Structure:
* Expansion: The star's outer layers expand dramatically, often becoming hundreds of times larger than its original size. This expansion is driven by the star's core becoming hotter and denser, increasing the outward pressure.
* Core Contraction: The core, now dominated by helium, contracts under its own gravity. This contraction further increases the core's temperature and density.
* Shell Burning: Hydrogen fusion begins in a shell surrounding the helium core, generating energy that causes the outer layers to expand further.
* Reduced Density: The outer layers become much less dense and cooler, resulting in the characteristic reddish appearance.
Behavior:
* Luminosity Increase: The star becomes much more luminous, even though the core is producing less energy. This is because the expanded outer layers are much larger, radiating more energy overall.
* Surface Temperature Decrease: The expansion and cooling of the outer layers result in a lower surface temperature, leading to the red color.
* Instability and Pulsation: The expanded outer layers are less stable and can pulsate, causing the star's brightness to vary.
* Mass Loss: Red giants experience significant mass loss through stellar wind, shedding material into space.
Other Notable Changes:
* Changes in Composition: As the star continues to evolve, it will eventually begin to fuse helium into heavier elements like carbon and oxygen.
* Evolutionary Path: The evolution of a red giant depends on its initial mass. Less massive stars will eventually shed their outer layers, becoming white dwarfs. More massive stars may undergo further fusion cycles, leading to even more dramatic changes and potentially ending their lives as supernovae.
In essence, the transition from a main sequence star to a red giant marks a significant stage in a star's life, involving major changes in its structure, behavior, and overall composition. It is a period of dramatic expansion, cooling, and instability, paving the way for the star's eventual fate.
