The life of a star is an ordered sequence of events dictated by gravity and nuclear physics. Our Sun, an average star, is currently in a stable phase, having spent approximately half of its existence generating light and heat. Like all stars, its finite internal fuel supply means its structure must inevitably change as it ages. These predictable changes mark a multi-stage process known as stellar evolution, leading to the end of its active life.
The Main Sequence State
The Sun is currently in its Main Sequence phase, a period of stability lasting about 4.6 billion years. During this time, the star maintains hydrostatic equilibrium: a balance between the inward force of gravity and the outward pressure from nuclear fusion. Energy is generated deep in the core, where temperatures reach approximately 15 million degrees Celsius. This heat fuses hydrogen atoms into helium via the proton-proton chain reaction, preventing the star from collapsing.
Transition to a Red Giant
The Sun’s stable phase will end approximately five billion years from now when the hydrogen fuel in its core is exhausted. Without fusion to support it, the inert helium core will begin to contract under gravity. This contraction causes the core’s temperature and density to rise dramatically. The intense heat generated will ignite the layer of fresh hydrogen gas surrounding it, initiating hydrogen shell burning.
This shell fusion releases more energy than the previous core fusion, destabilizing the star’s structure. The Sun’s outer layers will absorb this energy and expand rapidly, causing the star to transition into a Red Giant. Its surface temperature will cool to a deep orange-red, but its size will make it hundreds of times more luminous. The Sun’s radius is projected to swell to roughly 1 Astronomical Unit (AU), the current average distance between the Earth and the Sun.
This expansion will result in the engulfment and vaporization of the inner planets, Mercury and Venus. The Earth’s fate is less certain; models suggest its orbit will either be swallowed entirely or its surface will be sterilized by intense heat and radiation. Even if the rocky core survives, all water and atmosphere will be boiled away. Meanwhile, the helium core continues to contract and heat up until it reaches about 100 million degrees Celsius.
At this temperature, the helium nuclei will begin to fuse into carbon and oxygen in a runaway reaction known as the helium flash. This new core fusion temporarily stabilizes the star, causing it to shrink slightly and become less luminous for about 100 million years. However, the helium fuel will eventually run out, leading to intense helium shell burning around an inert carbon-oxygen core. During this final, unstable phase, the Sun will expand again, reaching its largest size and greatest luminosity.
Forming a Planetary Nebula
The Red Giant phase cannot be sustained indefinitely, as the fusion processes become erratic and unstable. The intense energy from the double shell-burning (hydrogen and helium) pushes the star’s outer atmosphere away. This final, unstable period is characterized by strong thermal pulses—periodic bursts of energy that cause the star to rapidly pulsate. Each pulse results in the Sun losing a significant amount of its mass into space.
These powerful stellar winds blow the star’s entire outer envelope away. Over a period lasting only a few tens of thousands of years, up to half of the Sun’s total mass will be ejected into the surrounding cosmos. This expelled material forms an expanding shell of gas and dust, which astronomers call a Planetary Nebula. The term is a historical misnomer, as these objects were mistaken for round, planetary disks when viewed through early telescopes.
The colors of the nebula are caused by the intense ultraviolet radiation emitted by the newly exposed stellar core. This radiation ionizes the expelled gas, causing the atoms to glow brightly. This final stage is an astronomically short-lived event. The nebula will continue to expand and disperse into interstellar space, enriching the galaxy with elements like carbon and oxygen that were synthesized inside the star.
Cooling into a White Dwarf
Once the outer layers of gas have been shed, all that remains is the dense, hot core of the former star. This remnant is known as a White Dwarf. The Sun’s core, composed primarily of carbon and oxygen ash, will be roughly the size of the Earth but contain approximately half of the Sun’s original mass. This results in a dense object where a single teaspoon of its material would weigh several tons.
The White Dwarf no longer generates energy through nuclear fusion, as it is not massive enough to ignite the carbon and oxygen fuel. Its stability against gravity is maintained 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 compression of the stellar material. The White Dwarf begins its cooling journey by slowly radiating away its residual heat.
This cooling process is projected to take trillions of years. Over this immense timescale, the White Dwarf will gradually dim, turning from white to yellow, then red, and finally fading out. Eventually, it will become a cold, dark, non-radiating remnant known as a Black Dwarf, a solid, crystallized sphere of carbon and oxygen.