The Sun is currently a main-sequence star, generating energy by fusing hydrogen into helium in its core. This process has powered the Solar System for approximately 4.5 billion years, maintaining a stable balance against its own gravity. In roughly five billion years, the Sun will exhaust its primary fuel source and embark on a dramatic transformation. This transformation will involve a massive physical expansion, the shedding of its outer atmosphere, and its ultimate reduction to a small, dense stellar remnant.
Transition to a Red Giant Star
The Sun’s transformation begins when the hydrogen fuel supply runs out in its central core. Without the outward pressure from fusion, the core can no longer support the overlying layers and begins to contract under gravity. This contraction heats the core immensely, igniting a new layer of hydrogen fusion in a shell surrounding the inert helium core.
The energy released by this intense shell burning is far greater than the energy previously generated. This massive increase in energy output causes the Sun’s outer layers to puff up dramatically and cool down. The star’s diameter will swell to an enormous size, expanding by over 200 times its current radius. This expansion causes the surface temperature to drop, shifting its color toward the red, thus earning it the name “Red Giant.”
The expansion will have catastrophic effects on the inner Solar System. Mercury and Venus are certain to be engulfed and vaporized. The Sun’s outer edge is predicted to extend out to about one astronomical unit (the current average distance to Earth). Earth will likely be engulfed by the Sun’s atmosphere, or at the very least, suffer a complete boil-off of its oceans and atmosphere, leaving behind a scorched, lifeless core.
Even if Earth avoids being consumed, the massive increase in solar luminosity will render the planet uninhabitable long before the Sun reaches its maximum size. The Sun will remain in this Red Giant phase for approximately 250 million years before beginning the next phase.
Shedding Outer Layers: The Planetary Nebula
Following the Red Giant phase, the Sun will enter a brief, unstable period known as the Asymptotic Giant Branch. During this stage, thermal pulses occur as helium fusion ignites and fades in shells around the core. These powerful pulsations cause the star’s outer envelope to be expelled into space through a stellar wind.
The Sun loses a significant fraction of its total mass, perhaps up to half, in these final episodes of mass loss. This ejected material forms a massive, rapidly expanding shell of gas and dust known as a planetary nebula. The term “planetary nebula” is a historical misnomer, as these objects have no relation to actual planets, but were mistaken for round, planet-like objects by early astronomers.
The expelled cloud is illuminated by the intensely hot, exposed core. This core emits powerful ultraviolet radiation that ionizes the surrounding gas cloud, causing it to glow brightly in various colors. The resulting structures often appear as colorful rings or bubbles. This phase is fleeting, lasting only for a few tens of thousands of years before the gas disperses and mixes with the interstellar medium.
The Final Form: A White Dwarf Star
After the planetary nebula dissipates, the last remnant of the Sun will be its exposed core, a dense, small object known as a white dwarf. This stellar cinder is composed primarily of carbon and oxygen, the products of the final stages of fusion. The white dwarf will be incredibly compact, containing about half the Sun’s original mass squeezed into a volume roughly the size of Earth.
This extreme density is possible because the white dwarf is no longer supported by the outward pressure of fusion, which has ceased. Instead, it is held up by a quantum mechanical effect called electron degeneracy pressure. This pressure arises from the inability of electrons to occupy the same quantum state, preventing further gravitational collapse.
The white dwarf radiates only its residual heat, making it extremely hot but dim due to its small surface area. Over the course of trillions of years, this stellar remnant will slowly cool down and fade away. The universe is not yet old enough for any white dwarfs to have completed this cooling process. Eventually, the Sun’s remnant will become a cold, dark object known as a black dwarf.