If I'm not mistaken, the smaller stars don't have as high a gravitational gradient and are fully convective so the helium ash and hydrogen fuel mix closer to un

If I'm not mistaken, the smaller stars don't have as high a gravitational gradient and are fully convective so the helium ash and hydrogen fuel mix closer to uniformly, allowing them to burn through all of it. Stars on the scale of sol slowly build up helium until it chokes out, compresses and starts burning helium at higher temperatures, causing the rest of it to expand.