Hydrogen:
* Decreasing: The Sun is currently fusing hydrogen into helium in its core. This process is what powers the Sun and provides its energy. As the Sun continues to burn, its hydrogen fuel will gradually deplete.
* Rate of Decrease: The rate of hydrogen consumption will increase as the Sun ages. This is because the core will become denser and hotter, leading to faster fusion rates.
* Eventually Gone: In about 5 billion years, the Sun will have exhausted most of the hydrogen in its core.
Helium:
* Increasing: As hydrogen is converted to helium, the amount of helium in the Sun's core will steadily increase.
* Fuel for Future Fusion: Helium will eventually become the dominant element in the core. However, helium fusion requires much higher temperatures and pressures than hydrogen fusion. This will mark a dramatic change in the Sun's evolution.
* Red Giant Phase: Once helium fusion begins, the Sun will expand dramatically into a red giant star. The increased energy output will push the outer layers of the Sun outward, engulfing Mercury, Venus, and potentially even Earth.
The Future of the Sun
* Beyond Helium: After the helium in the Sun's core is exhausted, the Sun will enter a series of unstable phases, eventually shedding its outer layers to form a planetary nebula.
* White Dwarf: The Sun's core will remain as a dense, hot white dwarf, slowly cooling over billions of years.
In summary:
The Sun's journey over the next few billion years will be a story of gradual depletion of hydrogen, accumulation of helium, and ultimately, a dramatic transformation into a red giant, followed by its eventual demise as a white dwarf.
