For the sun to turn into a red giant, which of the following should happen?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All of the above
The sun shines by burning the hydrogen in its core into helium through fusion.
Option (1) It will collapse under its own gravity. — Incorrect. This does happen, because the cooling core can no longer hold the sun up against gravity. But it is only one of the three steps.
Option (2) The hydrogen in its core will be depleted. — Incorrect. This is true, and it is the step that stops the hydrogen burning. Alone, it is not the full answer.
Option (3) It will start to cool. — Incorrect. The core does cool once fusion stops. Still, this option gives only one of the three steps.
Option (4) All of the above — Correct. The three happen one after another:
When the hydrogen in the core is depleted and has become helium, the hydrogen burning stops. The sun then begins to cool, so (2) and (3) hold.
Next, the sun starts to collapse under its own gravity, which raises the core temperature. Its outer envelope expands and turns it into a red giant, so (1) holds.
NCERT (2026-27):
Nuclear Fusion – Energy Generation in Stars – “… 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.”
Nuclear Fusion – Energy Generation in Stars – “… 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.”