A Main Sequence star can shine for only as long as it has nuclear fuel to burn in its core. The intense gravitational pressure generated by the weight of the star pressing inward on itself can only be held off by burning materials in the core, thus generating an outward force strong enough to support the structure of the star against collapse.

What happens when a star burns most of its hydrogen, and is not yet hot enough to convert higher order elements (helium, etc.) to produce energy? Unless it shares its solar system with a close companion, it has no way to secure additional fuel.

Without a counter-balancing force to oppose gravity, the star cannot support its own internal structure. It first collapses inward, forming a smaller, denser shape. The resultant increase in density and pressure elevates the temperature of the star, and allows for the next phase – helium burning. With the ignition of a new, hotter fuel, the outer layers of the star balloon outward again, and the star enters a giant phase.

Helium burning, or that of other higher order elements, can only be sustained for a brief period of time. At that point, a lower-mass star like our Sun has no alternatives but to succumb to the force of self-gravity. It collapses into a compact object, one which we call a white dwarf. Glowing hot upon formation, it will spend the rest of the age of the Universe slowly cooling, and fading away into the long dark of endless night.