As hydrogen fuel runs out and helium builds up in the center of stars, there is still a lot of hydrogen in the envelope (non-core) of a star. If there was a way to remove the helium and replenish it with fresh hydrogen from above, the lifespan of stars could be greatly lengthened. The convection zone does move material from the surface of the star to the interior but it cannot replenish the hydrogen in the core to cause more fusion. Why not