What causes red giants expand to become larger than main-sequence stars?

When stars first begin to fuse hydrogen to helium, they lie on the zero-age main sequence. The core of a red giant is contracting, but the outer layers are expanding as a result of hydrogen fusion in a shell outside the core. The star gets larger, redder, and more luminous as it expands and cools.

A) the enthralling of formation for pure element in its standard state is always positivehope this : )