The Sun, like all stars, undergoes profound transformations over billions of years. It is currently a main sequence star, maintaining a stable size and energy output. The total energy radiated by a star per second is its luminosity, a measure of its power. As the Sun ages, internal changes will cause its luminosity to shift dramatically, marking the beginning of its final stellar phases. This evolution will alter the star’s appearance and its influence on the solar system.
The Exit from the Main Sequence
The Sun has spent approximately \(4.6\) billion years in the main sequence phase, a period defined by the fusion of hydrogen into helium in its core. This fusion process generates the outward pressure that perfectly balances the inward force of gravity, establishing a state of hydrostatic equilibrium. This stable arrangement has allowed the Sun to maintain its current size and luminosity for eons.
This era ends when the supply of hydrogen fuel in the core is exhausted. When the core is filled with helium, nuclear fusion ceases. Without the outward pressure from fusion, the core begins to contract under its own immense gravity, causing its temperature to rise significantly.
This temperature rise heats the hydrogen shell surrounding the inert helium core. The conditions in this surrounding shell become hot and dense enough to ignite a new round of hydrogen fusion. This transition from core burning to shell burning is the mechanism that signals the end of the main sequence phase for a star like the Sun.
Characteristics of the Subgiant Stage
The subgiant stage is the transitional period following the cessation of core hydrogen fusion and preceding the expansion into a full red giant. This phase is characterized by the initial, noticeable swelling of the star’s outer layers. The Sun will begin to expand radially, growing from its current size.
This evolutionary phase is relatively brief in astronomical terms, lasting for about \(1.5\) billion years before the star moves onto the red giant branch. During this time, the Sun’s luminosity will increase considerably beyond its current output. The star will evolve to reach an approximate luminosity of \(2.2\) times its present energy output, expressed as \(2.2\) \(L\odot\) (solar luminosities).
This increase in energy output will cause the star to move upward and to the right on the Hertzsprung-Russell diagram, signifying both higher luminosity and a slightly cooler surface temperature due to the expansion. This doubling of energy output is a major shift from the Sun’s \(4.6\)-billion-year history of relative stability.
The Luminosity Shift and Its Physical Cause
The increase in luminosity during the subgiant phase is attributable to hydrogen shell burning. As the helium core contracts gravitationally, it generates immense heat. This rising heat acts upon the shell of hydrogen surrounding the core, raising its temperature and density to levels far greater than those previously seen in the main sequence core.
This compression accelerates the rate of nuclear fusion in the hydrogen shell dramatically. The fusion reactions in this shell become much more intense than the original fusion in the star’s core. This highly efficient shell fusion generates a significantly larger amount of outward thermal pressure.
The intense energy production from the shell pushes the star’s outer envelope outward, causing the star to expand and its overall energy emission to increase. This rapid, sustained fusion within the shell provides the physical cause for the Sun’s luminosity to climb to approximately \(2.2\) \(L\odot\) during this stage.
The core remains inert during this time, composed of helium not yet hot enough for fusion. The weight of the outer envelope continues to press down on the core, causing it to shrink and grow hotter, which, in turn, maintains the high rate of fusion in the surrounding hydrogen shell. This continuous compression-heating cycle drives the subgiant’s rising luminosity until the core reaches a critical state.
The Subsequent Red Giant Phase
The subgiant phase acts as a prelude to the Sun’s ultimate transformation into a red giant. The gravitational contraction of the inert helium core continues, relentlessly increasing the core’s temperature. This process culminates when the core reaches a temperature high enough to ignite helium fusion.
This ignition marks the star’s entry onto the Red Giant Branch, where evolution becomes more rapid and extreme. Once the Sun is fully a red giant, its luminosity will skyrocket far beyond the levels reached in the subgiant phase. Stellar models predict that the Sun will reach a peak luminosity of approximately \(2,300\) times its current output while on the Red Giant Branch.
The Sun’s radius will swell to an enormous size, potentially reaching \(170\) times its current diameter. This catastrophic expansion is a much more significant event than the moderate growth seen during the subgiant stage. The subgiant phase is merely the initial transition before the Sun fully embraces its destiny as a massive, luminous red giant.