Pierre Marchand
Pierre Marchand
The interstellar medium is filled with diffuse matter, that gathers into "molecular clouds". These giant structures several parsec wide can host thousands of solar mass of gas. Within these clouds, turbulent motions, gravity and feedback from nearby stars (particles, radiations) create overdense regions called "dense cores". If the gravitational force can overcome the support of the thermal and magnetic pressure, the core is unstable and collapses under its own weight. The matter becomes more and more hot and dense in the center, until the temperature is high enough to ignite the nuclear fusion reactions of hydrogen. The energy released by the fusion increases the thermal pressure within the gas. When it is sufficient to counter the pulling force of gravity, a star is born. The slight rotation of the gas in the dense cores is amplified as the gas collapses, by conservation of angular momentum. The centrifugal force creates a support against gravity, which leads to the formation of a disk. In this disk, dust grains and molecules aggregate, grow and eventually form planets, moons, asteroids. This disk is then called "protoplanetary disk".