1. The Sun's Current Phase
The Sun is currently in the life stage known as the main sequence.
In its core, the nuclei of hydrogen atoms fuse to form helium. The energy released during this nuclear fusion provides the Sun's light and heat.
During this period of the Sun's life, the amount of hydrogen in the core gradually decreases.
About 5 billion years from now, when most of the hydrogen in the core is depleted, the Sun's structure and way of producing energy will begin to change.
2. Transition to the Red Giant Phase
As hydrogen fusion in the core nears its end, the amount of helium at the center increases.
The core begins to contract under its own gravity and its temperature rises.
Meanwhile, fusion continues in the hydrogen-rich layer surrounding the core.
This energy production causes the Sun's outer layers to expand more and more.
Thus, the Sun moves toward becoming a star much larger than its current size.
3. The Sun Reaches Enormous Sizes
In the red giant phase, the Sun's outer layers will expand to extraordinary proportions.
The star's radius may reach tens or even hundreds of times the current radius of the Sun.
Due to this expansion, the Sun's outer surface will spread over a much larger area.
Although the Sun's total energy output increases, its surface temperature will be lower than it is today.
4. Why the Sun Will Appear Red
A star's color is closely related to its surface temperature.
Hotter stars appear more white or blue, while cooler stars tend toward yellow, orange, and red tones.
In the red giant phase, as the Sun's surface temperature drops, more of its light will shift to longer wavelengths.
For this reason, the Sun will transition from its current yellow-white appearance to a more reddish and orange look.
5. Core Contraction
While the Sun's outer layers expand, the central region contracts in contrast.
The contraction of the core causes the temperature to rise.
This process is one of the most important changes in the Sun's life.
Because when the temperature at the center rises high enough, conditions are created for the helium that has previously accumulated in the core to participate in nuclear fusion.
6. The Onset of Helium Fusion
When the core becomes sufficiently hot and dense, helium fusion begins.
Helium nuclei fuse to form carbon.
This process is a different stage of nuclear energy production than the hydrogen fusion during the Sun's main sequence period.
The Sun's mass is not large enough for helium fusion to proceed in a long chain toward heavier elements.
Therefore, carbon becomes an important final product in the Sun's evolution.
7. Changes in the Sun's Brightness
The Sun in its red giant phase will be much brighter than the Sun today.
One of the main reasons for this is the star's expanding outer layers and the strong continuation of hydrogen fusion around the core.
Even though the Sun's surface temperature drops, its enormous surface area causes the total energy output to increase significantly.
Therefore, the Sun's aging does not mean it will become a dimmer star.
8. The Future of Earth
The Sun's entry into the red giant phase will greatly alter the inner regions of the Solar System.
The Sun's increasing brightness will cause surface temperatures on Earth to rise excessively.
It will no longer be possible for the oceans to remain in liquid form for long, and Earth's current living conditions will disappear.
Although detailed models may yield different results regarding how much the Sun's outer layers will expand and how Earth's final position will be affected, Earth's habitability will end much earlier.
9. The Fate of Mercury and Venus
The Sun's expansion will directly affect the inner planets in particular.
Mercury and Venus will become very close to the Sun's expanding outer atmosphere.
Models predict that these planets may be engulfed by the Sun in the later stages of the red giant phase.
Whether Earth will face a similar fate depends on the Sun's maximum size and changes in the planets' orbits.
10. Toward the End of the Red Giant Phase
After most of the helium in the Sun's core is converted to carbon, a carbon-rich core will form at the center.
The Sun's mass is not high enough to sustain fusion of carbon into heavier elements.
As a result, nuclear energy production at the center gradually changes and the star moves toward an unstable structure.
The outer layers will then begin to separate from the star.
11. The Outer Layers Spread into Space
In the final stages of the Sun's life, its outer layers will spread into space.
As the hot core at the center is exposed, the expelled gas forms an expanding structure around it.
This cloud of gas is called a planetary nebula.
The term "planetary" here can be misleading; these structures are not planets and are not directly related to planets.
12. The Sun's Transformation into a White Dwarf
The remaining core of the Sun, after it loses its outer layers, will become a white dwarf.
A white dwarf is a hot stellar remnant with a size similar to Earth's but with very high density.
Since it is not massive enough to start a new period of nuclear fusion, it does not produce its own energy through long-term fusion.
Over time, it slowly cools by radiating its heat into space.
13. The Sun's Place in the Cycle of Life
The Sun's red giant phase is one of the natural stages of stellar life.
In medium-mass stars like the Sun, the life process begins with hydrogen fusion in the core; as hydrogen is depleted, the star expands into a red giant and later loses its outer layers to become a white dwarf.
During this process, the distribution of elements inside the star also changes.
The matter produced and released into space by the Sun can become part of the gas and dust used in the formation of other star systems and planets in the future.
14. Conclusion and Evaluation
The Sun's red giant phase is the result of a major structural change that begins with the depletion of hydrogen in the star's core.
As the core contracts and heats up, the outer layers will expand, the Sun's surface temperature will drop, but its total brightness will be much higher than today. When sufficient temperature is reached, helium fusion will begin in the core and carbon will form.
In the following stages, the Sun will shed its outer layers into space, leaving behind a hot, dense white dwarf.
The red giant phase is a significant stellar stage in the Sun's aging process, during which its size grows extraordinarily, its surface cools and reddens, and helium fusion begins in its core.