white dwarf
When a star like the Sun runs out of fuel, it does not explode and it does not vanish. It gently puffs off its outer layers and leaves behind its bare, exhausted core — a white-hot ember about the size of the Earth but holding nearly half the mass of the Sun. That ember is a white dwarf: the most common way for a star to end, and the fate awaiting our own Sun in roughly five billion years.
What makes a white dwarf strange is its density. Cram half a Sun into an Earth-sized ball and a single teaspoon of the stuff would weigh several tonnes. At that density the matter cannot behave like ordinary gas; instead the electrons are packed so tightly that a quantum rule forbids them from being squeezed any closer, and the pressure they exert — electron degeneracy pressure — holds the star up against gravity without needing any heat at all. A white dwarf no longer burns fuel; it simply shines from leftover warmth, starting blazing hot (over 100,000 kelvin for the youngest) and slowly fading over billions of years.
White dwarfs matter far beyond being stellar corpses. Because they have a strict maximum mass (the Chandrasekhar limit, about 1.4 solar masses), a white dwarf that gains too much matter from a companion star detonates as a Type Ia supernova — one of the standard candles used to measure the expansion of the universe. A common misconception is that white dwarfs are dim because they are small; the hottest ones are intensely luminous per square metre, but their tiny surface area makes the total light modest, so they hide in plain sight near brighter stars.
Sirius, the brightest star in the night sky, has a tiny white-dwarf companion called Sirius B. Though it once outshone Sirius itself, Sirius B is now a fading Earth-sized ember packing about one solar mass — so dense that its surface gravity is roughly 100,000 times Earth's.
Sirius B: a once-bright star reduced to an Earth-sized, super-dense cinder.
A white dwarf is the core of a low-to-medium-mass star (up to roughly 8 solar masses originally), not the remnant of every star. Heavier stars leave neutron stars or black holes instead.