Astronomy
Gravitational Collapse in Star-Forming Regions
Quick fact
Giant molecular clouds, the birthplaces of stars, can collapse to form hundreds or thousands of stars at once, with the process taking as little as a few hundred thousand years for the smallest stars.
Why this is interesting
A cloud of gas ten light-years across, colder than deep space, begins to fall inward. Within a few million years, a star ignites. How does a thin wisp of gas become the Sun, with nothing but gravity as the driving force?
Read the full explanation
Understanding Gravitational Collapse in Star-Forming Regions
Stars are born in enormous clouds of gas and dust called giant molecular clouds. These clouds are so cold (about 10-20 Kelvin) that the pressure within is barely enough to support them against their own gravity. When a region of the cloud becomes dense enough, gravity overcomes the internal pressure, and that region begins to contract. This is gravitational collapse. As the cloud shrinks, it heats up, but initially the heat escapes easily, so the collapse continues. The densest central region becomes a protostar, which continues to gather mass from the surrounding material. The whole process is a delicate balance between a force that pulls together (gravity) and a pressure that pushes apart (thermal pressure). The collapse occurs not all at once, but in fragments, each fragment potentially leading to a single star or a small group of stars.
A deeper explanation
The mechanism of gravitational collapse lies in the competition between gravity and pressure. Every parcel of gas in a cloud feels an inward gravitational force from all the matter around it. The gas also has internal pressure, which pushes outward. If gravity is stronger, the parcel moves inward. The condition for collapse is called the Jeans instability: a cloud will collapse if its mass is larger than a certain threshold for its temperature and density. This happens because a denser region has stronger gravity but its pressure does not increase as quickly, so the balance tips. Once collapse begins, it does not proceed uniformly. Small perturbations grow, breaking the cloud into smaller and smaller fragments—this is the key to forming many stars from one cloud. As a fragment contracts, it spins faster due to conservation of angular momentum, forming a flattened accretion disk around the central protostar. Eventually, when the center reaches high enough temperature and pressure, nuclear fusion begins, and the collapse stops as the outward pressure from fusion balances gravity. Thus, gravitational collapse is not just the beginning of star formation; it is a self-regulating process that sets the stage for every stage of a star's life.