1. Mass: The most significant factor is the mass of the star. Stars are formed from collapsing clouds of gas and dust. The more massive the cloud, the more gravity it exerts, pulling in more material and ultimately creating a larger, denser star. Massive stars have much stronger gravity, leading to higher pressures and temperatures in their cores, which in turn fuel much faster nuclear fusion reactions. This results in a larger and brighter star.
2. Age: As stars age, they evolve and their size changes. Stars like our Sun will spend most of their lives as main sequence stars, burning hydrogen into helium in their cores. As they age, they slowly expand and become red giants. This is because as they consume their hydrogen fuel, they eventually begin to fuse helium, leading to changes in their structure and causing them to swell. Eventually, they will shed their outer layers and become white dwarfs.
Therefore, a star's initial mass determines its eventual size throughout its lifespan, and its age influences its current size.
