The Sun is a G-type main-sequence star, currently generating energy by fusing hydrogen atoms into helium in its core. It has spent approximately 4.6 billion years in this stable phase.
The life cycle of any star is determined by its initial mass, which dictates the internal pressure and temperature governing its nuclear reactions. As an intermediate-mass star, the Sun’s eventual path is well-established through astronomical observation and modeling.
Our star is currently in its middle age, with an estimated remaining lifespan of about 5 billion years before it begins its profound metamorphosis. This transition will mark the end of its long period of equilibrium and initiate a series of dramatic changes.
The End of the Main Sequence
The Sun’s current stability stems from a delicate balance between the inward force of gravity and the outward pressure generated by core hydrogen fusion. This main-sequence phase will conclude when the available hydrogen fuel in the core is nearly exhausted and converted into inert helium “ash.” Once core hydrogen fusion ceases, the outward pressure supporting the star begins to diminish.
Without the thermal support from fusion, gravity causes the inert helium core to contract intensely. This contraction generates vast heat, significantly raising the temperature in the hydrogen gas shell surrounding the dense core. The elevated temperature ignites a new, rapidly burning layer of hydrogen fusion in this shell. This shell fusion generates far more energy than the previous core fusion, fundamentally destabilizing the star’s structure. The immense energy output pushes the star’s outer layers outward, initiating the first major physical transformation.
The Red Giant Expansion
The energy from the furiously burning hydrogen shell causes the Sun’s outer envelope to swell and cool dramatically, turning the star into a red giant. This expansion will be colossal, increasing the Sun’s radius by over 200 times its present size. The surface is expected to extend beyond the orbits of Mercury and Venus, engulfing both planets.
Models indicate the solar atmosphere will likely expand close to or even past Earth’s orbital distance. Although the Sun loses mass through an enhanced stellar wind, which slightly widens Earth’s orbit, the planet is still expected to be fully engulfed. Earth’s surface will be vaporized long before the expansion reaches it, due to the Sun’s increased luminosity preceding the full red giant stage.
Throughout this expansion, the dense helium core continues to contract and heat up. When the core temperature reaches approximately 100 million Kelvin, helium nuclei begin to fuse into carbon through the triple-alpha process. For a star of the Sun’s mass, this ignition occurs explosively in a runaway process called the “Helium Flash.” This immense energy is absorbed by the surrounding core material, causing it to expand and become non-degenerate. This helium fusion temporarily halts the star’s overall expansion and causes it to shrink slightly before entering its final, most luminous giant phase.
Shedding Outer Layers
After the core helium is depleted and converted into a carbon and oxygen core, the star begins its final ascent up the asymptotic giant branch (AGB). During this stage, the star is powered by two fusion shells: an outer shell of hydrogen fusion and an inner shell of helium fusion. This double-shell burning makes the star highly unstable, leading to thermal pulses and powerful stellar winds.
These extreme pulsations cause the Sun’s outer envelope, which is only weakly held by gravity, to be gently ejected into space over tens of thousands of years. The dispersed material forms an expanding, glowing cloud of gas known as a planetary nebula. The term “planetary nebula” is a historical misnomer, originating from the resemblance of these objects to gas planets when viewed through early telescopes.
The central remnant emits intense ultraviolet radiation that ionizes the ejected atoms, causing them to fluoresce in a brilliant, colorful display. This process distributes carbon, oxygen, and other elements synthesized inside the star into the interstellar medium. The formation of the nebula marks the end of the Sun’s life as a giant star, leaving only its remnant core behind.
The White Dwarf Remnant
The final stage of the Sun’s evolution is the exposure of its inert, dense core, which becomes a white dwarf. This compact remnant is primarily composed of carbon and oxygen, the products of earlier helium fusion. A white dwarf possesses a mass similar to the Sun but is compressed into a volume roughly the size of Earth.
This extreme compression results in material supported not by thermal pressure, but by electron degeneracy pressure, a quantum mechanical effect. The white dwarf no longer undergoes nuclear fusion, meaning it is essentially a cooling ember. It glows brightly due to the immense residual heat trapped within its matter.
Over the course of billions, or even trillions, of years, the white dwarf will slowly radiate this heat away into space. As its temperature plummets, its light will gradually fade from white to yellow, then red, and finally cease to be visible. The ultimate fate of this cooling remnant is to become a cold, dark celestial object known as a black dwarf, an end that the universe is not yet old enough to have witnessed.