Question

Why we can't live without sun?​

Answer 1

If the Sun disappeared or the Earth were flung out of its orbit the travel alone through interstellar space, then I think a small population of humans could survive (although we might need some advance warning).

While the surface of the Earth would freeze and become uninhabitable pretty quickly, there is enough internal heat in the core (left over from the formation of the Earth and generated by radioactive decay) that it would remain warm underground for billions of years. A small population (a few hundred people, maybe) could live in a suitably modified deep mine. With either nuclear power or geothermal power, they could power life support systems and grow food hydroponically. There are enough suitable deep mines for several of those survivor colonies.

Step-by-step explanation:

Answer 2

Without the sun there would be no vegetation on earth, because every plant needs light to live and grow. The sun even enables them to generate oxygen by means of photosynthesis, which humans and animals need to breathe. The earth's position in the solar system is optimal for the development of life.