The nebular hypothesis describes the prevailing scientific theory regarding the formation of stars and solar systems. According to this model, a stellar system originates from the gravitational collapse of a massive, rotating cloud of interstellar gas and dust, known as a nebula. As the cloud collapses, conservation of angular momentum causes the material to spin faster and flatten into a rotating protoplanetary disk. The gravitational energy is converted into thermal energy, causing the core of the cloud to heat up and increase in density. When the core reaches sufficient temperature and pressure, nuclear fusion begins, marking the birth of a protostar. Simultaneously, the material in the surrounding disk accretes to form planets, while the system eventually stabilizes into a mature star and its orbiting planetary bodies.