A star is born by Nuclear Fusion. “Nuclear Fusion is when the nuclei combine to form a more massive nuclei with a quick release of energy.” This is when enough matter, such as dust or gas, in a nebula join together during a shockwave, or a gravitational disturbance which pass through the Nucleus. A nebula is a cluster or gasses and dirt in our galaxy that is in the process of becoming a star. When the dust and gasses are pulled together, the gravity causes it to swirl and cluster up into a new born star (a protostar) and then turns into a Main Sequence star.
What is a Protostar?
This is a picture of a protostar
A protostar is a large mass of gasses or dusts that join together in the beginning creation of a star.