Stars do not all start out as giants and then become small over time because the evolution of a star depends on its initial mass. The size and lifetime of a star are determined by a delicate balance between two opposing forces: gravity, which tries to collapse the star inward, and the pressure generated by nuclear fusion in its core, which pushes outward.
Stars like our Sun, with masses between about 0.1 and 8 times that of the Sun, follow a relatively predictable evolutionary path. They begin their lives on the main sequence, where they fuse hydrogen into helium in their cores. During this phase, the outward pressure from fusion balances the gravitational forces, keeping the star stable and maintaining its size. This main sequence phase lasts for billions of years.
As a main sequence star exhausts its hydrogen fuel in the core, the balance between gravity and pressure is disrupted. Without enough fusion to counteract gravity, the star begins to contract. This contraction increases the temperature and pressure in the core, allowing the star to start fusing helium into heavier elements. This phase is known as the red giant phase.
During the red giant phase, the star expands significantly in size while becoming cooler in temperature. This expansion is a result of the increased fusion activity in the core, which produces more energy and pushes the outer layers of the star outward. The outer envelope of the star becomes less dense and much larger, causing the star to appear as a red giant.
Eventually, red giants can undergo further stages of stellar evolution depending on their mass. Low-mass stars like our Sun will shed their outer layers, forming a planetary nebula, and leave behind a dense core called a white dwarf. White dwarfs are small, hot remnants of low-mass stars that no longer undergo fusion.
On the other hand, more massive stars have different evolutionary paths. They can become red supergiants, undergo supernova explosions, or even collapse to form neutron stars or black holes.
So, the reason stars don't all start as giants and then become small like our Sun is because their initial mass determines their evolutionary path. Low-mass stars, like our Sun, spend the majority of their lives on the main sequence before transitioning to the red giant phase and eventually becoming white dwarfs. Higher-mass stars, however, have different evolutionary trajectories leading to different outcomes.