Why does the sun will increase in size as it begins to burn off its fule?

Answer 1

because helium is larger than hydrogen. Our sun will become a red giant, burn away Mercury, venus and possibly earth, then run out of gas and become a dwarf.

The death of a star is a complelex process and is very different for individual stats. A stars demise is dependent on it's mass. Since you are asking about stars that increase in size, I will answer your question adressing star of low to medium mass like our sun. The life of a star is spent in a balancing act between the force of gravity pulling the star inward and the outward force of pressuure of the gas, plasma, and heat caused by nuclear fusion inside the star. As hydrogen, the fuel you speak of, is converted to helium, nuclear fusion begins to decrease. As pressure drops, gravity takes over and causes the star to shrink. This is where things get interesting. As the star shrinks, pressure and heat begin to increase again and when the star reaches a critical pressure and temperature, helium begins to fuse into carbon, oxygen, and other medium elements. This "second" round of fusion releases copius amounts of energy that can, and often does, cause the outer layers of the star to swell up and dissipate into space. This is why, in 4+ billion years, our sun will expand and strip the Earth of its surface. Remeber, this is the scenario for low and medium size star. The end of larger is much more dramatic.

Answer 2

As the sun burns off its nuclear fuel, the core temperature increases, causing the outer layers to expand due to increased pressure. This expansion leads to an increase in the sun's size and brightness as it transitions into the red giant phase.