The Jeans instability is a concept in astrophysics that explains when and how interstellar gas clouds begin to collapse under their own gravity to eventually form stars. Named after British physicist Sir James Jeans, this instability occurs when the inward pull of gravity overcomes the outward pressure from the gas’s thermal energy.
A gas cloud will collapse if:
- It has enough mass (gravitational force is strong),
- It is cool (low internal pressure), and
- Its size exceeds a critical value, known as the Jeans length.
When these conditions are met, the cloud becomes gravitationally unstable—it starts to shrink and fragment, forming protostars in the densest regions. These protostars can eventually ignite nuclear fusion in their cores, becoming fully fledged stars.
The Jeans instability is a fundamental principle in the study of stellar formation and helps explain why stars are born in cold, dense regions of molecular clouds. It also plays a role in the evolution of galaxies and star clusters by influencing where and how stars emerge.