White Dwarf
A white dwarf is the small, extremely dense, hot remnant left after a low or medium mass star sheds its outer layers at the end of its life.
Definition
A white dwarf is what remains of a star like the Sun after it dies: a small, very hot, very dense ball about the size of Earth that no longer makes new energy and slowly cools over billions of years. It is left behind when a star of up to about $8$ solar masses ejects its outer layers as a planetary nebula, exposing the hot core; a typical white dwarf holds about $0.6$ solar masses in a body roughly Earth's size and shines only with leftover heat. It is supported against gravity by electron degeneracy pressure, a quantum effect that does not depend on temperature, and its carbon-oxygen interior reaches densities near $10^9\ \text{kg/m}^3$. The Chandrasekhar limit of about $1.4$ solar masses is the most that degeneracy pressure can support, and a white dwarf pushed over it by accretion from a companion detonates as a Type Ia supernova.
Example
Sirius, the brightest star in the night sky, has a white dwarf companion, Sirius B, with about $1.0$ solar masses packed into a radius of about $5{,}800\ \text{km}$, slightly smaller than Earth. Its surface temperature is about $25{,}000^\circ\text{C}$, far hotter than the Sun, yet it is dim because it is so small. Because white dwarf radius scales roughly as $R \propto M^{-1/3}$, a $1.2$ solar mass white dwarf is smaller than a $0.6$ solar mass one, and Type Ia supernovae have nearly uniform peak brightness because they all explode near the same $1.4$ solar mass limit, which makes them standard candles for cosmic distances.
Key Insight
A teaspoon of white dwarf material would weigh several tons on Earth, which shows how tightly a whole star's worth of matter is squeezed. A white dwarf is not a cold cinder; it starts hotter than the Sun's surface and takes longer than the current age of the universe to cool. The Chandrasekhar limit explains why the Sun will end as a white dwarf while a $20$ solar mass star cannot: above the limit no amount of electron pressure holds the core up, and only a neutron star or black hole remains.