On the occasion of Theo’s christening, 15 September 2002
Sometimes a cloud of gas and dust can be so large that its own gravity is strong enough to make it collapse into a ball. During the collapse, the atoms and molecules inside the gas are squeezed tightly together so that they become hot enough to fuse creating new atoms and emitting huge amounts of energy. When this happens the great ball of dust and gas begins to shine and a star is born.
This equation shows how much gas and dust is needed to start the collapse. The critical mass MJ is known as the Jeans mass after the astrophysicist James Jeans who worked out the details in the early 20th century. Any cloud that has less mass than the critical value remains a cloud but one that has more always collapses. The original density of the cloud is ρ0, T is its temperature, kB is the Boltzmann constant, G is the gravitational constant and m is the average mass of the molecules and atoms in the cloud.
A derivation of this equation is at