1. Core Hydrogen Fusion: Stars like the Sun fuse hydrogen into helium in their cores through nuclear reactions. As the star burns through its hydrogen fuel supply, the core becomes hotter and denser. This causes the fusion reactions to accelerate, leading to an increase in the star's luminosity and surface temperature.
2. Expansion and Cooling: As the core contracts, it heats up and causes the outer layers of the star to expand and cool. The star's surface area increases significantly, making it appear redder and larger. The star moves towards the red giant phase of its evolution on the Hertzsprung-Russell (H-R) diagram, which plots a star's luminosity against its surface temperature.
3. Core Helium Fusion: Once the hydrogen in the core is depleted, the star can no longer sustain its fusion reactions. The core contracts further under the force of gravity, leading to a rise in temperature and density. This ignites helium fusion in the core, marking the start of the red giant phase.
4. Shell Hydrogen Fusion: While helium fusion is occurring in the core, hydrogen fusion continues in a shell surrounding the core. The energy generated from both core helium and shell hydrogen fusion causes the star to become even larger and more luminous, further intensifying its red color.
5. Asymptotic Giant Branch (AGB) Phase: As a red giant, the star ascends the Asymptotic Giant Branch (AGB) on the H-R diagram. During this phase, the star's luminosity and temperature continue to increase while it undergoes significant mass loss. The mass loss occurs through stellar winds and pulsations, releasing gas and dust into the surrounding space, which can eventually form planetary nebulae.
6. Core Collapse: Eventually, the core of the red giant becomes dense enough to collapse under its own gravity. The collapse triggers a supernova explosion, or in the case of lower-mass stars like the Sun, a less powerful stellar event called a planetary nebula. The aftermath of these events leaves behind a compact stellar remnant, such as a white dwarf, neutron star, or black hole.
