Question

In which stage of the star does nuclear fusion reaction begin?

Answer 1

A reaction in which two or more nuclei combine and form one or more atomic nuclei along with subatomic particles like neutrons or protons, is called nuclear fusion reaction.

Complete step by step answer:
In case of protostar mechanism is as follows:
Nuclear Reaction in the core of protostar:
$4{}_{}^1H\xrightarrow{{4protons}}1{}^4He + photons + neutrinos$
The condition for fusion to occur in the core of a star is- when the temperature and density of the core are high enough. This happens in the early stage of the star or Protostar stage. During this stage, the star is still in the process of gathering mass from its parent molecular cloud.
Different fusion cycles occur through different phases. The first one involves the fusion of Hydrogen to Helium. An interesting thing to know is our Sun is in this stage.

Fusion reaction begins in the protostar stage of the star.

Additional Information:
There are seven main stages of the star:
1.A giant gas cloud
2.Protostar or baby star
3.The T-Tauri Phase
4.Main sequence star
5.Expansion into Red giant
6.Fusion of Heavier Element
7.Supernovae and Planetary Nebulae
Every stage implies very complex and different mechanisms going in it.

Note:
Inside the Sun, nuclear fusion reactions take place by squeezing 4 hydrogen nuclei together to result in a single Helium molecule. In this process, in order to stabilize, some mass of hydrogen is converted into energy, which is emitted out of the sun. This emitted heat energy we along with other planets receive. This Hydrogen fuel inside the core of any star is very limited and exhausted in finite time. After that star dies, similarly our Sun fuel would also be exhausted and we won’t get light after that.