How is the sun burning in space without oxygen?

Hint: The reaction which requires oxygen is known as combustion reaction. In combustion reaction heat is released in the form of energy. In space no oxygen is present than also sun burns but no chemical reaction takes place.

Complete step by step answer:
The combustion reaction is the chemical reaction which only takes place in the presence of oxygen where the carbon atom from the fuel binds with the oxygen present in the air to form carbon dioxide and carbon monoxide. Hydrogen also combines with oxygen to form water. In the combustion reaction energy is released which is observed as heat.
But the burning of the sun is not a chemical combustion, it is a nuclear fusion. The sun is considered as the giant hydrogen bomb. In the nuclear fusion, the nuclei of the atoms fused with each other to form a larger nuclei. The nuclear fusion does not involve oxygen.
There are two main forces taking place in nuclear fusion. These are the electromagnetic force and the strong nuclear force. The repulsive electromagnetic force present between the positive charge nuclei is weak whereas the attractive strong nuclear force is strong. When the two nuclei are far from each other the repulsive electromagnetic force dominates as the two nuclei get closer to each other the repulsive electromagnetic repulsion gets stronger and it becomes hard to push the nuclei together. As the two nuclei get close to each other, the attractive short range nuclear force dominates and the nuclei join to form a new nucleus. Due to this it requires a lot of pressure to push nuclei close enough so they fuse together.
The lighter elements like hydrogen and helium have little electromagnetic repulsion as they have little electric charge. In stars the fusion takes place by hydrogen fusing with itself or with other light elements. Gravity provides the pressure to ignite the nuclear fusion in stars. The gravity needs mass so a big mass of hydrogen is required to burn a star.

Note:
Don’t get confused as the sun is considered as one of the stars. In stars there is very little amount of oxygen or no oxygen so the oxygen present there was formed by hydrogen fusing repeatedly until the oxygen was formed.