Caption: The plot shows the evolution of the Sun's luminosity, radius, and effective temperature with time. Thus, the illustrates key aspects of the Sun's life phases.

Features:

1. The quantities are normalized to present-day values for the Sun.

2. The time-zero is the zero-age main sequence (ZAMS) phase and past ∼ 10 Gyr the Sun is evolving to its red giant phase.

3. The Sun's red giant phase is estimated to begin in about 5.4 Gyr (see Wikipedia: Sun: After core hydrogen exhaustion).

   The plot is NOT quite consistent with the estimate, but exact consistency between different astro sources is hard to find. Calculations of the model quantities vary somewhat between sources. This is a consequence of uncertainties in the modeling.

4. As you can see in the plot, soon after formation when the Sun just arrived on the main sequence, it was probably ∼ 30 % less bright than now.

   About 3.5 Gyr from now, the Sun will probably be ∼ 30 % brighter than now.

5. The secular increase (i.e., long-term increase) in the Sun's luminosity (i.e., the solar brightening) is a purely theoretical prediction since it is unobservably slow over human history. But it is as certain as pure theory can be since we understand main-sequence stars very well in their main behaviors.

   The solar brightening is a pretty modest change for the Sun for next 5 Gyr or so.

6. But alas, the solar brightening spells the doom of complex life on Earth and then later all life on Earth (see IAL 11: The Earth; Wikipedia: Future of Earth: Climate Impact; Rampino, M. R. & Caldiera, K. 1994, ARAA, 32, 83-114, The Goldilocks Problem: Climatic Evolution and Long-Term Habitability of Terrestrial Planets, R.J. Graham, Itay Halevy, Dorian Abbot, 2024, Substantial extension of the lifetime of the terrestrial biosphere):
   1. First, within a gigayear or so carbon dioxide (CO_2) will probably be removed from the Earth's atmosphere by weathering which locks it up in calcium carbonate (CaCO_3). The volcanic outgassing of CO_2 probably can't keep up with the weathering which speeds up as the Earth gets hotter with the solar brightening. With no CO_2, most plant life must cease (because NO CO_2, NO photosynthesis) and necessarily most animal life pari passu.

   2. Second, the extra heating of the Earth will cause the troposphere to extend much higher than it now does. Then convection will carry water vapor much higher where increased photodissociation by stronger ultraviolet (UV) light (see Wikipedia: Electromagnetic absorption by water: Electronic spectrum) will break up water molecules (H2O) and the freed hydrogen will undergo atmospheric escape. On the time scale of 2 Gyr, the Earth's liquid water will be gone. This will turn off weathering to calcium carbonate (CaCO_3). This will then lead volcanic outgassing to replenish the atmosphere CO_2 and then give the Earth a thick CO_2 dominated planetary atmosphere like the Venusian atmosphere which lead to a runaway greenhouse effect which will make Earth hellish like Venus.

   We don't have to worry about this melancholy story since it is on a time scale far longer than human history. We only contemplate it philosophically.