* Hydrogen Fuel Depletion: As the Sun continues to fuse hydrogen into helium in its core, the core will eventually run out of hydrogen fuel.
* Core Contraction: Without the outward pressure of hydrogen fusion, the Sun's core will begin to contract under its own gravity.
* Helium Fusion: The core's contraction will increase temperature and pressure, eventually reaching a point where helium fusion can begin.
* Expansion and Cooling: The helium fusion will cause the outer layers of the Sun to expand dramatically, transforming it into a red giant. The Sun will become much larger and cooler, its surface temperature dropping to about 3,000 K (compared to its current 5,500 K).
* Swallowing Planets: The Sun will expand so much that it will engulf Mercury, Venus, and likely Earth.
Important note: The red giant phase is just the beginning of a series of changes. The Sun will eventually shed its outer layers, forming a planetary nebula. The remaining core will become a white dwarf, a dense, hot object that slowly cools over billions of years.
