The minimum size of a cloud of gas, of given temperature and density, which is capable of collapsing under its own gravity. Gas clouds smaller than this do not collapse because their gas pressure more than balances their gravitational force. The Jeans length is proportional to the square root of the temperature, and inversely proportional to the square root of the density. For the dense gas characteristic of star-forming clouds it is a few tenths of a light year. It is named after J. H. Jeans.