Comparison of the current-day Sun and the Sun as a red giant in the future. by
Oona Räisänen. 2007.
Gravity as the Driving Force
The Sun has a mass of 2 * 10^30 kg. Gravity exerts a compression force on the Sun proportional to this immense mass. So why doesn’t the sun collapse under the weight of its gravity?
The pressure of the center of the Sun is about 340 billion times the air pressure on Earth at sea level. Temperatures at the Sun’s core reach 15 million Kelvin. The conditions at the Sun’s core allow nuclear reactions to occur.
We will leave the exact reactions for a different time. Nevertheless, the basic reaction for stars the size of our Sun is called the proton-proton chain:
The nuclear reactions inside the core result in energy and an outward pressure that combats the inward pressure of gravity. Gravity is the driving force behind the nuclear reactions that power the Sun, which in turn determines its size.
Hydrostatic Equilibrium
While the core of the Sun is able to fuse hydrogen into helium, the size of the Sun will be relatively stable. The outward pressure of the reaction matches the inward force of gravity exerted on a star proportional to its mass.
During this period, the Sun is in “Hydrostatic Equilibrium” along the main sequence. Eventually, the Sun’s core will run out of hydrogen to fuse. The core will begin to contract and core temperatures will increase.
Red Giants
Once the core of the Sun runs out of hydrogen material to fuse, the core will begin to collapse. The extreme temperature and pressure caused by the core collapsing allows layers of hydrogen just outside the core (which previously had no role in nuclear fusion) to begin reactions. This outer layer contains more volume. Additionally, the star uses a different fusion reaction that results in the star producing much greater net energy.
The Sun will expand and become a Red Giant due to the greater outward pressure exerted as a response to the force of gravity collapsing the star.
Post-Red Giant
Our Red Giant Sun will eventually lose much of its mass and its emitted material will become a planetary nebula. It will become a white dwarf and slowly cool.
Gravity initiates the process that forms nebulae and stars, influences the formation and size of the star, and determines the life cycle and death of the star. In this way, gravity is the catalyst for change, and the driving force, in the life of our Sun.
Sources
- Cain, Fraiser. “Why Do Red Giants Expand?”
- Milner. “Why Does A Star Expand As It Becomes a Red Giant?”
- Ryden, Barbara. Ohio State. “Inside The Sun.”
- Waller, Brad. “Why Doesn’t The Sun Collapse?”
- Wikipedia. “Proton-Proton Chain Reaction”
- Wikipedia. “Triple-Alpha Process.”
- Williams, Matt. “What is the Life Cycle Of The Sun?”
JSpin 
I really enjoyed your post! Your explanation of the sun’s lifetime and reasons for the future changes it will undergo was thorough and really helpful. I particularly loved the image you used to start the post that compares the sun’s current size to it’s expected future size. The sun is already so large it’s hard to fathom that the current size is a speck compared to the red giant size it will reach. Something that could also be interesting to think about are the other effects on the sun when it becomes a red giant, such as how temperature and luminosity will be impacted. Also it would be interesting to investigate how long the sun has until it runs out of hydrogen and moves off of the main sequence and how long it can exist as a red giant before it expels its planetary nebula.
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