Question

According to the nebular hypothesis, when did nuclear fusion begin?

Answer 1

There is evidence that Emanuel Swedenborg first proposed parts of the nebular hypothesis in 1734. Immanuel Kant, familiar with Swedenborg's work, developed the theory further in 1755, publishing his own Universal Natural History and Theory of the Heavens, wherein he argued that gaseous clouds (nebulae) slowly rotate, gradually collapse and flatten due to gravity, eventually forming stars and planets

Answer 2

According to nebular hypothesis, the solar system was formed 4.5 billion years ago in solar nebula.

The solar nebula- a molecular cloud of dust and gases collapsed under its own gravity. In order to conserve momentum, this collapsed core started spinning while gathering more and more mass. The temperature and pressure rose. When sufficient temperature and pressure is achieved, nuclear fusion reaction kicks in. While the rest of the part formed proto-planetary disk.