Our sun will end its life as a white dwarf, a dense, slowly cooling remnant about the size of Earth but with roughly the mass it has today. Before reaching that final state, it will pass through a dramatic red giant phase that will transform the inner solar system. The entire process will unfold over the next five billion years or so.
Where the Sun Is Now
The sun is currently 4.5 billion years old, roughly halfway through its expected lifespan. Stars of its mass burn through their hydrogen fuel over about nine to ten billion years, meaning there are still roughly five billion years of stable, steady energy production ahead. During this period, called the main sequence, the sun fuses hydrogen into helium in its core, and not much changes from one millennium to the next. Life on Earth exists in this window of stability.
The Red Giant Phase
Once the sun exhausts the hydrogen in its core, things change fast. The core, now mostly helium, begins to collapse under its own gravity. That collapse heats a shell of hydrogen surrounding the core to the point where it ignites and starts fusing on its own. The extra heat from this shell pushes the sun’s outer layers outward, inflating the star to several hundred times its current size. As the surface stretches, it cools and shifts in color from yellow-white to red. This is the red giant phase.
The sun’s expansion will be catastrophic for the inner solar system. It will puff up enough to melt, evaporate, and consume some of the inner rocky planets. Mercury and Venus are almost certainly doomed. Mars will likely survive. Earth sits in between, and its fate is genuinely uncertain. Astronomer Dimitri Veras at the University of Warwick has put it bluntly: he’s confident the sun will swallow Mercury and Venus and spare Mars, but Earth’s outcome is less clear. Recent computer models of the gravitational interactions between Earth and the aging sun suggest our planet probably won’t make it out intact.
Even if Earth somehow avoids being engulfed, it would be rendered completely uninhabitable long before the sun reaches its maximum size. The increased luminosity alone would boil away the oceans and strip the atmosphere well in advance of any physical contact.
Shedding Its Outer Layers
The end of the red giant phase is typically the most violent period in a sun-like star’s life. The bloated star throws off its outer layers in intense, episodic bursts. Over a relatively short window of about 10,000 years, the sun will shed a substantial fraction of its mass, somewhere between 10% and 100% of a full solar mass worth of material, pushing it out into space in expanding shells of glowing gas.
This expelled material forms what astronomers call a planetary nebula (a misleading name, since it has nothing to do with planets). These nebulae are some of the most visually striking objects in the universe: colorful, intricate clouds of gas lit up by the intense radiation of the exposed stellar core at their center. If any distant civilization were watching our solar system at that point, they’d see a brief but spectacular light show. Brief in cosmic terms, anyway. Planetary nebulae last only about 10,000 years before they dissipate into the surrounding space.
The White Dwarf Stage
After the outer layers are gone, all that remains is the sun’s core: a white dwarf. A typical white dwarf packs roughly the mass of the sun into an object only slightly bigger than Earth. That makes it one of the densest forms of matter in the universe, surpassed only by neutron stars and black holes. A teaspoon of white dwarf material would weigh several tons.
The sun’s white dwarf will be composed primarily of carbon and oxygen, the products of helium fusion that took place during the red giant phase. It won’t generate any new energy through fusion. Instead, it will simply glow from residual heat, like a coal pulled from a fire. Over billions of years, it will slowly radiate that heat into space, dimming and cooling.
The cooling process is extraordinarily slow. A simple calculation puts the time for a white dwarf to fade beyond detection at around 10 billion years. Even after the outer layers solidify, the star’s residual heat capacity keeps it glowing faintly for an immense stretch of time. Eventually, after tens of billions of years (possibly much longer than the current age of the universe), the white dwarf would cool to the point where it no longer emits significant light or heat. This hypothetical endpoint is sometimes called a black dwarf: a cold, dark lump of carbon drifting through space. No black dwarfs exist yet because the universe simply hasn’t been around long enough for any white dwarf to cool that far.
Why the Sun Won’t Explode
Stars that end their lives in supernova explosions are fundamentally more massive than the sun. The sun doesn’t have enough mass to trigger the kind of runaway core collapse that produces a supernova. There is a critical threshold, known as the Chandrasekhar limit, at about 1.4 times the sun’s mass. A white dwarf below that limit remains stable and simply cools. Only if a white dwarf somehow gains enough extra mass to approach that limit (by pulling material from a companion star, for example) can it detonate in a thermonuclear explosion. Since our sun is a single star with no close companion to feed it, its white dwarf will sit comfortably below that threshold and fade quietly.
The sun’s final chapter, then, is not a dramatic explosion but an impossibly long, slow fade. From a blazing yellow star to a swollen red giant, to a glowing shell of gas around a hot white core, and finally to a dark cinder cooling in the void for longer than the universe has currently existed.