When the Sun exhausts its hydrogen fuel, it will swell into a red giant star, eventually shed its outer layers into space, and leave behind a small, dense remnant called a white dwarf. This transformation will unfold over roughly a billion years, beginning about five billion years from now. The process is dramatic, involving phases where the Sun grows hundreds of times its current size, briefly reignites with helium fusion, and then quietly fades over trillions of years as a cooling ember no larger than Earth.
How the Sun Burns Now
The Sun is currently in its main sequence phase, the long, stable stretch of a star’s life powered by hydrogen fusion in its core. Deep inside, temperatures exceed 15 million degrees, and under that crushing pressure, hydrogen nuclei (protons) fuse together to form helium. This process, called the proton-proton chain, generates about 99 percent of the Sun’s energy output of roughly 3.84 × 10²⁶ watts.1Nature. Neutrinos from the primary proton–proton fusion process in the Sun The radiation produced by fusion pushes outward against the star’s own gravity, keeping the Sun in a balanced, stable state.
Modern estimates put the Sun’s total fusion lifetime at around 10 billion years. Since the Sun formed about 4.6 billion years ago, it is roughly halfway through its hydrogen-burning life. But the Sun will not simply run out of fuel one day like a car sputtering to a halt. Instead, changes begin gradually as the core’s hydrogen supply dwindles and the balance between gravity and radiation pressure starts to shift.
The Climb to Red Giant
As the Sun ages, helium ash accumulates in its core. Eventually, the core runs low enough on hydrogen that fusion there slows and then stops. Without that outward push, gravity compresses the core, heating it further. A thin shell of hydrogen surrounding the inert helium core reaches temperatures high enough to ignite, and this shell-burning phase actually produces more energy than the core fusion did. The extra energy causes the Sun’s outer layers to expand enormously.
Over hundreds of millions of years, the Sun will inflate into a red giant, growing so large that its surface could extend past the current orbit of Venus. Its surface temperature will drop as it expands, giving it a reddish hue, but its total luminosity will increase dramatically because of the sheer size of the radiating surface. Meanwhile, the inert helium core keeps contracting under its own weight, growing hotter and denser. In a star of the Sun’s mass, this core eventually becomes degenerate, meaning the electrons inside are packed so tightly that quantum mechanical effects, rather than thermal pressure, hold it up against gravity.
The Helium Flash
When the core temperature reaches about 100 million degrees, helium nuclei begin to fuse into carbon through a process called the triple-alpha reaction (three helium nuclei combine). In stars below about twice the Sun’s mass, this ignition happens under degenerate conditions, which means the core cannot expand and cool itself the way a normal gas would.2arXiv. Helium Ignition in the Cores of Low-Mass Stars The result is a thermonuclear runaway known as the helium flash, a brief but violent event where the core’s energy output spikes to extraordinary levels for a matter of minutes.
The helium flash itself is mostly invisible from the outside because the energy is absorbed by the surrounding layers of the star. But modeling work suggests it has real structural consequences. Strong convection during the flash can excite waves that propagate outward into the envelope, depositing enough energy to cause the star to expand by tens to hundreds of solar radii within just a few years.3Monthly Notices of the Royal Astronomical Society. Rapid expansion of red giant stars during core helium flash by waves propagation to the envelope and implications to exoplanets After the flash, the degeneracy in the core lifts, and the Sun settles into a more stable phase of helium burning in the core with hydrogen burning in a surrounding shell. This calmer period, sometimes called the horizontal branch phase, lasts roughly 100 million years.
The Second Giant Phase and Mass Loss
Once the helium in the core is exhausted and converted to carbon and oxygen, the Sun faces a similar problem again. An inert carbon-oxygen core sits at the center, surrounded now by two burning shells: an inner helium-burning shell and an outer hydrogen-burning shell. The star swells up again, this time onto the asymptotic giant branch (AGB), becoming an even more luminous and unstable giant than it was before.
