Question

How is nuclear fusion in a main-sequence star different from nuclear fusion in a giant star?

Answer 1

The main difference is the elements being fused and the time that such elements can be fused.

Explanation:

Larger stars have much higher core temperatures, which means the elements that can be fused are heavier. However, this comes at a cost of expending much more energy, so the lifetimes of giant stars will be shorter for fusing these elements.

Answer 2

There are different type of fusion reactions which take place in stars of different sizes and stages.

Explanation:

Main sequence stars are fusing Hydrogen into Helium in their cores. Smaller stars mainly use the relatively low temperature proton-proton chain reaction. Larger stars us the much hotter Carbon-Nitrogen-Oxygen (CNO) cycle.

When a large star runs out of Hydrogen in its core it seamlessly moves on to fusing Helium and heavier elements.

When a smaller star runs out of Hydrogen in its core, fusion stops and the core collapses. It heats up until Hydrogen fusion can take place in the Hydrogen shell around the Helium core. This causes the outer layer of the star to expand into a red giant.

So, a main sequence star is fusing Hydrogen into Helium in its core. A red giant has fusion reactions in a shell around the core.