What will cause the Sun to turn into a red giant?Report an ErrorThe Sun starting to fuse carbon.The Sun transitioning from fusion to fission.The Sun using up its hydrogen.The Sun emitting a planetary nebula.

The Sun will turn into a red giant when it uses up its hydrogen. Here's a step-by-step explanation:

1. The Sun, like all stars, is currently fusing hydrogen in its core to produce helium, a process that releases a large amount of energy. This energy is what makes the Sun shine.

2. Over billions of years, the Sun will eventually use up the hydrogen in its core.

3. When this happens, the core of the Sun will start to contract under its own gravity. This will cause the temperature and pressure in the core to increase.

4. The increased temperature and pressure will cause the hydrogen in a shell around the core to start fusing into helium.

5. The energy from this fusion process will cause the outer layers of the Sun to expand, and the Sun will become a red giant.

6. The Sun will stay in this red giant phase for a few million years, before eventually shedding its outer layers and becoming a white dwarf.

Note: The Sun will not start to fuse carbon or transition from fusion to fission. These processes occur in larger stars, not in a star of the Sun's mass. Also, the Sun will emit a planetary nebula, but this will happen after it has become a red giant, not as a cause of it becoming a red giant.