What happens to the star as it becomes brighter and brighter?

As a star becomes brighter, it typically undergoes changes in its core reactions, often increasing the fusion of hydrogen into helium. This heightened fusion rate generates more energy and causes the star to expand, potentially transforming it into a red giant. Eventually, depending on its mass, the star may shed its outer layers, forming a planetary nebula, or it might collapse into a supernova, leading to a neutron star or black hole. Throughout this process, the star's brightness can fluctuate significantly, reflecting its evolving internal dynamics.