The AGB phase is messy. The helium-burning shell is prone to periodic flare-ups called thermal pulses, where helium ignites in brief, intense flashes that temporarily boost the star’s luminosity and can dredge material from deeper layers up toward the surface.4PoS (ENAS 6). Study of the Thermal Pulsation of AGB Stars These pulses also help drive a powerful stellar wind. The outer layers of the star are cool enough for dust grains to form in the atmosphere, and when photons from the star hit those grains, momentum transfers from light to dust, dragging gas along with it.5Astronomy & Astrophysics. Dust driven mass loss from carbon stars as a function of stellar parameters This dust-driven wind gradually strips the Sun of its outer envelope, sending material streaming away into interstellar space.
Over the course of the AGB phase, the Sun will lose a substantial fraction of its total mass this way. The mass loss is critical: the Sun is not heavy enough to fuse carbon in its core (that requires a star several times more massive), so there is no next fuel source. Instead, the outer layers are blown away until only the hot, exposed core remains.
The Planetary Nebula Stage
As the last of the Sun’s envelope is ejected, the bare core is revealed. This core is extremely hot, with a surface temperature that can exceed 100,000 degrees. Its intense ultraviolet radiation ionizes the recently ejected gas surrounding it, causing it to glow. The result is a planetary nebula, one of the most visually striking objects in astronomy. Despite the name, planetary nebulae have nothing to do with planets; the term dates to the 18th century, when their round shapes reminded early astronomers of planetary discs through small telescopes.
Planetary nebulae are short-lived on cosmic timescales. The expanding shell of gas disperses into the surrounding space over roughly 10,000 to 20,000 years. During that brief window, though, the nebula can be spectacular, with complex shapes sculpted by the star’s magnetic field, rotation, and sometimes interactions with companion stars. The Sun’s planetary nebula will eventually fade and merge with the interstellar medium, recycling elements like carbon, nitrogen, and oxygen that were manufactured inside the star back into the galaxy, where they can become part of future stars and planets.
The White Dwarf Remnant
What remains after the nebula dissipates is a white dwarf: the Sun’s carbon-oxygen core, compressed into an object roughly the size of Earth but containing about 60 percent of the Sun’s original mass. The interior of a white dwarf is extraordinary. It consists of bare atomic nuclei sitting in a sea of degenerate electrons, and these nuclei arrange themselves into a structure more like a crystal lattice than a gas.6PubMed. Short-range ordering and equilibrium structure of binary crystal mixtures of atomic nuclei in white dwarf cores A teaspoon of white dwarf material would weigh several tons on Earth.
White dwarfs produce no new energy through fusion. They shine only because they are still hot from the energy stored during their active lives. Cooling is their only remaining act, and it proceeds extremely slowly. As the interior cools, the carbon and oxygen ions undergo a phase transition and crystallize, releasing latent heat that delays the cooling process by about a billion years.7Nature. Core crystallization and pile-up in the cooling sequence of evolving white dwarfs This crystallization has actually been observed: surveys of white dwarfs in our galaxy have found a statistical pile-up at certain luminosities, exactly where theory predicts cooling should temporarily stall as the star crystallizes.
Over trillions of years, far longer than the current age of the universe, the white dwarf will eventually radiate away all its residual heat and become a cold, dark object sometimes called a black dwarf. No black dwarfs exist yet, because the universe is not old enough for any white dwarf to have cooled that far.
Will Earth Survive?
This is the question most people jump to, and the honest answer is that we are not sure. The fate of Earth during the Sun’s giant phases depends on a tug-of-war between two effects. As the Sun loses mass through its stellar wind, its gravitational grip on the planets weakens, and Earth’s orbit should expand outward. But as the Sun’s surface swells to enormous size, tidal interactions between the bloated Sun and Earth tend to drag the planet inward. Which effect wins depends on details that remain uncertain.
A recent reassessment of this problem found that Earth’s survival is highly sensitive to the tidal model used and to the assumed rate of mass loss during the AGB phase. Using updated prescriptions for how tidal forces dissipate energy, the study found that Earth survives both the red giant branch and the AGB phases. But with earlier, less refined tidal models, Earth gets swallowed during the AGB. Similarly, if the Sun’s mass loss during the AGB is slower than expected, Earth is engulfed; if the mass loss is faster, Earth escapes.8Astronomy & Astrophysics. The fate of Earth during the Sun’s giant phases Mercury and Venus will almost certainly be consumed regardless. Earth is the borderline case.
