Our Sun, like all stars, undergoes predictable transformations. It is currently in a stable phase but will evolve through distinct stages, each marked by changes in its size, temperature, and energy output. Understanding these phases helps anticipate the Sun’s future impact on our solar system.
The Sun’s Main Sequence Life
The Sun is currently in the main sequence stage, its longest and most stable period. During this phase, it generates energy by fusing hydrogen into helium in its core. This creates an outward pressure that balances gravity, maintaining the Sun’s stable size and luminosity.
Our Sun is a G-type main-sequence star, approximately 4.6 billion years old. It is about halfway through its expected 10-billion-year main sequence lifespan. Over this timescale, the Sun’s luminosity has gradually increased and will continue to do so. The continuous consumption of hydrogen fuel in its core sets the stage for its eventual evolution away from the main sequence.
Transition to the Subgiant Phase
The Sun’s departure from the main sequence begins when it exhausts hydrogen fuel in its core. As core hydrogen fusion diminishes, the helium core can no longer sustain outward pressure against gravity and begins contracting. This compression significantly increases the core’s temperature and density.
This heating ignites a new phase of hydrogen fusion in a shell surrounding the helium core. The energy released from this shell burning is more intense than previous core fusion, causing the Sun’s outer layers to expand significantly. As these outer layers expand, they also cool, marking the star’s transition into the subgiant phase. This transitional period is relatively brief compared to the main sequence, lasting several hundred million to about a billion years.
Characteristics of the Subgiant Stage
During the subgiant phase, the Sun will undergo significant physical changes. Its outer envelope will expand considerably, growing to approximately 1.6 to 3 times its current radius. Despite this expansion, the Sun’s surface temperature will slightly decrease, cooling from about 5,777 Kelvin to around 4,900 Kelvin. This cooling results from energy spreading over a much larger surface area.
The Sun’s luminosity will increase during this stage. While the surface temperature cools, the increased surface area leads to a greater total energy output. The Sun’s luminosity during this phase is expected to be approximately 2.2 times its current luminosity. Subgiants are generally more luminous than main sequence stars of the same spectral class but are not yet as bright as true giant stars.
The Sun’s Further Evolution and Earth’s Fate
After the subgiant phase, the Sun will continue its evolution into a red giant star. This next stage involves a dramatic expansion, with the Sun’s outer layers swelling to engulf Mercury, Venus, and likely Earth. The Sun’s radius could expand to reach approximately Earth’s current orbital distance. This red giant phase is projected to last for about 1 to 2 billion years.
Long before the Sun becomes a red giant, life on Earth will cease to exist due to steadily increasing solar radiation. The heightened energy output will cause Earth’s oceans to boil away and its atmosphere to escape into space. Following its red giant phase, the Sun will shed its outer layers, forming a planetary nebula. The remaining core will then contract into a dense, compact white dwarf, slowly cooling over billions of years.