The Sun is a G-type main-sequence star (G2V) currently about halfway through its predicted existence. Stellar evolution is a predictable process determined primarily by a star’s initial mass. The Sun is estimated to have a total lifespan of approximately 10 billion years, transitioning through distinct phases that dramatically alter its structure and output. This journey begins with the gravitational collapse of interstellar matter and concludes with a stellar remnant.
Stellar Birth: From Molecular Cloud to Protostar
The Sun began its life within a dense pocket of gas and dust known as a giant molecular cloud, or nebula. Gravity acts upon slight overdensities within these structures, causing a region to collapse. This gravitational contraction increases the density and pressure at the center of the material.
As the material spirals inward, it forms a dense, hot core called a protostar, surrounded by a disk of leftover matter. The core’s temperature and pressure increase dramatically, fueled by the energy released from the gravitational collapse. This phase continues until the core reaches the 10 million Kelvin threshold necessary to ignite nuclear fusion.
The Main Sequence: Hydrogen Fusion and Stability
The Sun is currently in the main sequence phase, a stable state that accounts for about 90% of its entire life, having lasted approximately 4.6 billion years. This period is defined by hydrostatic equilibrium, where the inward force of gravity is perfectly counteracted by the outward pressure generated by nuclear fusion in the core.
During this phase, the Sun’s core fuses hydrogen atoms into helium, primarily through the proton-proton chain. This constant conversion of mass into energy sustains the star’s luminosity and temperature. The Sun will remain on the main sequence for about another 5 billion years, burning hydrogen fuel at a steady rate.
As hydrogen fuel is consumed, the core slowly accumulates denser helium “ash.” Although the Sun appears stable, it is gradually getting hotter and brighter; its luminosity will almost double by the end of this phase. This slow change signals the transition to the next stage of stellar evolution.
The Red Giant Phase: Expansion and Transformation
The main sequence ends when the Sun depletes the hydrogen fuel in its core. Without the outward pressure of fusion, the core begins to contract under gravity, causing its temperature to rise significantly.
The intense heat ignites the remaining hydrogen in a shell surrounding the inert helium core, a process called shell burning. This shell fusion generates energy rapidly, creating enormous outward thermal pressure. The star’s outer layers respond by expanding hundreds of times their current size, turning the Sun into a Red Giant.
This expansion will engulf the orbits of Mercury and Venus, and it is highly likely that Earth will also be swallowed by the expanding solar atmosphere. The helium core continues to contract until it reaches 100 million Kelvin, triggering the violent ignition of helium fusion into carbon, known as the Helium Flash. After the flash, the Sun will spend a short period stably fusing helium in its core, before expanding once more as the core helium runs out.
Final Stages: Planetary Nebula and White Dwarf
Following the Red Giant phase, the Sun’s mass is insufficient to generate the temperatures needed to fuse carbon into heavier elements. The star’s internal structure becomes unstable, leading to a series of thermal pulses. These pulses gently expel the Sun’s outer layers of gas and plasma into space, forming a glowing, expanding shell called a Planetary Nebula.
The planetary nebula phase is short-lived, lasting only about 10,000 years before the gas dissipates into the interstellar medium. The remaining stellar core is an extremely hot, dense remnant known as a White Dwarf. This core, composed primarily of carbon and oxygen, will be roughly the size of Earth but contain approximately 62% of the Sun’s original mass.
The White Dwarf is held stable by electron degeneracy pressure, a quantum mechanical effect that resists further gravitational collapse. Without any fusion reactions, this stellar corpse will slowly cool and fade over trillions of years. Eventually, the white dwarf will become a cold, dark object known as a Black Dwarf.