In 1877, English mathematician and astronomer James Jeans was born.
One of Jeans’ major discoveries, named Jeans length, is a critical radius of an interstellar cloud in space. It is a function of the temperature and density of the cloud as well as the mass of the particles composing the cloud. A cloud that is smaller than its Jeans length will not have sufficient gravity to overcome the repulsive gas pressure forces and condense to form a star. A cloud that is larger than its Jeans length will collapse.
Learn more at http://en.wikipedia.org/wiki/James_Hopwood_Jeans.