar nurseries in two ways. One is to observe active stellar nurseries such as the Orion Nebula. By observing protostars, young stars, planetary disks, etc. within a stellar nursery we can piece together a timeline of how stars and solar systems form. The other method is to study stellar nurseries computationally. On a large computer cluster we can simulate the gravitational collapse of large molecular clouds. Once such simulation can be seen in the video below.
This particular model starts with a molecular cloud of about 50 solar masses initially in a rough sphere about a light year across. As you watch the video you will see two driving mechanisms at work. The first is gravity as it tries to collapse the molecular cloud together. If the cloud were a perfect sphere initially at rest it might succeed, but in this case the cloud has a bit of turbulence, just as real molecular clouds would. This turbulence means there is a bit of rotational motion (angular momentum) scattered about the cloud.
When something rotates, it will tend to rotate at a constant rate unless it changes shape. Angular momentum basically depends on an object’s shape as well as rotational speed, so when shape changes, rotational speed changes as well. A good example of this can be seen in figure skating. When a skater has their arms and legs stretched out they spin rather slowly, but when they jump into the air and pull their arms and legs close to their body they spin rather quickly.
The same thing happens in the molecular cloud. As gravity collapses the cloud, the turbulent parts rotate faster and faster. Of course when something rotates quickly it has a tendency to fly apart. The interaction of gravity and angular momentum means that the molecular cloud will tend to collapse into clumps, but those clumps will also tend to fling apart into smaller, more dense clumps. These small clumps are flung out of the molecular cloud to become stars and planetary systems.
Since computer models can be run multiple times, we can model different initial conditions to see how things change. We can also do things such as add more or less dust to the model cloud, or change the amount of different elements. By comparing these models to observed stellar nurseries we can learn more about the process of stellar evolution.
To see more videos, and for more technical details of these models, check out the website of Matthew Bate.