How do red giants differ from main sequence stars?

Red giants differ from main sequence stars primarily in size, temperature, and stage of stellar evolution.

Red giants are a type of star that represent a later stage in a star's life cycle, following the main sequence stage. They are characterised by their large size and relatively cool surface temperature, which gives them a distinctive red appearance. In contrast, main sequence stars, like our Sun, are in the prime of their life, burning hydrogen in their core to produce energy.

The difference in size between red giants and main sequence stars is significant. Red giants can be up to 100 times larger than main sequence stars. This increase in size is due to the star exhausting its hydrogen fuel and beginning to burn helium, causing the outer layers of the star to expand. Despite their larger size, red giants are cooler than main sequence stars. This is because the energy produced by burning helium is spread over a larger surface area, resulting in a lower surface temperature and a red colour.

The stage of stellar evolution also differentiates red giants from main sequence stars. Main sequence stars are in a stable phase, where the gravitational pull inward is balanced by the outward pressure from nuclear fusion in the core. This stage can last billions of years. Once the hydrogen in the core is depleted, the star enters the red giant phase. During this phase, the star burns helium in a shell around the core, causing the star to expand and cool.

In summary, red giants and main sequence stars are at different stages in their life cycle, with red giants being older and in a phase of expansion and cooling. They are larger in size, cooler, and burn different fuel in their core compared to main sequence stars.