We get used to the notion of a huge yellow ball orbiting around the Earth in approximately twenty-four hours. It is the essence of all of us in the beginning. The reason is as follows. The life on the Earth depends on the Sun and its unique location to our planet. However, is the Sun same in all this time without a little fluctuation in its brightness.
Brightness depends on the energy level of an object, for instance, the hotter iron has a higher luminosity than the cold one. Which is why the Sun has a different luminosity level than its present luminosity level. Nuclear reactions give the Sun energy to shine, especially hydrogen reactions in its core. Hydrogen turns into helium because of the pressure of the mass of the Sun and nuclear reactions continue to create heavier elements until the core cannot preserve itself with the electron degeneracy pressure occurred by the nuclear reactions but it is not related to the subject right now. When nuclear combustion began in the core the young Sun, the luminosity of our star shot up briefly to about 20 times its current level. Then it diminished and stabilized at about 70% of its present-day value.(1) In other words, as the Sun has started to burn its fuel – hydrogen – because of its enormous mass, the Sun gained a sudden energy shift due to the speed of the nuclear reactions in its core, and it caused an increase in the luminosity level of the Sun. But, afterwards, the Sun lost the sudden energy shift, and therefore stabilized. Nevertheless, the luminosity level of the Sun gradually increases even now you are reading this. The reason is as follows. Nuclear reactions perpetuate the deterioration in maintaining the core of the Sun by using the elements heavier than hydrogen as a fuel resource even after the Sun enters the red giant phase about 6.5 billion years from now. We know the exact time of the collapse of the Sun because nucleocosmochronology allows us to determine the age of celestial bodies from the decay of their radioactive elements(2). In other words, it is the notion of evaluating the electromagnetic radiation radiated by celestial bodies.
The Sun will not be as bright as it is now, it will be brighter than we could imagine until it consumes all life by its light surrounding the Earth as fires.
(1) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "Has the Sun always been as bright as it is now?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 40. Print.
(2) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "How old is the Sun?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 40. Print.
Figure - 18.1 https://cdn.pixabay.com/photo/2016/06/16/14/50/sundown-1461426_960_720.jpg
Figure - 18.2 https://www.worldatlas.com/r/w728-h425-c728x425/upload/4c/55/b8/untitled-design-163.jpg
Figure - 18.3 https://www.nationalgeographic.com/science/space/solar-system/the-sun/
Figure - 18.4 http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/imgast/mseq.png
Figure - 18.5 http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/imgast/Carboncyc.gif
Figure - 18.6 https://www.universetoday.com/wp-content/uploads/2013/07/Red_Giant_Earth_warm.jpg