Most scientists believe that the Universe was created by a 'Big Bang'.
As the universe expanded, it cooled and uncharged atoms formed.
The force of gravity pulled matter into stars and galaxies. Gravitational force attracts more dust and matter around stars and this may have formed planets.
Evetually dust and gas were pulled together by Gravitational Attraction to form the stars.the resulting heat started Nuclear Fusion reactions, so they began to emit light and other radiation.
The Life of a Star
Stars form from clouds of dust and gas.
Gravitational forces make the clouds become increasingly dense, forming a 'Protostar'.
As a Protostar becomes denser, it gets hotter. If it becomes hot enough, the nuclei of hydrogen atoms start to fuse together.
Stars radiate energy because of the hydrogen fusion in the core. This is the main stage in the life of a star. It can continue for billions of years until the star runs out of hydrogen nuclei to fuse together.
The inward force of gravity is balanced by the outward pressure of radiation from the core, so the star is stable. During this stable period the star is called a 'main sequence star'.
The Life of a Star (Cont.)
Low Mass Star:
Protostar ---> Main Sequence Star ---> Red Giant ---> White Dwarf ---> Cold(black) Dwarf
High Mass Star, after the Red Giant stage:
Red Giant ---> Supernova ---> Neutron Star ---> Black Hole (If there is sufficient mass)
How The Chemical Elements Formed
Elements as heavy as Iron are formed inside a star as a result of Nuclear Fusion during the early stages of a star's life.
Elements heavier than Iron are formed in the final stages of a star's life during the explosion of a Supernova.