Originally, it was believed that chemical energy was the source of the Sun's output (which would produce heat & light,) it showed that the Sun had a short finite life.
In the 19th cenury, geologists discovered that the Earth was millions of years old which cause difficulties for the original theory.
The process in stars cause the chemical composition of the universe to change over time. This happens as the quantity of heavy metals is increasing as a result of nuclear fussions in stars in which light elements join to produce heavier nuclei.
Stars are created by gravity pulling togher dust/hydrogen unil the core becomes very hot. The atoms move very fast to produce fussion of hydrogen.
The nebula is the start of a star, it will collapse due to gravitational attraction. Then all the gas/dust will gravitate towards the dense hot central blob, the protostar is made. In the core of the protostar it will glow and produce a large outward radiation pressure force, this happens as both are produced by fussion with a release of energy (heat/light.)…