The Sun, the star at the center of our solar system, is a massive, nearly perfect sphere of hot plasma that provides the energy necessary for life on Earth. Its existence and eventual demise are governed by stellar evolution, which dictates the life cycle of stars based on their mass. Our Sun is considered a G-type main-sequence star, informally known as a yellow dwarf, meaning its lifespan is relatively long compared to more massive stars. This stellar life cycle spans vast cosmic timescales, beginning as a cloud of gas and dust and ending billions of years later as a cold, dark remnant.
Stellar Genesis
The Sun’s life began approximately 4.6 billion years ago within a giant, cold, and dense accumulation of gas and dust known as a molecular cloud, or stellar nursery. This immense cloud, containing the remnants of earlier stars, began to collapse under gravity. This initial gravitational collapse may have been triggered by a shockwave from a nearby supernova explosion or by a collision with another cloud.
As the vast cloud collapsed, the material at the center gathered into a denser, hotter core, forming a protostar. The remaining matter flattened into a spinning disk around this core, which would eventually form the planets. For millions of years, the protostar continued to contract, converting gravitational energy into heat, until the temperature and pressure in the core reached an extreme threshold.
The critical moment of stellar ignition occurred when the core temperature surpassed about 15 million degrees Celsius. At this immense heat and pressure, hydrogen atoms began to fuse together to form helium, a process called nuclear fusion. This fusion reaction released a tremendous amount of energy, which created an outward pressure that finally halted the gravitational collapse. Once the star achieved this balance, it began its long, stable phase.
Main Sequence Stability
The Sun is currently in the main sequence phase, a stage where it has spent the majority of its life and will remain for about 10 billion years in total. This phase is defined by hydrostatic equilibrium, where the outward pressure generated by nuclear fusion perfectly counters the inward pull of gravity. The energy source maintaining this stability is the continuous fusion of hydrogen into helium occurring deep within the star’s core.
Every second, the Sun’s core converts about 600 billion kilograms of hydrogen into helium, simultaneously transforming about 4 billion kilograms of mass into energy, while providing the light and heat that radiate outward. Over the billions of years it has spent on the main sequence, the Sun has been remarkably stable, allowing conditions for life to evolve on Earth.
However, the core composition is gradually changing as hydrogen is consumed, leading to a slow increase in the Sun’s overall luminosity. This means the Sun is slowly getting brighter and hotter, a change that will eventually have profound effects on the inner solar system.
The Red Giant Transformation
The Sun’s stability will ultimately end in approximately five billion years when the hydrogen fuel in its core is finally exhausted. Without the outward pressure from hydrogen fusion, the inert helium core will begin to contract under gravity, causing its temperature and density to increase. This core contraction heats the layer of hydrogen surrounding the core, igniting a new shell of fusion that burns hydrogen much more intensely than before.
The immense energy released by this hydrogen shell burning will cause the Sun’s outer layers to expand and cool, signaling its transition into a red giant star. As the Sun expands, its radius will swell to about 100 times its current size. The outer atmosphere of the star will expand past the current orbits of Mercury and Venus, engulfing both planets.
While the Earth’s orbit may slightly expand due to the Sun’s mass loss during this phase, it is highly likely that our planet will also be engulfed by the star’s atmosphere. Even if the Earth avoids being swallowed, the heat and radiation from the Sun’s swollen surface will render the planet uninhabitable. The Red Giant phase is a short but violent period, lasting for roughly a billion years before the next stage begins.
The Final Stages
After the red giant phase, the Sun’s helium core will contract until it becomes hot enough to ignite helium fusion, transforming helium into carbon and oxygen. Once this secondary fuel is depleted, the Sun will not have enough mass to generate the necessary heat and pressure to fuse carbon, leading to a final, unstable period. The star will then begin to shed its outer layers due to thermal pulses and stellar winds.
This expelled material will drift outward into space, forming a shell of gas known as a Planetary Nebula. The nebula is illuminated by the hot, dense core left behind at the center, which is now a White Dwarf. This remnant is small, about the size of Earth, but contains nearly half of the Sun’s original mass, composed primarily of carbon and oxygen.
As all nuclear fusion has ceased, the white dwarf will slowly radiate away its residual thermal energy and cool down over trillions of years. Eventually, the white dwarf will become so cool that it no longer emits light, transitioning into a cold, dark object known as a black dwarf, completing the stellar life cycle.