The Sun has existed in a state of stable equilibrium for approximately 4.6 billion years, known as the main sequence phase. During this time, the star has generated outward pressure by converting hydrogen into helium deep within its core through nuclear fusion. This energy production perfectly counterbalances the inward crushing force of its own gravity, maintaining a constant size and temperature. The central question of stellar evolution is what happens when the Sun, which is about halfway through its lifespan, finally depletes the hydrogen fuel in its core, forcing a dramatic transformation over the next few billion years.
The End of Core Fusion
The Sun’s long period of stability will end when the hydrogen fuel in its core is exhausted and converted into helium ash. This helium cannot fuse at the current core temperature, causing the primary outward pressure from fusion to cease. Gravity then gains the upper hand, initiating a collapse of the helium core.
This core contraction causes a profound increase in temperature and density in the layers of gas surrounding the core. This surrounding region still contains fresh hydrogen, which is compressed and heated until fusion begins in a shell around the core. This new hydrogen shell fusion produces energy at a much higher rate than the previous core fusion. The resulting surge of thermal energy pushes outward against the star’s outer layers, signaling the end of the Sun’s main sequence life.
The Red Giant Phase and Helium Ignition
The energy output from the newly ignited hydrogen shell causes the Sun’s outer envelope to swell, marking the beginning of the Red Giant phase. The star’s radius will expand hundreds of times its current size, and its surface temperature will cool, giving it a reddish-orange hue despite its increased luminosity. This expansion will easily engulf the orbits of Mercury and Venus, and may even extend past the Earth’s current orbit.
As the Sun expands, the helium core at its center continues to contract under the weight of the outer layers. This contraction drives the core temperature upward until it reaches approximately 100 million Kelvin. At this extreme temperature, helium nuclei begin to fuse into carbon.
For a star with the Sun’s mass, this ignition of helium occurs in a rapid, uncontrolled thermal runaway event called the Helium Flash. This sudden release of energy causes the core to expand, which temporarily reduces the star’s overall luminosity and size as it enters a new, stable phase of helium core fusion. This period of stability, where the Sun fuses helium in its core and hydrogen in a surrounding shell, will only last for about 100 million years, a brief moment compared to its 10-billion-year main sequence life.
The Formation of a Planetary Nebula
The temporary stability achieved by core helium fusion eventually ends when the helium fuel is depleted, leaving behind a core of carbon and oxygen ash. Gravity causes the core to contract, and fusion shifts outward into two shells: an inner shell of helium fusion and an outer shell of hydrogen fusion. The star enters the Asymptotic Giant Branch phase, a second period of expansion where it becomes larger and more luminous than the first Red Giant phase.
During this phase, the star becomes unstable, experiencing periodic bursts of energy known as thermal pulses from the helium shell. These bursts cause the star to lose its outer layers of gas and dust through strong stellar winds. A significant fraction of the Sun’s mass, up to 45 percent, will be ejected into space over tens of thousands of years.
The ejected material forms an expanding shell of gas around the star’s remaining hot core. This glowing shell is called a planetary nebula, a misnomer dating back to when early telescopes made these objects appear round like planets. The remnant core, which is incredibly hot, emits intense ultraviolet radiation that ionizes the surrounding gas, causing the nebula to glow brightly.
The Final Remnant: A White Dwarf
The planetary nebula is a short-lived event, dissipating into interstellar space after 20,000 years, leaving behind the stellar core. This exposed core is known as a White Dwarf. It is composed primarily of carbon and oxygen, the products of the star’s final fusion stages.
This object is dense, packing about half of the Sun’s original mass into a sphere approximately the size of the Earth. Collapse is resisted by electron degeneracy pressure. This pressure arises because electrons cannot occupy the same quantum state, creating resistance to compression.
The White Dwarf no longer generates heat through fusion, as it lacks the mass to ignite the carbon and oxygen fuel. Instead, it radiates away its stored thermal energy, glowing from residual heat. Over trillions of years, this hot, dense stellar corpse will gradually cool and dim until its light fades completely, becoming a cold, dark object known as a Black Dwarf.