The Sun will eventually become a white dwarf, a dense, slowly cooling remnant about the size of Earth but carrying roughly half its current mass. Before reaching that final state, it will spend billions of years swelling into a red giant, shedding its outer layers into space, and leaving behind a crystallizing core that will fade over trillions of years into a cold, dark object called a black dwarf.
Right now, the Sun is about 4.5 billion years old and roughly halfway through its life as a hydrogen-burning star. It has another 5 to 6 billion years of relatively stable behavior ahead of it before things start to change dramatically.
Why the Sun Will Eventually Run Out of Fuel
The Sun generates energy by fusing hydrogen into helium deep in its core, where temperatures exceed 15 million degrees. This process has been running steadily for 4.5 billion years, but the supply of hydrogen in the core is finite. Stars like the Sun burn for about 9 to 10 billion years total, which means the clock is roughly half expired.
As the core’s hydrogen supply dwindles over the next several billion years, the fusion reactions that hold the Sun up against its own gravity will slow down. The core will begin to contract under its own weight, and that contraction will set off a chain of events that transforms the Sun from the stable yellow star we know into something far larger and far more dramatic.
The Red Giant Phase
About 6 billion years from now, the Sun will enter its red giant phase. As hydrogen fusion slows in the core, the core contracts and heats up, which paradoxically causes the outer layers of the Sun to expand enormously. The Sun’s radius will swell to roughly 166 times its current size, reaching about 0.77 astronomical units (an astronomical unit is the current Earth-Sun distance). Its surface will cool to around 3,100 degrees Kelvin, giving it a deep red-orange color, while its total brightness will climb to over 2,300 times what it is today.
That expansion is enough to swallow Mercury entirely. Venus and Earth won’t be consumed, but they’ll be pushed outward as the Sun loses mass during this phase. Earth would migrate to about 1.69 times its current distance from the Sun, and Venus to about 1.22. Being pushed outward won’t save them from the heat, though. The Sun’s enormous luminosity at this stage would strip away atmospheres and bake planetary surfaces.
During the red giant phase, something explosive happens inside the core. For a star of the Sun’s mass, the core becomes so compressed that matter enters a strange physical state called degeneracy. When the core temperature finally hits around 200 million degrees, helium ignites in a runaway thermonuclear event known as the helium flash. Within seconds, the entire core is engulfed, briefly releasing energy equivalent to 100 billion times the Sun’s current output. Despite its ferocity, this event is contained within the core and doesn’t blow the star apart. Instead, it stabilizes helium fusion, and the Sun settles into a quieter phase of burning helium into carbon.
Shedding Its Outer Layers
As the Sun progresses through its giant phases, it loses mass at an extraordinary rate. Late-stage giant stars shed material at rates far exceeding what nuclear burning consumes, essentially blowing their outer layers into space through intense stellar winds. For the Sun, this means roughly half its total mass will be expelled over time.
The ejected material forms a glowing shell of gas expanding outward from the dying star, known as a planetary nebula (a misleading name that has nothing to do with planets). These nebulae are some of the most visually striking objects in the galaxy, often appearing as colorful rings or hourglass shapes illuminated by the ultraviolet radiation of the hot core left behind. The nebula itself contains only a few tenths of a solar mass of gas and doesn’t last long in cosmic terms. Within about 10,000 years, it disperses into the surrounding space.
The White Dwarf Stage
Once the outer layers are gone, what remains is the Sun’s exposed core: a white dwarf. This remnant will be about as massive as half the current Sun but compressed into an object roughly the size of Earth. A teaspoon of white dwarf material would weigh several tons. The star is held up not by fusion (which has stopped entirely) but by a quantum mechanical effect called electron degeneracy pressure, where electrons resist being squeezed any closer together.
The Sun’s white dwarf will be composed primarily of carbon and oxygen, the products of helium fusion, with thin outer layers of hydrogen and helium. It will start out extremely hot, with a surface temperature of tens of thousands of degrees, making it glow white or bluish-white despite its tiny size.
One thing the Sun will never do is explode as a supernova. That fate is reserved for much more massive stars. A white dwarf can only remain stable below a mass of about 1.4 times the Sun’s current mass, a boundary known as the Chandrasekhar limit. The Sun’s remnant will fall well below this threshold, so there’s no risk of further collapse into a neutron star or black hole.
Cooling Into a Black Dwarf
With no fusion to generate new energy, a white dwarf simply radiates its stored heat into space and slowly cools. The timeline for this process is staggering. After about 2 billion years, the white dwarf’s surface temperature drops to around 8,000 degrees and its brightness fades to just 1/4,000th of the Sun’s current luminosity. Five billion years in, the entire core crystallizes into a solid lattice structure, essentially becoming an Earth-sized diamond of roughly 6 billion trillion trillion carats.
After 15 billion years of cooling (longer than the current age of the universe), the surface drops to just a few thousand degrees and the luminosity falls to about 1/100,000th of the Sun’s current output. Eventually, through a process called Debye cooling, the crystallized core loses heat rapidly and the object fades almost completely from visibility. At this point, it earns the name “black dwarf,” a cold, dark, crystallized remnant drifting through space.
No black dwarfs exist yet anywhere in the universe. The cosmos is only 13.8 billion years old, and even the oldest white dwarfs haven’t had enough time to cool to that point. The Sun’s transformation into a black dwarf lies not billions but trillions of years in the future, making it the longest chapter in a story that began with a collapsing cloud of gas 4.5 billion years ago.