  When a star exhausts the hydrogen fuel in its core, the core contracts, and
the envelope may expand to $\sim 100$ times its original size. Consequently,
stars with $\lesssim 8$ solar masses become red giants, while those with
$\gtrsim 8$ solar masses evolve into red supergiants. However, the physics
driving this extreme expansion is unclear. This letter presents a new mechanism
grounded in the instability of stellar envelopes to explain the expansion. If
the gravitational energy released by core contraction can trigger the initial
expansion to a critical radius where the envelope becomes unstable, the star
then continues expanding until stabilizing, thereby becoming a red giant or
supergiant. We demonstrate that the envelope instability can explain several
elusive features in the Hertzsprung-Russell diagram, such as the Hertzsprung
gap between the main sequence and the red giant branch, as well as the
dichotomy of the supergiant populations in red and blue.
