Question

If fire needs oxygen then how sun is burning in space? As there is no oxygen in space.

Answer 1

Answer. The answer is much simpler than it seems. Sun “burns”, i.e. emits light and creates heat because of thermonuclear reactions [1]. Fire “burns”, i.e. emits lite and produces heat because of chemical reactions [2]. Thermonuclear reactions are due do interactions of atomic nuclei, and chemical ones are due to interactions of molecules, so that is the main difference.
[1] https://en.wikipedia.org/wiki/Sun#Structure_and_energy_production
[2] https://en.wikipedia.org/wiki/Fire