Stellar evolution studies have played a key role in the understanding of the Universe. Observation and theoretical models, together with computer simulations have revealed unimaginable truths. Stars form inside giant molecular clouds, which are nothing more than very dense regions of dust and gas. Under the influence of gravity, fragments of debris form protostars. When there is enough heat generated, the nuclear fusion inside a dense region will create a main sequence star. Apart from helium and hydrogen, the majority of elements were created in the nucleus of stars.
The starting mass influences the development of a star. The bigger the mass, the greater the luminosity, thus the faster hydrogen will reach its core. In time, hydrogen will evolve into helium, that will allow the start to continue its development. The helium fusion happens at a higher core temperature. This enables the star to expend its size and increase the density of its inner layers and the core. The result of this process are red giants, that have a life span equal to the time the hydrogen at the core needs to consume itself in the burn. When these giant stars include heavier elements into their structure, they might encounter a brief period of development decreasing phase.
When a star becomes eight times bigger than the Sun, their core collapses and they become supernovas. If the mass is at least three time the size of the Sun, it can generate a black hole. When a star is smaller, it is called a white dwarf and eventually its mass will become a planetary nebulae.
These evolutionary steps are important in the formation of the universe, as the nebulae and the supernova explosions are the main form of metals distribution into the interstellar space. Without these processes, the new stars, planets and galaxies would be formed from hydrogen and helium alone.