Question

We have learnt that oxygen is required for something to burn but there is no oxygen in space then how do the stars burn?

Answer 1

The way the Sun "burns" fuel is completely different from the way a fire on Earth burns (the term "burning" is a bit misleading when used to talk about stars). The Sun gets its energy by smashing small light elements together to make heavier elements; most of a star's life is spent smashing hydrogen atoms together to make helium. The burning that a star does, then, is a nuclear reaction, and not a chemical one like the fires on Earth (when a candle burns, the atoms themselves remain unchanged: just the molecules are affected).

So, the Sun can "burn" hydrogen to helium without the need for oxygen. It should be noted that in the presence of carbon, nitrogen and oxygen, stars heavier than the Sun may burn hydrogen to helium by using the C, N and O as catalysts. Even in these stars, however, an absence of oxygen does not prevent nuclear burning.