What structural characteristic is typical for stars below about $0.4 ext{M}_{\odot}$?
They are convective throughout their entire volume.
Stars located at the very low-mass end of the main sequence, specifically those with masses under approximately $0.4$ times the mass of the Sun ($ ext{M}_{ ext{odot}}$), exhibit a unique internal structure unlike both Sun-like stars and massive stars. These low-mass stars are convective throughout their entire structure, meaning the material is constantly mixing from the core all the way to the photosphere. This continuous mixing ensures that the helium produced by fusion is not trapped in a central region but is distributed more uniformly throughout the star's volume. Because the primary fuel source (hydrogen) is constantly being replenished in the fusion zone via this mixing, these stars can sustain hydrogen burning for proportionally much longer periods, with main-sequence lifespans that can extend to trillions of years, greatly exceeding the current age of the universe.
