CBSE NOTES CLASS 12 PHYSICS
CHAPTER 13 NUCLEUS
The proton-proton (p, p) cycle in the sun
The final balanced reaction after combining (i) to (iv)
As the hydrogen in the core gets depleted and becomes helium, the core starts to cool. The star begins to collapse under its own gravity which increases the temperature of the core. If this temperature increases to about 108 K, fusion takes place again, this time of helium nuclei into carbon. This process can generate through fusion higher and higher mass number elements. But elements more massive than those near the peak of the binding energy curve in cannot be produced.
The age of the sun is about 5×109 y and it is estimated that there is enough hydrogen in the sun to keep it going for another 5 billion years.
After that, the hydrogen burning will stop and the sun will begin to cool and will start to collapse under gravity, which will raise the core temperature. The outer envelope of the sun will expand, turning it into the so called red giant.
Controlled thermonuclear reaction may be possible in future.