when a red giant and a white dwarf orbit each other, the red star’s outer gas gets pulled onto the white dwarf due to gravity. Hydrogen from the red star collects on the white dwarf’s surface, and a thermonuclear explosion happens once enough hydrogen piles up, igniting a bright burst called a nova.
When a red giant’s core contracts and the outer layers expand, the outer gas layers are blown away into space and glow. This outer gas layer that the star shed is called a planetary nebula. The leftover core is called a white dwarf, which is created from a star with a low or medium mass--like our sun. This is the final stage of evolution for such stars, and they are characterized by their low luminosity and high density. A white dwarf typically has a mass around 0.5-0.7M ☉.
When electrons resist being squeezed into a smaller volume, electron degeneracy pressure is created, acting as a repulsive force. This pressure prevents white dwarfs from collapsing under gravity.
White dwarfs have a theoretical maximum mass of 1.4 solar masses. This is called the Chandrasekhar Limit, and any white dwarf less than or equal to its mass is considered to be stable. Once the limit is reached, the electron degeneracy pressure within the core is no longer able to support it against gravity and the further collapse of the star.