The Sun’s final moments, billions of years in the future, represent a predictable end to its current existence. This transformation is a natural stage in the evolution of stars like our Sun, driven by stellar physics. For the last 4.6 billion years, the Sun has been a stable, middle-aged G-type main-sequence star. Approximately five billion years from now, the next chapter in its life will begin, fundamentally reshaping the solar system.
The Sun’s Final Fuel Crisis
The end of the Sun’s main sequence stability will be marked by the exhaustion of its primary fuel source in the core. For billions of years, the Sun maintained a balance where outward pressure from hydrogen fusion counteracted gravity. When hydrogen atoms in the deep core are converted into helium, this outward pressure diminishes. The fusion reaction ceases, causing the core to lose its ability to support the weight of the layers above it.
Gravitational forces cause the inert helium core to contract rapidly, increasing its density and temperature. This intense heating profoundly affects the surrounding layers, specifically a shell of hydrogen just outside the core. The heat ignites this shell, causing hydrogen fusion to start in this new, hot region. This process, known as shell burning, is far more energetic than previous core fusion, generating a surge of energy.
The enormous energy output from this hydrogen-burning shell pushes the Sun’s outer layers outward, initiating immense expansion. This internal transformation drives the Sun into its next, larger phase of stellar evolution. This shift in energy production marks the Sun’s departure from its long period of stability on the main sequence.
The Era of the Red Giant
The increased energy from shell burning will cause the Sun’s outer atmosphere to swell, transforming it into a red giant. The Sun’s radius could expand to roughly 100 times its current size during this phase. This vast expansion will consume the orbits of the innermost planets, Mercury and Venus. The Sun’s outer layers are expected to extend out to approximately one astronomical unit, the current average distance between the Earth and the Sun.
The fate of Earth is less certain, as it may be physically engulfed by the Sun’s expanding outer layers or orbit dangerously close to the star’s surface. Even if Earth avoids being swallowed, the planet will be rendered uninhabitable long before the Sun reaches its maximum size. The Sun’s luminosity will increase by a factor of thousands, causing the surface temperature on Earth to skyrocket. This intense heat will boil away all the oceans and sterilize the surface.
The increased radiation will also cause the planet’s crust to melt, turning Earth into a molten cinder orbiting the massive, bloated star. As the Sun expands, it loses a significant amount of mass through a powerful stellar wind. This mass loss weakens the Sun’s gravitational hold on the remaining planets, causing their orbits to drift outward. Even with this orbital expansion, Earth will be a desolate rock, its atmosphere and water lost forever, having experienced a runaway greenhouse effect years earlier.
Cooling into a White Dwarf
The red giant phase is temporary, lasting for a relatively short time on a cosmic scale. The intense helium core, which has been building up for billions of years, will eventually ignite into helium fusion, creating carbon and oxygen. This new fusion source temporarily stabilizes the star, but this fuel is consumed quickly. Following this short-lived second fusion phase, the Sun will become unstable, and its outer layers will be shed into space in episodic bursts.
These ejected outer layers form a beautiful, expanding shell of gas and dust known as a planetary nebula. The name is a historical misnomer from early astronomers who thought the structures resembled planets through small telescopes, though they have nothing to do with planets themselves. This shedding process leaves behind the star’s hot, dense core. This remnant core is called a white dwarf, a small object roughly the size of Earth but containing about half of the Sun’s original mass.
The white dwarf no longer generates energy through fusion; it glows intensely from the residual heat of its former life. It is supported against gravity by electron degeneracy pressure, a quantum mechanical effect that prevents further collapse. Over trillions of years, this dense stellar corpse will slowly cool and fade. Eventually, it will become a cold, dark, non-radiating object called a black dwarf, marking the final stage of the Sun’s evolution.