Even if Earth avoids being physically swallowed, it would be utterly transformed. The intense luminosity of the red giant Sun would boil away the oceans, strip the atmosphere, and likely melt the surface. Survival as a rocky body orbiting a white dwarf is not the same as survival as a habitable world.
A New Habitable Zone in the Outer Solar System
While the inner solar system gets scorched, something more interesting happens farther out. As the Sun brightens on the red giant branch, its habitable zone, the range of distances where liquid water could exist on a surface, migrates outward past Jupiter and Saturn and eventually beyond the Kuiper Belt.9The Astrophysical Journal. HABITABLE ZONES OF POST-MAIN SEQUENCE STARS Icy moons that are frozen solid today could experience a temporary thaw.
Europa, Jupiter’s ice-covered moon believed to harbor a subsurface ocean, would find itself in the habitable zone of the red giant Sun. Modeling suggests its surface ice would sublimate readily, and the different hemispheres of Europa would display interesting asymmetric behavior as the ice responds to changing light and heat.10Monthly Notices of the Royal Astronomical Society. Life after death: Europa in the evolving habitable zone of a Red Sun Saturn’s moon Titan presents an even more tantalizing scenario. Titan already has a thick atmosphere and abundant organic chemistry but is far too cold for liquid water. Researchers have found that as the Sun reddens and its ultraviolet output drops, Titan’s atmospheric haze production would plummet, allowing more heat to reach the surface. A window of several hundred million years could open, roughly six billion years from now, when liquid water-ammonia oceans could form on Titan’s surface and react with the organic compounds already there.11PubMed. Titan under a red giant sun: a new kind of “habitable” moon That window exceeds the time it took life to emerge on Earth.
These outer-solar-system habitable periods are temporary, lasting only as long as the Sun’s red giant phase puts those moons in the right temperature range. But they represent a genuinely different kind of habitable environment from what we usually imagine: cold water-ammonia mixtures warmed by methane greenhouses under a dim red star, rather than the warm water-and-carbon-dioxide conditions found on Earth today.
What Orbits a White Dwarf
After the Sun becomes a white dwarf, the surviving planets will still be there, just in wider orbits because of the mass the Sun has shed. Jupiter and Saturn, being far enough away to avoid the red giant’s reach, should survive largely intact, though their orbital distances will increase. The asteroid belt and Kuiper Belt objects will also persist, though their orbits will be reshuffled.
We have direct evidence that planetary systems survive stellar death. Observations of white dwarfs elsewhere in the galaxy show signs that many have accreted smaller bodies, and a small number host gaseous debris discs visible through emission lines, with at least one confirmed planetesimal found orbiting within such a disc.12PubMed. A planetesimal orbiting within the debris disc around a white dwarf star The chemical fingerprints of these accreted bodies tell us about their composition, revealing that rocky, metal-rich objects, essentially the shattered remains of asteroids and possibly planets, are falling onto white dwarfs throughout the galaxy. This means the Sun’s white dwarf remnant will likely still interact gravitationally with the leftover debris of our solar system for billions of years.
Where the Sun’s White Dwarf Will Drift
White dwarfs in the Sun’s neighborhood tend to follow well-behaved galactic orbits. Surveys of white dwarfs within about 300 parsecs of the Sun show that the vast majority, around 90 to 95 percent, belong to the thin disk of the Milky Way and follow nearly circular paths around the galactic center, much like the Sun does now.13EDP Sciences (Astronomy & Astrophysics). Kinematic properties of white dwarfs – Section: Abstract The remaining 5 to 10 percent follow more eccentric orbits and belong to the thick disk, while a tiny handful are halo members with trajectories that carry them far above and below the galactic plane.
The Sun’s white dwarf will not be flung out of the galaxy or sent careening through space. It will quietly continue orbiting the Milky Way’s center, growing imperceptibly dimmer over timescales that dwarf the current age of the universe. Its crystallizing core will gradually lock into a solid lattice of carbon and oxygen nuclei, a diamond-like object the size of a planet, glowing ever more faintly until, in the unimaginably distant future, it emits no detectable light at all. By that point, the galaxy itself will look nothing like it does today, and the Sun’s long story will have reached its final, silent chapter.